Pub. No. 9
THE AMERICAN PRACTICAL NAVIGATOR
AN EPITOME OF NAVIGATION
ORIGINALLY BY
NATHANIEL BOWDITCH, LL.D.
1995 EDITION
Prepared and published by the
NATIONAL IMAGERY AND MAPPING AGENCY Bethesda, Maryland
© COPYRIGHT 1995 BY THE UNITED STATES GOVERNMENT. NO COPYRIGHT CLAIMED UNDER TITLE 17 U.S.C.
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For sale by the National Ocean Service and its authorized Sales Agents

Last painting by Gilbert Stuart (1828). Considered by the family of Bowditch to be the best of various paintings made, although it was unfinished when the artist died.

NATHANIEL BOWDITCH
(1773-1838)

Nathaniel Bowditch was born on March 26, 1773, in Salem, Mass., fourth of the seven children of shipmaster Habakkuk Bowditch and his wife, Mary.
Since the migration of William Bowditch from England to the Colonies in the 17th century, the family had resided at Salem. Most of its sons, like those of other families in this New England seaport, had gone to sea, and many of them became shipmasters. Nathaniel Bowditch himself sailed as master on his last voyage, and two of his brothers met untimely deaths while pursuing careers at sea.
It is reported that Nathaniel Bowditch’s father lost two ships at sea, and by late Revolutionary days he returned to the trade of cooper, which he had learned in his youth. This provided insufficient income to properly supply the needs of his growing family, and hunger and cold were often experienced. For many years the nearly destitute family received an annual grant of 15 to 20 dollars from the Salem Marine Society. By the time Nathaniel had reached the age of 10, the family’s poverty necessitated his leaving school and joining his father in the cooper’s trade.
Nathaniel was unsuccessful as a cooper, and when he was about 12 years of age, he entered the first of two shipchandlery firms by which he was employed. It was during the nearly 10 years he was so employed that his great mind first attracted public attention. From the time he began school Bowditch had an all-consuming interest in learning, particularly mathematics. By his middle teens he was recognized in Salem as an authority on that subject. Salem being primarily a shipping town, most of the inhabitants sooner or later found their way to the ship chandler, and news of the brilliant young clerk spread until eventually it came to the attention of the learned men of his day. Impressed by his desire to educate himself, they supplied him with books that he might learn of the discoveries of other men. Since many of the best books were written by Europeans, Bowditch first taught himself their languages. French, Spanish, Latin, Greek, and German were among the two dozen or more languages and dialects he studied during his life. At the age of 16 he began the study of Newton’s Principia, translating parts of it from the Latin. He even found an error in that classic, and though lacking the confidence to announce it at the time, he later published his findings and had them accepted.
During the Revolutionary War a privateer out of Beverly, a neighboring town to Salem, had taken as one of its prizes an English vessel which was carrying the philosophical library of a famed Irish scholar, Dr. Richard Kirwan. The books were brought to the Colonies and there bought by a group of educated Salem men who used them to found the Philosophical Library Company, reputed to have been the best library north

of Philadelphia at the time. In 1791, when Bowditch was 18, two Harvard-educated ministers, Rev. John Prince and Rev. William Bentley, persuaded the Company to allow Bowditch the use of its library. Encouraged by these two men and a thirdNathan Read, an apothecary and also a Harvard man-Bowditch studied the works of the great men who had preceded him, especially the mathematicians and the astronomers. By the time he became of age, this knowledge, acquired before and after his long working hours and in his spare time, had made young Bowditch the outstanding mathematician in the Commonwealth, and perhaps in the country.
In the seafaring town of Salem, Bowditch was drawn to navigation early, learning the subject at the age of 13 from an old British sailor. A year later he began studying surveying, and in 1794 he assisted in a survey of the town. At 15 he devised an almanac reputed to have been of great accuracy. His other youthful accomplishments included the construction of a crude barometer and a sundial.
When Bowditch went to sea at the age of 21, it was as captain’s writer and nominal second mate, the officer’s berth being offered him because of his reputation as a scholar. Under Captain Henry Prince, the ship Henry sailed from Salem in the winter of 1795 on what was to be a year-long voyage to the Ile de Bourbon (now called Reunion) in the Indian Ocean.
Bowditch began his seagoing career when accurate time was not available to the average naval or merchant ship. A reliable marine chronometer had been invented some 60 years before, but the prohibitive cost, plus the long voyages without opportunity to check the error of the timepiece, made the large investment an impractical one. A system of determining longitude by “lunar distance,” a method which did not require an accurate timepiece, was known, but this product of the minds of mathematicians and astronomers was so involved as to be beyond the capabilities of the uneducated seamen of that day. Consequently, ships navigated by a combination of dead reckoning and parallel sailing (a system of sailing north or south to the latitude of the destination and then east or west to the destination). The navigational routine of the time was “lead, log, and lookout.”
To Bowditch, the mathematical genius, computation of lunar distances was no mystery, of course, but he recognized the need for an easier method of working them in order to navigate ships more safely and efficiently. Through analysis and observation, he derived a new and simplified formula during his first trip.
John Hamilton Moore’s The Practical Navigator was the leading navigational text when Bowditch first went to sea, and had been for many years. Early in his first voyage, however, the captain’s writer-second mate began turning up

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errors in Moore’s book, and before long he found it necessary to recompute some of the tables he most often used in working his sights. Bowditch recorded the errors he found, and by the end of his second voyage, made in the higher capacity of supercargo, the news of his findings in The New Practical Navigator had reached Edmund Blunt, a printer at Newburyport, Mass. At Blunt’s request, Bowditch agreed to participate with other learned men in the preparation of an American edition of the thirteenth (1798) edition of Moore’s work. The first American edition was published at Newburyport by Blunt in 1799. This edition corrected many of the errors that Moore had failed to correct. Although most of the errors were of little significance to practical navigation as they were errors in the fifth and sixth places of logarithm tables, some errors were significant.
The most significant error was listing the year 1800 as a leap year in the table of the sun’s declination. The consequence was that Moore gave the declination for MARCH 1, 1800, as 7°11'. Since the actual value was 7° 33', the calculation of a meridian altitude would be in error by 22 minutes of latitude.
Bowditch’s principal contribution to the first American edition was his chapter “The Method of finding the Longitude at Sea,” which was his new method for computing the lunar distance. Following publication of the first American edition, Blunt obtained Bowditch’s services in checking the American and English editions for further errors. Blunt then published a second American edition of Moore’s thirteenth edition in 1800. When preparing a third American edition for the press, Blunt decided that Bowditch had revised Moore’s work to such an extent that Bowditch should be named as author. The title was changed to The New American Practical Navigator and the book was published in 1802 as a first edition. Bowditch vowed while writing this edition to “put down in the book nothing I can’t teach the crew,” and it is said that every member of his crew including the cook could take a lunar observation and plot the ship’s position.
Bowditch made a total of five trips to sea, over a period of about nine years, his last as master and part owner of the three-masted Putnam. Homeward bound from a 13-month voyage to Sumatra and the Ile de France (now called Mauritius) the Putnam approached Salem harbor on December 25, 1803, during a thick fog without having had a celestial observation since noon on the 24th. Relying upon his dead reckoning, Bowditch conned his wooden-hulled ship to the entrance of the rocky harbor, where he had the good fortune to get a momentary glimpse of Eastern Point, Cape Ann, enough to confirm his position. The Putnam proceeded in, past such hazards as “Bowditch’s Ledge” (named after a great-grandfather who had wrecked his ship on the rock more than a century before) and anchored safely at 1900 that evening. Word of the daring feat, performed when other masters were hove-to outside the harbor, spread along the coast and added greatly to Bowditch’s reputation. He was, indeed, the “practical navigator.”

His standing as a mathematician and successful shipmaster earned him a lucrative (for those times) position ashore within a matter of weeks after his last voyage. He was installed as president of a Salem fire and marine insurance company at the age of 30, and during the 20 years he held that position the company prospered. In 1823 he left Salem to take a similar position with a Boston insurance firm, serving that company with equal success until his death.
From the time he finished the “Navigator” until 1814, Bowditch’s mathematical and scientific pursuits consisted of studies and papers on the orbits of comets, applications of Napier’s rules, magnetic variation, eclipses, calculations on tides, and the charting of Salem harbor. In that year, however, he turned to what he considered the greatest work of his life, the translation into English of Mecanique Celeste, by Pierre Laplace. Mecanique Celeste was a summary of all the then known facts about the workings of the heavens. Bowditch translated four of the five volumes before his death, and published them at his own expense. He gave many formula derivations which Laplace had not shown, and also included further discoveries following the time of publication. His work made this information available to American astronomers and enabled them to pursue their studies on the basis of that which was already known. Continuing his style of writing for the learner, Bowditch presented his English version of Mecanique Celeste in such a manner that the student of mathematics could easily trace the steps involved in reaching the most complicated conclusions.
Shortly after the publication of The New American Practical Navigator, Harvard College honored its author with the presentation of the honorary degree of Master of Arts, and in 1816 the college made him an honorary Doctor of Laws. From the time the Harvard graduates of Salem first assisted him in his studies, Bowditch had a great interest in that college, and in 1810 he was elected one of its Overseers, a position he held until 1826, when he was elected to the Corporation. During 1826-27 he was the leader of a small group of men who saved the school from financial disaster by forcing necessary economies on the college’s reluctant president. At one time Bowditch was offered a Professorship in Mathematics at Harvard but this, as well as similar offers from West Point and the University of Virginia, he declined. In all his life he was never known to have made a public speech or to have addressed any large group of people.
Many other honors came to Bowditch in recognition of his astronomical, mathematical, and marine accomplishments. He became a member of the American Academy of Arts and Sciences, the East India Marine Society, the Royal Academy of Edinburgh, the Royal Society of London, the Royal Irish Academy, the American Philosophical Society, the Connecticut Academy of Arts and Sciences, the Boston Marine Society, the Royal Astronomical Society, the Palermo Academy of Science, and the Royal Academy of Berlin.
Nathaniel Bowditch outlived all of his brothers and sisters by nearly 30 years. Death came to him on March 16, 1838, in his sixty-fifth year. The following eulogy by the

iv

Salem Marine Society indicates the regard in which this distinguished American was held by his contemporaries:
“In his death a public, a national, a human benefactor has departed. Not this community, nor our country only, but the whole world, has reason to do honor to his memory. When the voice of Eulogy shall be still, when the tear of Sorrow shall cease to flow, no monument will be needed to keep alive his memory among men; but as long as ships shall sail, the needle point to the north, and the stars go through their wonted courses in the heavens, the name of Dr. Bowditch will be revered as of one who helped his fellow-men in a time of need, who was and is a guide to them over the pathless ocean, and of one who forwarded the great interests of mankind.”
The New American Practical Navigator was revised by Nathaniel Bowditch several times after 1802 for subsequent

editions of the book. After his death, Jonathan Ingersoll Bowditch, a son who made several voyages, took up the work and his name appeared on the title page from the eleventh edition through the thirty-fifth, in 1867. In 1868 the newly organized U.S. Navy Hydrographic Office bought the copyright. Revisions have been made from time to time to keep the work in step with navigational improvements. The name has been altered to the American Practical Navigator, but the book is still commonly known as “Bowditch.” A total of more than 900,000 copies has been printed in about 75 editions during the nearly two centuries since the book was first published in 1802. It has lived because it has combined the best techniques of each generation of navigators, who have looked to it as their final authority.

v

Original title page of The New American Practical Navigator, First Edition, published in 1802. vi

PREFACE

The Naval Observatory library in Washington, D.C., is unnaturally quiet. It is a large circular room, filled with thousands of books. Its acoustics are perfect; a mere whisper from the room’s open circular balcony can be easily heard by those standing on the ground floor. A fountain in the center of the ground floor softly breaks the room’s silence as its water stream slowly splashes into a small pool. A library clerk will lead you into a small antechamber where there is a vault containing the Observatory’s most rare books. In this vault, one can find an original 1802 first edition of the New American Practical Navigator.
One cannot hold this small, delicate, slipcovered book without being impressed by the nearly 200-year unbroken chain of publication that it has enjoyed. It sailed on U.S. merchantmen shortly after the quasi-war with France and during British impressment of merchant seamen that led to the War of 1812. It sailed on U.S. Naval vessels during operations against Mexico in the 1840’s, on ships of both the Union and Confederate fleets during the Civil War, and with the U.S. Navy in Cuba in 1898. It went with the Great White Fleet around the world, across the North Atlantic to Europe during both World Wars, to Asia during the Korean and Vietnam Wars, and to the Middle East during Operation Desert Storm.
As navigational requirements and procedures have changed throughout the years, Bowditch has changed with them. Originally devoted almost exclusively to celestial navigation, it now also covers a host of modern topics. It is as practical today as it was when Nathaniel Bowditch, master of the Putnam, gathered the crew on deck and taught them the mathematics involved in calculating lunar distances. It is that practicality that has been the publication’s greatest strength. It is that practicality that makes the publication as useful today as it was in the age of sail.
Seafarers have long memories. In no other profession is tradition more closely guarded. Even the oldest and most cynical acknowledge the special bond that connects those who have made their livelihood plying the sea. This bond is not comprised of a single strand; rather, it is a rich and varied tapestry that stretches from the present back to the birth of our nation and its seafaring culture. As this book is a part of that tapestry, it should not be lightly regarded; rather, it should be preserved, as much for its historical importance as for its practical utility.
Since antiquity, mariners have gathered available navigation information and put it into a text for others to follow. One of the first attempts at this involved volumes of Spanish and Portuguese navigational manuals translated into English between about 1550 to 1750. Writers and translators of the time “borrowed” freely in compiling nav-

igational texts, a practice which continues today with works such as Sailing Directions and Pilots.
Colonial and early American navigators depended exclusively on English navigation texts because there were no American editions. The first American navigational text, Orthodoxal Navigation, was completed by Benjamin Hubbard in 1656. The first American navigation text published in America was Captain Thomas Truxton’s Remarks, Instructions, and Examples Relating to the Latitude and Longitude; also the Variation of the Compass, Etc., Etc., published in 1794.
The most popular navigational text of the late 18th century was John Hamilton Moore’s The New Practical Navigator. Edmund M. Blunt, a Newburyport publisher, decided to issue a revised copy of this work for American navigators. Blunt convinced Nathaniel Bowditch, a locally famous mariner and mathematician, to revise and update The New Practical Navigator. Several other men also assisted in the revision. Blunt’s The New Practical Navigator was published in 1799. Blunt also published a second American edition of Hamilton’s book in 1800.
By 1802, when Blunt was ready to publish a third edition, Nathaniel Bowditch and others had corrected so many errors in Hamilton’s work that Blunt decided to issue the work as a first edition of the New American Practical Navigator. It is to that 1802 work that the current edition of the American Practical Navigator traces its pedigree.
The New American Practical Navigator stayed in the Bowditch and Blunt family until the government bought the copyright in 1867. Edmund M. Blunt published the book until 1833; upon his retirement, his sons, Edmund and George, took over publication.The elder Blunt died in 1862; his son Edmund followed in 1866. The next year, 1867, George Blunt sold the copyright to the government for $25,000. The government has published Bowditch ever since. George Blunt died in 1878.
Nathaniel Bowditch continued to correct and revise the book until his death in 1838. Upon his death, the editorial responsibility for the American Practical Navigator passed to his son, J. Ingersoll Bowditch. Ingersoll Bowditch continued editing the Navigator until George Blunt sold the copyright to the government. He outlived all of the principals involved in publishing and editing the Navigator, dying in 1889.
The U.S. government has published some 52 editions since acquiring the copyright to the book that has come to be known simply by its original author’s name, “Bowditch”. Since the government began production, the book has been known by its year of publishing, instead of by the edition number. During a revision in 1880 by Commander Phillip H. Cooper, USN, the name was changed to American Prac-

vii

tical Navigator. Bowditch’s original method of taking “lunars” was finally dropped from the book in 1914. After several more minor revisions and printings, Bowditch was extensively revised between 1946 and 1958.
The present volume, while retaining the basic format of the 1958 version, reorganizes the subjects, deletes obsolete text, and adds new material to keep pace with the extensive changes in navigation that have taken place in the electronic age.
This 1995 edition of the American Practical Navigator incorporates extensive changes in organization, format, and content. Recent advances in navigational electronics, communications, positioning, and other technologies have transformed the way navigation is practiced at sea, and it is clear that even more changes are forthcoming. The changes to this edition of BOWDITCH are intended to ensure that this publication remains the premier reference work for practical marine navigation. Concerted efforts were made to return to Nathaniel Bowditch’s original intention “to put down in the book nothing I can’t teach the crew.” To this end, many complex formulas and equations have been eliminated, and emphasis placed on the capabilities and limitations of various navigation systems and how to use them, instead of explaining complex technical and theoretical details. This edition replaces but does not cancel former editions, which may be retained and consulted as to navigation methods not discussed herein.
The former Volume II has been incorporated into this volume to save space and production cost. A larger page size has also been chosen for similar reasons. These two changes allow us to present a single, comprehensive navigation science reference which explains modern navigational methods while respecting traditional ones. The goal of the changes is to put as much useful information before the navigator as possible in the most understandable and readable format.
TAB 1, FUNDAMENTALS, has been reorganized to include an overview of the types and phases of marine navigation and the organizations which support and regulate it. It includes chapters relating to the structure, use and limitations of nautical charts; chart datums and their importance; and other material of a basic nature. The former chapter on the history of navigation has been largely removed. Historical facts are included in the text where necessary to explain present practices or conventions.
TAB 2, PILOTING, now emphasizes the practical aspects of navigating a vessel in restricted waters.
TAB 3, ELECTRONIC NAVIGATION, returns to the position it held in the 1958 edition. Electronic systems are now the primary means of positioning of the modern navigator. Chapters deal with each of the several electronic methods of navigation, organized by type.
TAB 4, CELESTIAL NAVIGATION, has been streamlined and updated. The text in this section contains updated examples and problems and a completely re-edited sight reduction chapter. Extracts from necessary tables have been

added to the body of the text for easier reference. TAB 5, NAVIGATIONAL MATHEMATICS, includes
chapters relating to such topics as basic navigational mathematics and computer use in the solution of navigation problems.
TAB 6, NAVIGATIONAL SAFETY, discusses aspects of the new distress and safety communications systems now in place or being implemented in the next several years, as well as navigation regulations, emergency navigation procedures, and distress communications.
TAB 7, OCEANOGRAPHY, is updated and consolidated, but largely unchanged from the former edition.
TAB 8, MARINE METEOROLOGY, (formerly WEATHER) incorporates new weather routing and forecasting methods and material from former appendices. Included are new color plates of the Beaufort Sea States (Courtesy of Environment Canada).
The Glossary has been extensively edited and updated with modern navigational terms, including computer terminology.
This edition was produced largely electronically from start to finish, using the latest in publishing software and data transfer techniques to provide a very flexible production system. This ensures not only that this book is the most efficiently produced ever, but also that it can be easily updated and improved when it again becomes dated, as it surely will.
The masculine pronoun “he” used throughout is meant to refer to both genders.
This book may be kept corrected using the Notice to Mariners and Summary of Corrections. Suggestions and comments for changes and additions may be sent to:
NAVIGATION DIVISION ST D 44 DMA HYDROGRAPHIC/TOPOGRAPHIC CENTER 4600 SANGAMORE ROAD BETHESDA, MD 20816-5003
This book could not have been produced without the expertise of dedicated personnel from many organizations, among them: U.S. Coast Guard, U.S. Naval Academy, U.S. Naval Oceanographic Office, Fleet Training Center (Norfolk), Fleet Numerical Meteorology and Oceanography Center (Monterey), the U.S. Naval Observatory, U.S. Merchant Marine Academy, U.S. Coast and Geodetic Survey, the National Ocean Service, and the National Weather Service. In addition to official government expertise, we appreciate the contributions of private organizations, in particular the Institute of Navigation, and other organizations and individuals too numerous to mention by name. Mariners worldwide can be grateful for the experience, dedication, and professionalism of the people who generously gave their time in this effort.
THE EDITORS

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CHAPTER 1

INTRODUCTION TO MARINE NAVIGATION

DEFINITIONS

100. The Art And Science Of Navigation
Marine navigation blends both science and art. A good navigator gathers information from every available source, evaluates this information, determines a fix, and compares that fix with his pre-determined “dead reckoning” position. A navigator constantly evaluates the ship’s position, anticipates dangerous situations well before they arise, and always keeps “ahead of the vessel.” The modern navigator must also understand the basic concepts of the many navigation systems used today, evaluate their output’s accuracy, and arrive at the best possible navigational decisions.
Navigation methods and techniques vary with the type of vessel, the conditions, and the navigator’s experience. Navigating a pleasure craft, for example, differs from navigating a container ship. Both differ from navigating a naval vessel. The navigator uses the methods and techniques best suited to the vessel and conditions at hand.
Some important elements of successful navigation cannot be acquired from any book or instructor. The science of navigation can be taught, but the art of navigation must be developed from experience.
101. Types Of Navigation
Methods of navigation have changed through history. Each new method has enhanced the mariner’s ability to complete his voyage safely and expeditiously. One of the most important judgments the navigator must make involves choosing the best method to use. Commonly recognized types of navigation are listed below.
• Dead reckoning (DR) determines position by advancing a known position for courses and distances. A position so determined is called a dead reckoning (DR) position. It is generally accepted that only course and speed determine the DR position. Correcting the DR position for leeway, current effects, and steering error result in an estimated position (EP). An inertial navigator develops an extremely accurate EP.

• Celestial navigation involves reducing celestial measurements to lines of position using tables, spherical trigonometry, and almanacs. It is used primarily as a backup to satellite and other electronic systems in the open ocean.
• Radio navigation uses radio waves to determine position by either radio direction finding systems or hyperbolic systems.
• Radar navigation uses radar to determine the distance from or bearing of objects whose position is known. This process is separate from radar’s use as a collision avoidance system.
• Satellite navigation uses artificial earth satellites for determination of position.
Electronic integrated bridge concepts are driving future navigation system planning. Integrated systems take inputs from various ship sensors, electronically display positioning information, and provide control signals required to maintain a vessel on a preset course. The navigator becomes a system manager, choosing system presets, interpreting system output, and monitoring vessel response.
In practice, a navigator synthesizes different methodologies into a single integrated system. He should never feel comfortable utilizing only one method when others are available for backup. Each method has advantages and disadvantages. The navigator must choose methods appropriate to each particular situation.
With the advent of automated position fixing and electronic charts, modern navigation is almost completely an electronic process. The mariner is constantly tempted to rely solely on electronic systems. This would be a mistake. Electronic navigation systems are always subject to failure, and the professional mariner must never forget that the safety of his ship and crew may depend on skills that differ little from those practiced generations ago. Proficiency in conventional piloting and celestial navigation remains essential.

• Piloting involves navigating in restricted waters with frequent determination of position relative to geographic and hydrographic features.

102. Phases Of Navigation Four distinct phases define the navigation process. The 1

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INTRODUCTION TO MARINE NAVIGATION

mariner should choose the system mix that meets the accuracy requirements of each phase.

• Coastal Phase: Navigating within 50 miles of the coast or inshore of the 200 meter depth contour.

• Inland Waterway Phase: Piloting in narrow canals, channels, rivers, and estuaries.
• Harbor/Harbor Approach Phase: Navigating to a harbor entrance and piloting in harbor approach channels.

• Ocean Phase: Navigating outside the coastal area in the open sea.
The navigator’s position accuracy requirements, his fix interval, and his systems requirements differ in each phase. The following table can be used as a general guide for selecting the proper system(s).

DR Piloting Celestial Radio Radar Satellite

Inland Waterway

Harbor/Harbor Approach

Coastal

X

X

X

X

X

X

X

X

X

X

X

X

X*

X

X

Table 102. The relationship of the types and phases of navigation. * Differential GPS may be used if available.

Ocean
X X X X

NAVIGATIONAL TERMS AND CONVENTIONS

103. Important Conventions And Concepts
Throughout the history of navigation, numerous terms and conventions have been established which enjoy worldwide recognition. The professional navigator, to gain a full understanding of his field, should understand the origin of certain terms, techniques, and conventions. The following section discusses some of the important ones.
Defining a prime meridian is a comparatively recent development. Until the beginning of the 19th century, there was little uniformity among cartographers as to the meridian from which to measure longitude. This did not lead to any problem because there was no widespread method for determining longitude accurately.
Ptolemy, in the 2nd century AD, measured longitude eastward from a reference meridian 2 degrees west of the Canary Islands. In 1493, Pope Alexander VI established a line in the Atlantic west of the Azores to divide the territories of Spain and Portugal. For many years, cartographers of these two countries used this dividing line as the prime meridian. In 1570 the Dutch cartographer Ortelius used the easternmost of the Cape Verde Islands. John Davis, in his 1594 The Seaman’s Secrets, used the Isle of Fez in the Canaries because there the variation was zero. Mariners paid little attention to these conventions and often reckoned their longitude from several different capes and ports during a

voyage. The meridian of London was used as early as 1676, and
over the years its popularity grew as England’s maritime interests increased. The system of measuring longitude both east and west through 180° may have first appeared in the middle of the 18th century. Toward the end of that century, as the Greenwich Observatory increased in prominence, English cartographers began using the meridian of that observatory as a reference. The publication by the Observatory of the first British Nautical Almanac in 1767 further entrenched Greenwich as the prime meridian. An unsuccessful attempt was made in 1810 to establish Washington, D.C. as the prime meridian for American navigators and cartographers. In 1884, the meridian of Greenwich was officially established as the prime meridian. Today, all maritime nations have designated the Greenwich meridian the prime meridian, except in a few cases where local references are used for certain harbor charts.
Charts are graphic representations of areas of the earth for use in marine or air navigation. Nautical charts depict features of particular interest to the marine navigator. Charts have probably existed since at least 600 BC. Stereographic and orthographic projections date from the 2nd century BC. In 1569 Gerardus Mercator published a chart using the mathematical principle which now bears his name. Some 30 years later, Edward Wright published cor-

INTRODUCTION TO MARINE NAVIGATION

3

rected mathematical tables for this projection, enabling cartographers to produce charts on the Mercator projection. This projection is still widely in use.
Sailing directions or pilots have existed since at least the 6th century BC. Continuous accumulation of navigational data, along with increased exploration and trade, led to increased production of volumes through the Middle Ages. “Routiers” were produced in France about 1500; the English referred to them as “rutters.” In 1584 Lucas Waghenaer published the Spieghel der Zeevaerdt (The Mariner’s Mirror), which became the model for such publications for several generations of navigators. They were known as “Waggoners” by most sailors. Modern pilots and sailing directions are based on extensive data collection and compilation efforts begun by Matthew Fontaine Maury beginning in 1842.
The compass was developed about 1000 years ago. The origin of the magnetic compass is uncertain, but Norsemen used it in the 11th century. It was not until the 1870s that Lord Kelvin developed a reliable dry card marine compass. The fluid-filled compass became standard in 1906.
Variation was not understood until the 18th century, when Edmond Halley led an expedition to map lines of variation in the South Atlantic. Deviation was understood at least as early as the early 1600s, but correction of compass error was not possible until Matthew Flinders discovered that a vertical iron bar could reduce errors. After 1840, British Astronomer Royal Sir George Airy and later Lord Kelvin developed combinations of iron masses and small magnets to eliminate most magnetic compass error.
The gyrocompass was made necessary by iron and steel ships. Leon Foucault developed the basic gyroscope in 1852. An American (Elmer Sperry) and a German (Anshutz Kampfe) both developed electrical gyrocompasses in the early years of the 20th century.
The log is the mariner’s speedometer. Mariners originally measured speed by observing a chip of wood passing down the side of the vessel. Later developments included a wooden board attached to a reel of line. Mariners measured speed by noting how many knots in the line unreeled as the ship moved a measured amount of time; hence the term knot. Mechanical logs using either a small paddle wheel or a rotating spinner arrived about the middle of the 17th century. The taffrail log still in limited use today was developed in 1878. Modern logs use electronic sensors or spinning devices that induce small electric fields proportional to a vessel’s speed. An engine revolution counter or shaft log often measures speed onboard large ships. Doppler speed logs are used on some vessels for very accurate speed readings. Inertial and satellite systems also provide highly accurate speed readings.
The Metric Conversion Act of 1975 and the Omnibus Trade and Competitiveness Act of 1988 established the metric system of weights and measures in the United States. As a result, the government is converting charts to

the metric format. Considerations of expense, safety of navigation, and logical sequencing will require a conversion effort spanning many years. Notwithstanding the conversion to the metric system, the common measure of distance at sea is the nautical mile.
The current policy of the Defense Mapping Agency Hydrographic/Topographic Center (DMAHTC) and the National Ocean Service (NOS) is to convert new compilations of nautical, special purpose charts, and publications to the metric system. This conversion began on January 2, 1970. Most modern maritime nations have also adopted the meter as the standard measure of depths and heights. However, older charts still on issue and the charts of some foreign countries may not conform to this standard.
The fathom as a unit of length or depth is of obscure origin. Posidonius reported a sounding of more than 1,000 fathoms in the 2nd century BC. How old the unit was then is unknown. Many modern charts are still based on the fathom, as conversion to the metric system continues.
The sailings refer to various methods of mathematically determining course, distance, and position. They have a history almost as old as mathematics itself. Thales, Hipparchus, Napier, Wright, and others contributed the formulas that permit computation of course and distance by plane, traverse, parallel, middle latitude, Mercator, and great circle sailings.
104. The Earth
The earth is an oblate spheroid (a sphere flattened at the poles). Measurements of its dimensions and the amount of its flattening are subjects of geodesy. However, for most navigational purposes, assuming a spherical earth introduces insignificant error. The earth’s axis of rotation is the line connecting the North Pole and the South Pole.
A great circle is the line of intersection of a sphere and a plane through its center. This is the largest circle that can be drawn on a sphere. The shortest line on the surface of a sphere between two points on the surface is part of a great circle. On the spheroidal earth the shortest line is called a geodesic. A great circle is a near enough approximation to a geodesic for most problems of navigation. A small circle is the line of intersection of a sphere and a plane which does not pass through the center. See Figure 104a.
The term meridian is usually applied to the upper branch of the half-circle from pole to pole which passes through a given point. The opposite half is called the lower branch.
A parallel or parallel of latitude is a circle on the surface of the earth parallel to the plane of the equator. It connects all points of equal latitude. The equator is a great circle at latitude 0°. See Figure 104b. The poles are single points at latitude 90°. All other parallels are small circles.

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Figure 104a. The planes of the meridians meet at the polar axis.

Figure 104b. The equator is a great circle midway between the poles.

105. Coordinates
Coordinates, termed latitude and longitude, can define any position on earth. Latitude (L, lat.) is the angular distance from the equator, measured northward or southward along a meridian from 0° at the equator to 90° at the poles. It is designated north (N) or south (S) to indicate the direction of measurement.
The difference of latitude (l, DLat.) between two places is the angular length of arc of any meridian between their parallels. It is the numerical difference of the latitudes if the places are on the same side of the equator; it is the sum of the latitudes if the places are on opposite sides of the equator. It may be designated north (N) or south (S) when appropriate. The middle or mid-latitude (Lm) between two places on the same side of the equator is half the sum of their latitudes. Mid-latitude is labeled N or S to indicate whether it is north or south of the equator.
The expression may refer to the mid-latitude of two places on opposite sides of the equator. In this case, it is equal to half the difference between the two latitudes and takes the name of the place farthest from the equator. However, this usage is misleading because it lacks the significance usually associated with the expression. When the places are on opposite sides of the equator, two mid-latitudes are generally used. Calculate these two mid-latitudes by averaging each latitude and 0°.
Longitude (l, long.) is the angular distance between

the prime meridian and the meridian of a point on the earth, measured eastward or westward from the prime meridian through 180°. It is designated east (E) or west (W) to indicate the direction of measurement.
The difference of longitude (DLo) between two places is the shorter arc of the parallel or the smaller angle at the pole between the meridians of the two places. If both places are on the same side (east or west) of Greenwich, DLo is the numerical difference of the longitudes of the two places; if on opposite sides, DLo is the numerical sum unless this exceeds 180°, when it is 360° minus the sum. The distance between two meridians at any parallel of latitude, expressed in distance units, usually nautical miles, is called departure (p, Dep.). It represents distance made good east or west as a craft proceeds from one point to another. Its numerical value between any two meridians decreases with increased latitude, while DLo is numerically the same at any latitude. Either DLo or p may be designated east (E) or west (W) when appropriate.
106. Distance On The Earth
Distance, as used by the navigator, is the length of the rhumb line connecting two places. This is a line making the same angle with all meridians. Meridians and parallels which also maintain constant true directions may be considered special cases of the rhumb line. Any other rhumb line spirals toward the pole, forming a loxodromic curve or

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107. Direction On The Earth

Figure 106. A loxodrome
loxodrome. See Figure 106. Distance along the great circle connecting two points is customarily designated great-circle distance. For most purposes, considering the nautical mile the length of one minute of latitude introduces no significant error.
Speed (S) is rate of motion, or distance per unit of time. A knot (kn.), the unit of speed commonly used in navigation, is a rate of 1 nautical mile per hour. The expression speed of advance (SOA) is used to indicate the speed to be made along the intended track. Speed over the ground (SOG) is the actual speed of the vessel over the surface of the earth at any given time. To calculate speed made good (SMG) between two positions, divide the distance between the two positions by the time elapsed between the two positions.

Direction is the position of one point relative to another. Navigators express direction as the angular difference in degrees from a reference direction, usually north or the ship’s head. Course (C, Cn) is the horizontal direction in which a vessel is steered or intended to be steered, expressed as angular distance from north clockwise through 360°. Strictly used, the term applies to direction through the water, not the direction intended to be made good over the ground.
The course is often designated as true, magnetic, compass, or grid according to the reference direction. Track made good (TMG) is the single resultant direction from the point of departure to point of arrival at any given time. Course of advance (COA) is the direction intended to be made good over the ground, and course over ground (COG) is the direction between a vessel’s last fix and an EP. A course line is a line drawn on a chart extending in the direction of a course. It is sometimes convenient to express a course as an angle from either north or south, through 90° or 180°. In this case it is designated course angle (C) and should be properly labeled to indicate the origin (prefix) and direction of measurement (suffix). Thus, C N35°E = Cn 035° (000° + 35°), C N155°W = Cn 205° (360° - 155°), C S47°E = Cn 133° (180° - 47°). But Cn 260° may be either C N100°W or C S80°W, depending upon the conditions of the problem.
Track (TR) is the intended horizontal direction of travel with respect to the earth. The terms intended track and trackline are used to indicate the path of intended travel. See Figure 107a. The track consists of one or a series of course lines, from the point of departure to the destination, along which it is intended to proceed. A great circle which a vessel intends to follow is called a great-circle track, though it consists of a series of straight lines approximating a great circle.

Figure 107a. Course line, track, track made good, and heading.

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Heading (Hdg., SH) is the direction in which a vessel is pointed, expressed as angular distance from 000° clockwise through 360°. Do not confuse heading and course. Heading constantly changes as a vessel yaws back and forth across the course due to sea, wind, and steering error.
Bearing (B, Brg.) is the direction of one terrestrial point from another, expressed as angular distance from 000° (North) clockwise through 360°. When measured through 90° or 180° from either north or south, it is called bearing angle (B). Bearing and azimuth are sometimes used interchangeably, but the latter more accurately refers to the horizontal direction of a point on the celestial sphere from

a point on the earth. A relative bearing is measured relative to the ship’s heading from 000° (dead ahead) clockwise through 360°. However, it is sometimes conveniently measured right or left from 0° at the ship’s head through 180°. This is particularly true when using the table for Distance of an Object by Two Bearings.
To convert a relative bearing to a true bearing, add the true heading:
True Bearing = Relative Bearing + True Heading. Relative Bearing = True Bearing – True Heading.

Figure 107b. Relative Bearing.

DEVELOPMENT OF NAVIGATION

108. Latitude And Longitude Determination
Navigators have made latitude observations for thousands of years. Accurate sun declination tables have been published for centuries, enabling experienced seamen to compute latitude to within 1 or 2 degrees. Mariners still use meridian observations of the sun and highly refined ex-meridian techniques. Those who today determine their latitude by measuring the altitude of Polaris are using a method well known to 15th century navigators.
A method of finding longitude eluded mariners for centuries. Several solutions independent of time proved too cumbersome. The lunar distance method, which determines GMT by observing the moon’s position among the stars, became popular in the 1800s. However, the mathematics required by most of these processes were far above the

abilities of the average seaman. It was apparent that the solution lay in keeping accurate time at sea.
In 1714, the British Board of Longitude was formed, offering a small fortune in reward to anyone who could provide a solution to the problem.
An Englishman, John Harrison, responded to the challenge, developing four chronometers between 1735 and 1760. The most accurate of these timepieces lost only 15 seconds on a 156 day round trip between London and Barbados. The Board, however, paid him only half the promised reward. The King finally intervened on Harrison’s behalf, and Harrison received his full reward of £20,000 at the advanced age of 80.
Rapid chronometer development led to the problem of determining chronometer error aboard ship. Time balls, large black spheres mounted in port in prominent locations,

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were dropped at the stroke of noon, enabling any ship in harbor which could see the ball to determine chronometer error. By the end of the U.S. Civil War, telegraph signals were being used to key time balls. Use of radio signals to send time ticks to ships well offshore began in 1904, and soon worldwide signals were available.
109. The Navigational Triangle
Modern celestial navigators reduce their celestial observations by solving a navigational triangle whose points are the elevated pole, the celestial body, and the zenith of the observer. The sides of this triangle are the polar distance of the body (codeclination), its zenith distance (coaltitude), and the polar distance of the zenith (colatitude of the observer).
A spherical triangle was first used at sea in solving lunar distance problems. Simultaneous observations were made of the altitudes of the moon and the sun or a star near the ecliptic and the angular distance between the moon and the other body. The zenith of the observer and the two celestial bodies formed the vertices of a triangle whose sides were the two coaltitudes and the angular distance between the bodies. Using a mathematical calculation the navigator “cleared” this distance of the effects of refraction and parallax applicable to each altitude. This corrected value was then used as an argument for entering the almanac. The almanac gave the true lunar distance from the sun and several stars at 3 hour intervals. Previously, the navigator had set his watch or checked its error and rate with the local mean time determined by celestial observations. The local mean time of the watch, properly corrected, applied to the Greenwich mean time obtained from the lunar distance observation, gave the longitude.
The calculations involved were tedious. Few mariners could solve the triangle until Nathaniel Bowditch published his simplified method in 1802 in The New American Practical Navigator.
Reliable chronometers were available in 1802, but their high cost precluded their general use aboard most ships. However, most navigators could determine their longitude using Bowditch’s method. This eliminated the need for parallel sailing and the lost time associated with it. Tables for the lunar distance solution were carried in the American nautical almanac until the second decade of the 20th century.
110. The Time Sight
The theory of the time sight had been known to mathematicians since the development of spherical trigonometry, but not until the chronometer was developed could it be used by mariners.
The time sight used the modern navigational triangle. The codeclination, or polar distance, of the body could be determined from the almanac. The zenith distance (coaltitude) was determined by observation. If the colatitude were known, three

sides of the triangle were available. From these the meridian angle was computed. The comparison of this with the Greenwich hour angle from the almanac yielded the longitude.
The time sight was mathematically sound, but the navigator was not always aware that the longitude determined was only as accurate as the latitude, and together they merely formed a point on what is known today as a line of position. If the observed body was on the prime vertical, the line of position ran north and south and a small error in latitude generally had little effect on the longitude. But when the body was close to the meridian, a small error in latitude produced a large error in longitude.
The line of position by celestial observation was unknown until discovered in 1837 by 30-year-old Captain Thomas H. Sumner, a Harvard graduate and son of a United States congressman from Massachusetts. The discovery of the “Sumner line,” as it is sometimes called, was considered by Maury “the commencement of a new era in practical navigation.” This was the turning point in the development of modern celestial navigation technique. In Sumner’s own words, the discovery took place in this manner:
Having sailed from Charleston, S. C., 25th November, 1837, bound to Greenock, a series of heavy gales from the Westward promised a quick passage; after passing the Azores, the wind prevailed from the Southward, with thick weather; after passing Longitude 21° W, no observation was had until near the land; but soundings were had not far, as was supposed, from the edge of the Bank. The weather was now more boisterous, and very thick; and the wind still Southerly; arriving about midnight, 17th December, within 40 miles, by dead reckoning, of Tusker light; the wind hauled SE, true, making the Irish coast a lee shore; the ship was then kept close to the wind, and several tacks made to preserve her position as nearly as possible until daylight; when nothing being in sight, she was kept on ENE under short sail, with heavy gales; at about 10 AM an altitude of the sun was observed, and the Chronometer time noted; but, having run so far without any observation, it was plain the Latitude by dead reckoning was liable to error, and could not be entirely relied on. Using, however, this Latitude, in finding the Longitude by Chronometer, it was found to put the ship 15' of Longitude E from her position by dead reckoning; which in Latitude 52° N is 9 nautical miles; this seemed to agree tolerably well with the dead reckoning; but feeling doubtful of the Latitude, the observation was tried with a Latitude 10' further N, finding this placed the ship ENE 27 nautical miles, of the former position, it was tried again with a Latitude 20' N of the dead reckoning; this also placed the ship still further ENE, and still 27 nautical miles further; these three positions were then seen to lie in the direction of Small’s light.
It then at once appeared that the observed altitude must have happened at all the three points, and at Small’s light, and at the ship, at the same instant of time;

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Figure 110. The first celestial line of position, obtained by Captain Thomas Sumner in 1837.

and it followed, that Small’s light must bear ENE, if the Chronometer was right. Having been convinced of this truth, the ship was kept on her course, ENE, the wind being still SE., and in less than an hour, Small’s light was made bearing ENE 1/2 E, and close aboard.
In 1843 Sumner published a book, A New and Accurate Method of Finding a Ship’s Position at Sea by Projection on Mercator’s Chart. He proposed solving a single time sight twice, using latitudes somewhat greater and somewhat less than that arrived at by dead reckoning, and joining the two positions obtained to form the line of position.
The Sumner method required the solution of two time sights to obtain each line of position. Many older navigators preferred not to draw the lines on their charts, but to fix their position mathematically by a method which Sumner had also devised and included in his book. This was a tedious but popular procedure.

igator had no choice but to solve each triangle by tedious, manual computations.
Lord Kelvin, generally considered the father of modern navigational methods, expressed interest in a book of tables with which a navigator could avoid tedious trigonometric solutions. However, solving the many thousands of triangles involved would have made the project too costly. Computers finally provided a practical means of preparing tables. In 1936 the first volume of Pub. No. 214 was made available; later, Pub. No. 249 was provided for air navigators. Pub. No. 229, Sight Reduction Tables for Marine Navigation, has replaced Pub. No. 214.
Modern calculators are gradually replacing the tables. Scientific calculators with trigonometric functions can easily solve the navigational triangle. Navigational calculators readily solve celestial sights and perform a variety of voyage planning functions. Using a calculator generally gives more accurate lines of position because it eliminates the rounding errors inherent in tabular inspection and interpolation.

111. Navigational Tables

112. Electronics And Navigation

Spherical trigonometry is the basis for solving every

Perhaps the first application of electronics to naviga-

navigational triangle, and until about 80 years ago the nav- tion involved sending telegraphic time signals in 1865 to

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check chronometer error. Transmitting radio time signals for at sea chronometer checks dates to 1904.
Radio broadcasts providing navigational warnings, begun in 1907 by the U.S. Navy Hydrographic Office, helped increase the safety of navigation at sea.
By the latter part of World War I the directional properties of a loop antenna were successfully used in the radio direction finder. The first radiobeacon was installed in 1921. Early 20th century experiments by Behm and Langevin led to the U.S. Navy’s development of the first practical echo sounder in 1922.
Today, electronics touches almost every aspect of navigation. Hyperbolic systems, satellite systems, and electronic charts all require an increasingly sophisticated electronics suite. These systems’ accuracy and ease of use make them invaluable assets to the navigator. Indeed, it is no exaggeration to state that, with the advent of the electronic chart and differential GPS, the mariner will soon be able to navigate from port to port using electronic navigation equipment alone.
113. Development Of Radar
As early as 1904, German engineers were experimenting with reflected radio waves. In 1922 two American scientists, Dr. A. Hoyt Taylor and Leo C. Young, testing a communication system at the Naval Aircraft Radio Laboratory, noted fluctuations in the signals when ships passed between stations on opposite sides of the Potomac River. In 1935 the British began work on radar. In 1937 the USS Leary tested the first seagoing radar. In 1940 United States and British scientists combined their efforts. When the British revealed the principle of the multicavity magnetron developed by J. T. Randall and H. A. H. Boot at the University of Birmingham in 1939, microwave radar became practical. In 1945, at the close of World War II, radar became available for commercial use.

114. Development Of Hyperbolic Radio Aids
Various hyperbolic systems were developed from World War II, including Loran A. This was replaced by the more accurate Loran C system in use today. Using very low frequencies, the Omega navigation system provides worldwide, though less accurate, coverage for a variety of applications including marine navigation. Various short range and regional hyperbolic systems have been developed by private industry for hydrographic surveying, offshore facilities positioning, and general navigation.
115. Other Electronic Systems
The Navy Navigation Satellite System (NAVSAT) fulfilled a requirement established by the Chief of Naval Operations for an accurate worldwide navigation system for all naval surface vessels, aircraft, and submarines. The system was conceived and developed by the Applied Physics Laboratory of The Johns Hopkins University. The underlying concept that led to development of satellite navigation dates to 1957 and the first launch of an artificial satellite into orbit. NAVSAT has been replaced by the far more accurate and widely available Global Positioning System (GPS).
The first inertial navigation system was developed in 1942 for use in the V2 missile by the Peenemunde group under the leadership of Dr. Wernher von Braun. This system used two 2-degree-of-freedom gyroscopes and an integrating accelerometer to determine the missile velocity. By the end of World War II, the Peenemunde group had developed a stable platform with three single-degree-of-freedom gyroscopes and an integrating accelerometer. In 1958 an inertial navigation system was used to navigate the USS Nautilus under the ice to the North Pole.

NAVIGATION ORGANIZATIONS

116. Governmental Roles
Navigation only a generation ago was an independent process, carried out by the mariner without outside assistance. With compass and charts, sextant and chronometer, he could independently travel anywhere in the world. The increasing use of electronic navigation systems has made the navigator dependent on many factors outside his control. Government organizations fund, operate, and regulate satellites, Loran, and other electronic systems. Governments are increasingly involved in regulation of vessel movements through traffic control systems and regulated areas. Understanding the governmental role in supporting and regulating navigation is vitally important to the mariner. In the United States, there are a number of official organizations which support the interests of navigators. Some have a policy-making role; others build and operate

navigation systems. Many maritime nations have similar organizations performing similar functions. International organizations also play a significant role.
117. The Coast And Geodetic Survey
The U.S. Coast and Geodetic Survey was founded in 1807 when Congress passed a resolution authorizing a survey of the coast, harbors, outlying islands, and fishing banks of the United States. President Thomas Jefferson appointed Ferdinand Hassler, a Swiss immigrant and professor of mathematics at West Point, the first Director of the “Survey of the Coast.” The survey became the “Coast Survey” in 1836.
The approaches to New York were the first sections of the coast charted, and from there the work spread northward and southward along the eastern seaboard. In 1844 the work

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was expanded and arrangements made to chart simultaneously the gulf and east coasts. Investigation of tidal conditions began, and in 1855 the first tables of tide predictions were published. The California gold rush necessitated a survey of the west coast. This survey began in 1850, the year California became a state. Coast Pilots, or Sailing Directions, for the Atlantic coast of the United States were privately published in the first half of the 19th century. In 1850 the Survey began accumulating data that led to federally produced Coast Pilots. The 1889 Pacific Coast Pilot was an outstanding contribution to the safety of west coast shipping.
In 1878 the survey was renamed “Coast and Geodetic Survey.” In 1970 the survey became the “National Ocean Survey,” and in 1983 it became the “National Ocean Service.” The Office of Charting and Geodetic Services accomplished all charting and geodetic functions. In 1991 the name was changed back to the original “Coast and Geodetic Survey,” organized under the National Ocean Service along with several other environmental offices. Today it provides the mariner with the charts and coast pilots of all waters of the United States and its possessions, and tide and tidal current tables for much of the world. Its administrative order requires the Coast and Geodetic Survey to plan and direct programs to produce charts and related information for safe navigation of the Nation’s waterways, territorial seas, and national airspace. This work includes all activities related to the National Geodetic Reference System; surveying, charting, and data collection; production and distribution of charts; and research and development of new technologies to enhance these missions.
118. The Defense Mapping Agency
In the first years of the newly formed United States of America, charts and instruments used by the Navy and merchant mariners were left over from colonial days or were obtained from European sources. In 1830 the U.S. Navy established a “Depot of Charts and Instruments” in Washington, D. C. It was a storehouse from which available charts, sailing directions, and navigational instruments were issued to Naval ships. Lieutenant L. M. Goldsborough and one assistant, Passed Midshipman R. B. Hitchcock, constituted the entire staff.
The first chart published by the Depot was produced from data obtained in a survey made by Lieutenant Charles Wilkes, who had succeeded Goldsborough in 1834. Wilkes later earned fame as the leader of a United States expedition to Antarctica. From 1842 until 1861 Lieutenant Matthew Fontaine Maury served as Officer in Charge. Under his command the Depot rose to international prominence. Maury decided upon an ambitious plan to increase the mariner’s knowledge of existing winds, weather, and currents. He began by making a detailed record of pertinent matter included in old log books stored at the Depot. He then inaugurated a hydrographic reporting program among shipmasters, and the thousands of reports received, along

with the log book data, were compiled into the “Wind and Current Chart of the North Atlantic” in 1847. This is the ancestor of today’s Pilot Chart. The United States instigated an international conference in 1853 to interest other nations in a system of exchanging nautical information. The plan, which was Maury’s, was enthusiastically adopted by other maritime nations. In 1854 the Depot was redesignated the “U.S. Naval Observatory and Hydrographical Office.” In 1861, Maury, a native of Virginia, resigned from the U.S. Navy and accepted a commission in the Confederate Navy at the beginning of the Civil War. This effectively ended his career as a navigator, author, and oceanographer. At war’s end, he fled the country. Maury’s reputation suffered from his embracing the Confederate cause. In 1867, while Maury was still absent from the country to avoid arrest for treason, George W. Blunt, an editor of hydrographic publications, wrote:
In mentioning what our government has done towards nautical knowledge, I do not allude to the works of Lieutenant Maury, because I deem them worthless. . . . They have been suppressed since the rebellion by order of the proper authorities, Maury’s loyalty and hydrography being alike in quality.
After Maury’s return to the United States in 1868, he served as an instructor at the Virginia Military Institute. He continued at this position until his death in 1873. Since his death, his reputation as one of America’s greatest hydrographers has been restored.
In 1866 Congress separated the Observatory and the Hydrographic Office, broadly increasing the functions of the latter. The Hydrographic Office was authorized to carry out surveys, collect information, and print every kind of nautical chart and publication “for the benefit and use of navigators generally.”
The Hydrographic Office purchased the copyright of The New American Practical Navigator in 1867. The first Notice to Mariners appeared in 1869. Daily broadcast of navigational warnings was inaugurated in 1907. In 1912, following the sinking of the Titanic, the International Ice Patrol was established.
In 1962 the U.S. Navy Hydrographic Office was redesignated the U.S. Naval Oceanographic Office. In 1972 certain hydrographic functions of the latter office were transferred to the Defense Mapping Agency Hydrographic Center. In 1978 the Defense Mapping Agency Hydrographic/Topographic Center (DMAHTC) assumed hydrographic and topographic chart production functions. DMAHTC provides support to the U.S. Department of Defense and other federal agencies on matters concerning mapping, charting, and geodesy. It continues to fulfill the old Hydrographic Office’s responsibilities to “navigators generally.”

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119. The United States Coast Guard
Alexander Hamilton established the U.S. Coast Guard as the Revenue Marine, later the Revenue Cutter Service, on August 4, 1790. It was charged with enforcing the customs laws of the new nation. A revenue cutter, the Harriet Lane, fired the first shot from a naval unit in the Civil War at Fort Sumter. The Revenue Cutter Service became the U.S. Coast Guard when combined with the Lifesaving Service in 1915. The Lighthouse Service was added in 1939, and the Bureau of Marine Inspection and Navigation was added in 1942. The Coast Guard was transferred from the Treasury Department to the Department of Transportation in 1967.
The primary functions of the Coast Guard include maritime search and rescue, law enforcement, and operation of the nation’s aids to navigation system. In addition, the Coast Guard is responsible for port safety and security, merchant marine inspection, and marine pollution control. The Coast Guard operates a large and varied fleet of ships, boats, and aircraft in performing its widely ranging duties.
Navigation systems operated by the Coast Guard include the system of some 40,000 lighted and unlighted beacons, buoys, and ranges in U.S. waters; the U.S. stations of the Loran C system; the Omega navigation system; radiobeacons and racons; differential GPS (DGPS) services in the U.S.; and Vessel Traffic Services (VTS) in major ports and harbors of the U.S.
120. The United States Navy
The U.S. Navy was officially established in 1798. Its role in the development of navigational technology has been singular. From the founding of the Naval Observatory to the development of the most advanced electronics, the U.S. Navy has been a leader in developing devices and techniques designed to make the navigator’s job safer and easier.
The development of almost every device known to navigation science has been deeply influenced by Naval policy. Some systems are direct outgrowths of specific Naval needs; some are the result of technological improvements shared with other services and with commercial maritime industry.
121. The United States Naval Observatory
One of the first observatories in the United States was built in 1831-1832 at Chapel Hill, N.C. The Depot of Charts and Instruments, established in 1830, was the agency from which the U.S. Navy Hydrographic Office and the U.S. Naval Observatory evolved 36 years later. Under Lieutenant Charles Wilkes, the second Officer in Charge, the Depot about 1835 installed a small transit instrument for rating chronometers.
The Mallory Act of 1842 provided for the establishment of a permanent observatory. The director was

authorized to purchase everything necessary to continue astronomical study. The observatory was completed in 1844 and the results of its first observations were published two years later. Congress established the Naval Observatory as a separate agency in 1866. In 1873 a refracting telescope with a 26 inch aperture, then the world’s largest, was installed. The observatory, located in Washington, D.C., has occupied its present site since 1893.
122. The Royal Greenwich Observatory
England had no early privately supported observatories such as those on the continent. The need for navigational advancement was ignored by Henry VIII and Elizabeth I, but in 1675 Charles II, at the urging of John Flamsteed, Jonas Moore, Le Sieur de Saint Pierre, and Christopher Wren, established the Greenwich Royal Observatory. Charles limited construction costs to £500, and appointed Flamsteed the first Astronomer Royal, at an annual salary of £100. The equipment available in the early years of the observatory consisted of two clocks, a “sextant” of 7 foot radius, a quadrant of 3 foot radius, two telescopes, and the star catalog published almost a century before by Tycho Brahe. Thirteen years passed before Flamsteed had an instrument with which he could determine his latitude accurately.
In 1690 a transit instrument equipped with a telescope and vernier was invented by Romer; he later added a vertical circle to the device. This enabled the astronomer to determine declination and right ascension at the same time. One of these instruments was added to the equipment at Greenwich in 1721, replacing the huge quadrant previously used. The development and perfection of the chronometer in the next hundred years added to the accuracy of observations.
Other national observatories were constructed in the years that followed: at Berlin in 1705, St. Petersburg in 1725, Palermo in 1790, Cape of Good Hope in 1820, Parramatta in New South Wales in 1822, and Sydney in 1855.
123. The International Hydrographic Organization
The International Hydrographic Organization (IHO) was originally established in 1921 as the International Hydrographic Bureau (IHB). The present name was adopted in 1970 as a result of a revised international agreement among member nations. However, the former name, International Hydrographic Bureau, was retained for the IHO’s administrative body of three Directors and a small staff at the organization’s headquarters in Monaco.
The IHO sets forth hydrographic standards to be agreed upon by the member nations. All member states are urged and encouraged to follow these standards in their surveys, nautical charts, and publications. As these standards are uniformly adopted, the products of the world’s hydrographic and oceanographic offices become more uniform. Much has been done in the field of standardization since the

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Bureau was founded. The principal work undertaken by the IHO is:
• To bring about a close and permanent association between national hydrographic offices.
• To study matters relating to hydrography and allied sciences and techniques.
• To further the exchange of nautical charts and documents between hydrographic offices of member governments.
• To circulate the appropriate documents. • To tender guidance and advice upon request, in par-
ticular to countries engaged in setting up or expanding their hydrographic service. • To encourage coordination of hydrographic surveys with relevant oceanographic activities. • To extend and facilitate the application of oceanographic knowledge for the benefit of navigators. • To cooperate with international organizations and scientific institutions which have related objectives.
During the 19th century, many maritime nations established hydrographic offices to provide means for improving the navigation of naval and merchant vessels by providing nautical publications, nautical charts, and other navigational services. There were substantial differences in hydrographic procedures, charts, and publications. In 1889, an International Marine Conference was held at Washington, D. C., and it was proposed to establish a “permanent international commission.” Similar proposals were made at the sessions of the International Congress of Navigation held at St. Petersburg in 1908 and again in 1912.
In 1919 the hydrographers of Great Britain and France cooperated in taking the necessary steps to convene an international conference of hydrographers. London was selected as the most suitable place for this conference, and on July 24, 1919, the First International Conference opened, attended by the hydrographers of 24 nations. The object of the conference was “To consider the advisability of all maritime nations adopting similar methods in the preparation, construction, and production of their charts and all hydrographic publications; of rendering the results in the most convenient form to enable them to be readily used; of instituting a prompt system of mutual exchange of hydrographic information between all countries; and of providing an opportunity to consultations and discussions to be carried out on hydrographic subjects generally by the hydrographic experts of the world.” This is still the major purpose of the International Hydrographic Organization.
As a result of the conference, a permanent organization was formed and statutes for its operations were prepared. The International Hydrographic Bureau, now the International Hydrographic Organization, began its activities in 1921 with 18 nations as members. The Principality of Monaco was selected because of its easy communication with the rest of the world and also because of the generous offer of Prince Albert I of

Monaco to provide suitable accommodations for the Bureau in the Principality. There are currently 59 member governments. Technical assistance with hydrographic matters is available through the IHO to member states requiring it.
Many IHO publications are available to the general public, such as the International Hydrographic Review, International Hydrographic Bulletin, Chart Specifications of the IHO, Hydrographic Dictionary, and others. Inquiries should be made to the International Hydrographic Bureau, 7 Avenue President J. F. Kennedy, B.P. 445, MC98011, Monaco, CEDEX.
124. The International Maritime Organization
The International Maritime Organization (IMO) was established by United Nations Convention in 1948. The Convention actually entered into force in 1959, although an international convention on marine pollution was adopted in 1954. (Until 1982 the official name of the organization was the Inter-Governmental Maritime Consultative Organization.) It is the only permanent body of the U. N. devoted to maritime matters, and the only special U. N. agency to have its headquarters in the UK.
The governing body of the IMO is the Assembly of 137 member states, which meets every two years. Between Assembly sessions a Council, consisting of 32 member governments elected by the Assembly, governs the organization. Its work is carried out by the following committees:
• Maritime Safety Committee, with subcommittees for:
• Safety of Navigation • Radiocommunications • Life-saving • Search and Rescue • Training and Watchkeeping • Carriage of Dangerous Goods • Ship Design and Equipment • Fire Protection • Stability and Load Lines/Fishing Vessel Safety • Containers and Cargoes • Bulk Chemicals • Marine Environment Protection Committee • Legal Committee • Technical Cooperation Committee • Facilitation Committee
IMO is headed by the Secretary General, appointed by the council and approved by the Assembly. He is assisted by some 300 civil servants.
To achieve its objectives of coordinating international policy on marine matters, the IMO has adopted some 30 conventions and protocols, and adopted over 700 codes and recommendations. An issue to be adopted first is brought before a committee or subcommittee, which submits a draft to a conference. When the conference adopts the final text, it is submitted

INTRODUCTION TO MARINE NAVIGATION

13

to member governments for ratification. Ratification by a specified number of countries is necessary for adoption; the more important the issue, the more countries must ratify. Adopted conventions are binding on member governments.
Codes and recommendations are not binding, but in most cases are supported by domestic legislation by the governments involved.
The first and most far-reaching convention adopted by the IMO was the Convention of Safety of Life at Sea (SOLAS) in 1960. This convention actually came into force in 1965, replacing a version first adopted in 1948. Because of the difficult process of bringing amendments into force internationally, none of subsequent amendments became binding. To remedy this situation, a new convention was adopted in 1974, and became binding in 1980. Among the regulations is V-20, requiring the carriage of up-to-date charts and publications sufficient for the intended voyage.
Other conventions and amendments were also adopted, such as the International Convention on Load Lines (adopted 1966, came into force 1968), a convention on the tonnage measurement of ships (adopted 1969, came into force 1982), The International Convention on Safe Containers (adopted 1972, came into force 1977), and the convention on International Regulations for Preventing Collisions at Sea (COLREGS) (adopted 1972, came into force 1977).
The 1972 COLREGS convention contained, among other provisions, a section devoted to Traffic Separation Schemes, which became binding on member states after having been adopted as recommendations in prior years.
One of the most important conventions is the International Convention for the Prevention of Pollution from Ships (MARPOL 73/78), which was first adopted in 1973, amended by Protocol in 1978, and became binding in 1983. This convention built on a series of prior conventions and agreements dating from 1954, highlighted by several severe pollution disasters involving oil tankers. The MARPOL convention reduces the amount of oil discharged into the sea by ships, and bans discharges completely in certain areas. A related convention known as the London Dumping Convention regulates dumping of hazardous chemicals and other debris into the sea.
IMO also develops minimum performance standards for a wide range of equipment relevant to safety at sea. Among such standards is one for the Electronic Chart Display and Information System (ECDIS), the digital display deemed the operational and legal equivalent of the conventional paper chart.
Texts of the various conventions and recommendations, as well as a catalog and publications on other subjects, are available from the Publications Section of the IMO at 4 Albert Embankment, London SE1 7SR, United Kingdom.
125. The International Association Of Lighthouse Authorities

to navigation services of more than 80 member countries for technical coordination, information sharing, and coordination of improvements to visual aids to navigation throughout the world. It was established in 1957 to provide a permanent organization to support the goals of the Technical Lighthouse Conferences, which had been convening since 1929. The General Assembly of IALA meets about every 4 years. The Council of 20 members meets twice a year to oversee the ongoing programs.
Five technical committees maintain the permanent programs:
• The Marine Marking Committee • The Radionavigation Systems Committee • The Vessel Traffic Services (VTS) Committee • The Reliability Committee • The Documentation Committee
IALA committees provide important documentation to the IHO and other international organizations, while the IALA Secretariat acts as a clearing house for the exchange of technical information, and organizes seminars and technical support for developing countries.
Its principle work since 1973 has been the implementation of the IALA Maritime Buoyage System, described in Chapter 5, Visual Aids to Navigation. This system replaced some 30 dissimilar buoyage systems in use throughout the world with 2 major systems.
IALA is based near Paris, France in Saint-Germaineen-Laye.
126. The Radio Technical Commission for Maritime Services
The Radio Technical Commission for Maritime Services is a non-profit organization which serves as a focal point for the exchange of information and the development of recommendations and standards related to all aspects of maritime telecommunications.
Specifically, RTCM:
• Promotes ideas and exchanges information on maritime telecommunications.
• Facilitates the development and exchange of views among government, business, and the public.
• Conducts studies and prepares reports on maritime telecommunications issues to improve efficiency and capabilities.
• Suggests minimum essential rules and regulations for effective telecommunications.
• Makes recommendations on important issues. • Pursues other activities as permitted by its by-laws
and membership.

The International Association of Lighthouse Au-

Both government and non-government organizations

thorities (IALA) brings together representatives of the aids are members, including many from foreign nations. The or-

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INTRODUCTION TO MARINE NAVIGATION

ganization consists of a Board of Directors, the Assembly consisting of all Members, Officers, staff, technical advisors, and standing and special committees.
Working committees are formed as needed to develop official RTCM recommendations regarding technical standards and policies in the maritime field. Currently committees exist for maritime safety information, electronic charts, emergency position-indicating radiobeacons (EPIRB’s) and personal locator beacons, survival craft telecommunications, differential GPS, and GLONASS. Ad hoc committees address short-term concerns such as regulatory proposals.
RTCM headquarters is in Washington D.C.
127. The National Marine Electronic Association
The National Marine Electronic Association (NMEA) is a professional trade association founded in

1957 whose purpose is to coordinate the efforts of marine electronics manufacturers, technicians, government agencies, ship and boat builders, and other interested groups. In addition to certifying marine electronics technicians and professionally recognizing outstanding achievements by corporate and individual members, the NMEA sets standards for the exchange of digital data by all manufacturers of marine electronic equipment. This allows the configuration of integrated navigation system using equipment from different manufacturers.
NMEA works closely with RTCM and other private organizations and with government agencies to monitor the status of laws and regulations affecting the marine electronics industry.
It also sponsors conferences and seminars, and publishes a number of guides and periodicals for members and the general public.

CHAPTER 2

GEODESY AND DATUMS IN NAVIGATION

GEODESY, THE BASIS OF CARTOGRAPHY

200. Definition
Geodesy is the science concerned with the exact positioning of points on the surface of the earth. It also involves the study of variations of the earth’s gravity, the application of these variations to exact measurements on the earth, and the study of the exact size and shape of the earth. These factors were unimportant to early navigators because of the relative inaccuracy of their methods. The precise accuracies of today’s navigation systems and the global nature of satellite and other long-range positioning methods demand a more complete understanding of geodesy than has ever before been required.
201. The Shape Of The Earth
The irregular topographic surface is that upon which actual geodetic measurements are made. The measurements, however, are reduced to the geoid. Marine navigation measurements are made on the ocean surface which approximates the geoid.
The geoid is a surface along which gravity is always

equal and to which the direction of gravity is always perpendicular. The latter is particularly significant because optical instruments containing level devices are commonly used to make geodetic measurements. When properly adjusted, the vertical axis of the instrument coincides with the direction of gravity and is, therefore, perpendicular to the geoid.
The geoid is that surface to which the oceans would conform over the entire earth if free to adjust to the combined effect of the earth’s mass attraction and the centrifugal force of the earth’s rotation. The ideal ocean surface would be free of ocean currents and salinity changes. Uneven distribution of the earth’s mass makes the geoidal surface irregular.
The geoid refers to the actual size and shape of the earth, but such an irregular surface has serious limitations as a mathematical earth model because:
• It has no complete mathematical expression. • Small variations in surface shape over time intro-
duce small errors in measurement. • The irregularity of the surface would necessitate a
prohibitive amount of computations.

Figure 201. Geiod, ellipsoid, and topographic surface of the earth, and deflection of the vertical due to differences in mass. 15

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GEODESY AND DATUMS IN NAVIGATION

The surface of the geoid, with some exceptions, tends to rise under mountains and to dip above ocean basins.
For geodetic, mapping, and charting purposes, it is necessary to use a regular or geometric shape which closely approximates the shape of the geoid either on a local or global scale and which has a specific mathematical expression. This shape is called the ellipsoid.
The separations of the geoid and ellipsoid are called geoidal heights, geoidal undulations, or geoidal separations.
The irregularities in density and depths of the material making up the upper crust of the earth also result in slight alterations of the direction of gravity. These alterations are reflected in the irregular shape of the geoid, the surface that is perpendicular to a plumb line.
Since the earth is in fact flattened slightly at the poles and bulges somewhat at the equator, the geometric figure used in geodesy to most nearly approximate the shape of the earth is the oblate spheroid or ellipsoid of revolution. This is the three dimensional shape obtained by rotating an ellipse about its minor axis.

This ratio is about 1/300 for the earth.
f = -a----–a----b-- .
The ellipsoidal earth model has its minor axis parallel to the earth’s polar axis.
203. Ellipsoids And The Geoid As Reference Surfaces
Since the surface of the geoid is irregular and the surface of the ellipsoid is regular, no one ellipsoid can provide other than an approximation of part of the geoidal surface. Figure 203 illustrates an example. The ellipsoid that fits well in North America does not fit well in Europe; therefore, it must be positioned differently.

202. Defining The Ellipsoid

An ellipsoid of revolution is uniquely defined by specifying two parameters. Geodesists, by convention, use the semimajor axis and flattening. The size is represented by the radius at the equator, the semimajor axis. The shape of the ellipsoid is given by the flattening, which indicates how closely an ellipsoid approaches a spherical shape. The flattening is the ratio of the difference between the semimajor and semiminor axes of the ellipsoid and the semimajor axis. See Figure 202. If a and b represent the semimajor and semiminor axes, respectively, of the ellipsoid, and f is the flattening,

Figure 203. The geoid and two ellipsoids, illustrating how the ellipsoid which fits well in North America will not fit
well in Europe, and must have a different origin. (exaggerated for clarity)

Figure 202. An ellipsoid of revolution, with semimajor axis (a), and semiminor axis (b).

A number of reference ellipsoids are used in geodesy and mapping because an ellipsoid is mathematically simpler than the geoid.
204. Coordinates
The astronomic latitude is the angle between the

GEODESY AND DATUMS IN NAVIGATION

17

plumb line at a station and the plane of the celestial equator. It is the latitude which results directly from observations of celestial bodies, uncorrected for deflection of the vertical component in the meridian (north-south) direction. Astronomic latitude applies only to positions on the earth. It is reckoned from the astronomic equator (0°), north and south through 90°.
The astronomic longitude is the angle between the plane of the celestial meridian at a station and the plane of the celestial meridian at Greenwich. It is the longitude which results directly from observations of celestial bodies, uncorrected for deflection of the vertical component in the prime vertical (east-west) direction. These are the coordinates observed by the celestial navigator using a sextant and a very accurate clock based on the earth’s rotation.
Astronomic observations by geodesists are made with optical instruments (theodolite, zenith camera, prismatic astrolabe) which all contain leveling devices. When properly adjusted, the vertical axis of the instrument coincides with the direction of gravity, and is, therefore, perpendicular to the geoid. Thus, astronomic positions are referenced to the geoid. Since the geoid is an irregular, non-mathematical surface, astronomic positions are wholly independent of each other.
The geodetic latitude is the angle which the normal to the ellipsoid at a station makes with the plane of the geodetic equator. In recording a geodetic position, it is essential that the geodetic datum on which it is based be also stated. A geodetic latitude differs from the corresponding astronomic latitude by the amount of the meridian component of the local deflection of the vertical.
The geodetic longitude is the angle between the plane

of the geodetic meridian at a station and the plane of the geodetic meridian at Greenwich. A geodetic longitude differs from the corresponding astronomic longitude by the prime vertical component of the local deflection of the vertical divided by the cosine of the latitude. The geodetic coordinates are used for mapping.
The geocentric latitude is the angle at the center of the ellipsoid (used to represent the earth) between the plane of the equator, and a straight line (or radius vector) to a point on the surface of the ellipsoid. This differs from geodetic latitude because the earth is approximated more closely by a spheroid than a sphere and the meridians are ellipses, not perfect circles.
Both geocentric and geodetic latitudes refer to the reference ellipsoid and not the earth. Since the parallels of latitude are considered to be circles, geodetic longitude is geocentric, and a separate expression is not used.
Because of the oblate shape of the ellipsoid, the length of a degree of geodetic latitude is not everywhere the same, increasing from about 59.7 nautical miles at the equator to about 60.3 nautical miles at the poles.
A horizontal geodetic datum usually consists of the astronomic and geodetic latitude, and astronomic and geodetic longitude of an initial point (origin); an azimuth of a line (direction); the parameters (radius and flattening) of the ellipsoid selected for the computations; and the geoidal separation at the origin. A change in any of these quantities affects every point on the datum.
For this reason, while positions within a given datum are directly and accurately relateable, those from different datums must be transformed to a common datum for consistency.

TYPES OF GEODETIC SURVEY

205. Triangulation
The most common type of geodetic survey is known as triangulation. Triangulation consists of the measurement of the angles of a series of triangles. The principle of triangulation is based on plane trigonometry. If the distance along one side of the triangle and the angles at each end are accurately measured, the other two sides and the remaining angle can be computed. In practice, all of the angles of every triangle are measured to provide precise measurements. Also, the latitude and longitude of one end of the measured side along with the length and direction (azimuth) of the side provide sufficient data to compute the latitude and longitude of the other end of the side.
The measured side of the base triangle is called a baseline. Measurements are made as carefully and accurately as possible with specially calibrated tapes or wires of Invar, an alloy highly resistant to changes in length resulting from changes in temperature. The tape or wires are checked periodically against standard measures of length.

To establish an arc of triangulation between two widely separated locations, the baseline may be measured and longitude and latitude determined for the initial points at each location. The lines are then connected by a series of adjoining triangles forming quadrilaterals extending from each end. All angles of the triangles are measured repeatedly to reduce errors. With the longitude, latitude, and azimuth of the initial points, similar data is computed for each vertex of the triangles, thereby establishing triangulation stations, or geodetic control stations. The coordinates of each of the stations are defined as geodetic coordinates.
Triangulation is extended over large areas by connecting and extending series of arcs to form a network or triangulation system. The network is adjusted in a manner which reduces the effect of observational errors to a minimum. A denser distribution of geodetic control is achieved in a system by subdividing or filling in with other surveys.
There are four general classes or orders of triangulation. First-order (primary) triangulation is the most precise and exact type. The most accurate instruments and rigorous

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GEODESY AND DATUMS IN NAVIGATION

computation methods are used. It is costly and time-consuming, and is usually used to provide the basic framework of control data for an area, and the determination of the figure of the earth. The most accurate first-order surveys furnish control points which can be interrelated with an accuracy ranging from 1 part in 25,000 over short distances to approximately 1 part in 100,000 for long distances.
Second-order triangulation furnishes points closer together than in the primary network. While second-order surveys may cover quite extensive areas, they are usually tied to a primary system where possible. The procedures are less exacting and the proportional error is 1 part in 10,000.
Third-order triangulation is run between points in a secondary survey. It is used to densify local control nets and position the topographic and hydrographic detail of the area. Triangle error can amount to 1 part in 5,000.
The sole accuracy requirement for fourth-order triangulation is that the positions be located without any appreciable error on maps compiled on the basis of the control. Fourth-order control is done primarily as mapping control.

leveling. It is often the only practical method of establishing accurate elevation control in mountainous areas.
In barometric leveling, differences in height are determined by measuring the differences in atmospheric pressure at various elevations. Air pressure is measured by mercurial or aneroid barometer, or a boiling point thermometer. Although the accuracy of this method is not as great as either of the other two, it obtains relative heights very rapidly at points which are fairly far apart. It is used in reconnaissance and exploratory surveys where more accurate measurements will be made later or where a high degree of accuracy is not required.

206. Trilateration, Traverse, And Vertical Surveying

Trilateration involves measuring the sides of a chain of triangles or other polygons. From them, the distance and direction from A to B can be computed. Figure 206 shows this process.
Traverse involves measuring distances and the angles between them without triangles for the purpose of computing the distance and direction from A to B. See Figure 206.
Vertical surveying is the process of determining elevations above mean sea-level. In geodetic surveys executed primarily for mapping, geodetic positions are referred to an ellipsoid, and the elevations of the positions are referred to the geoid. However, for satellite geodesy the geoidal heights must be considered to establish the correct height above the geoid.
Precise geodetic leveling is used to establish a basic network of vertical control points. From these, the height of other positions in the survey can be determined by supplementary methods. The mean sea-level surface used as a reference (vertical datum) is determined by averaging the hourly water heights for a specified period of time at specified tide gauges.
There are three leveling techniques: differential, trigonometric, and barometric. Differential leveling is the most accurate of the three methods. With the instrument locked in position, readings are made on two calibrated staffs held in an upright position ahead of and behind the instrument. The difference between readings is the difference in elevation between the points.
Trigonometric leveling involves measuring a vertical angle from a known distance with a theodolite and computing the elevation of the point. With this method, vertical measurement can be made at the same time horizontal angles are measured for triangulation. It is, therefore, a somewhat more economical method but less accurate than differential

GEODESY AND DATUMS IN NAVIGATION

19

Figure 206. Triangulation, trilateration, and traverse.
DATUM CONNECTIONS

207. Definitions
A datum is defined as any numerical or geometrical quantity or set of such quantities which serves as a reference point to measure other quantities.
In geodesy, as well as in cartography and navigation, two types of datums must be considered: a horizontal datum and a vertical datum. The horizontal datum forms the basis for computations of horizontal position. The vertical datum provides the reference to measure heights. A horizontal datum may be defined at an origin point on the ellipsoid (local datum) such that the center of the ellipsoid coincides with the Earth’s center of mass (geocentric datum). The coordinates for points in specific geodetic surveys and triangulation networks are computed from certain initial quantities, or datums.
208. Preferred Datums
In areas of overlapping geodetic triangulation networks, each computed on a different datum, the coordinates of the points given with respect to one datum will differ from those given with respect to the other. The differences can be used to derive transformation formulas. Datums are connected by developing transformation formulas at common points, either between overlapping control networks or by satellite connections.
Many countries have developed national datums which differ from those of their neighbors. Accordingly, national maps and charts often do not agree along national borders.

The North American Datum, 1927 (NAD 27) has been used in the United States for about 50 years, but it is being replaced by datums based on the World Geodetic System. NAD 27 coordinates are based on the latitude and longitude of a triangulation station (the reference point) at Mead’s Ranch in Kansas, the azimuth to a nearby triangulation station called Waldo, and the mathematical parameters of the Clarke Ellipsoid of 1866. Other datums throughout the world use different assumptions as to origin points and ellipsoids.
The origin of the European Datum is at Potsdam, Germany. Numerous national systems have been joined into a large datum based upon the International Ellipsoid of 1924 which was oriented by a modified astrogeodetic method. European, African, and Asian triangulation chains were connected, and African measurements from Cairo to Cape Town were completed. Thus, all of Europe, Africa, and Asia are molded into one great system. Through common survey stations, it was also possible to convert data from the Russian Pulkova, 1932 system to the European Datum, and as a result, the European Datum includes triangulation as far east as the 84th meridian. Additional ties across the Middle East have permitted connection of the Indian and European Datums.
The Ordnance Survey of Great Britain 1936 Datum has no point of origin. The data was derived as a best fit between retriangulation and original values of 11 points of the earlier Principal Triangulation of Great Britain (1783-1853).
Tokyo Datum has its origin in Tokyo. It is defined in terms of the Bessel Ellipsoid and oriented by a single astro-

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GEODESY AND DATUMS IN NAVIGATION

Figure 208. Major geodetic datum blocks.

nomic station. Triangulation ties through Korea connect the Japanese datum with the Manchurian datum. Unfortunately, Tokyo is situated on a steep slope on the geoid, and the singlestation orientation has resulted in large systematic geoidal separations as the system is extended from its initial point.
The Indian Datum is the preferred datum for India and several adjacent countries in Southeast Asia. It is computed

on the Everest Ellipsoid with its origin at Kalianpur, in central India. It is largely the result of the untiring work of Sir George Everest (1790-1866), Surveyor General in India from 1830 to 1843. He is best known by the mountain named after him, but by far his most important legacy was the survey of the Indian subcontinent.

MODERN GEODETIC SYSTEMS

209. Development Of The World Geodetic System
By the late 1950’s the increasing range and sophistication of weapons systems had rendered local or national datums inadequate for military purposes; these new weapons required datums at least continental in scope. In response to these requirements, the U.S. Department of Defense generated a geocentric reference system to which different geodetic networks could be referred and established compatibility between the coordinates of sites of interest. Efforts of the Army, Navy, and Air Force were

combined leading to the development of the DoD World Geodetic System of 1960 (WGS 60).
In January 1966, a World Geodetic System Committee was charged with the responsibility for developing an improved WGS needed to satisfy mapping, charting, and geodetic requirements. Additional surface gravity observations, results from the extension of triangulation and trilateration networks, and large amounts of Doppler and optical satellite data had become available since the development of WGS 60. Using the additional data and improved techniques, the Committee produced WGS 66 which

GEODESY AND DATUMS IN NAVIGATION

21

served DoD needs following its implementation in 1967. The same World Geodetic System Committee began
work in 1970 to develop a replacement for WGS 66. Since the development of WGS 66, large quantities of additional data had become available from both Doppler and optical satellites, surface gravity surveys, triangulation and trilateration surveys, high precision traverses, and astronomic surveys.
In addition, improved capabilities had been developed in both computers and computer software. Continued research in computational procedures and error analyses had produced better methods and an improved facility for handling and combining data. After an extensive effort extending over a period of approximately three years, the Committee completed the development of the Department of Defense World Geodetic System 1972 (WGS 72).
Further refinement of WGS 72 resulted in the new World Geodetic System of 1984 (WGS 84). As of 1990, WGS 84 is being used for chart making by DMA. For surface navigation, WGS 60, 66, 72 and the new WGS 84 are essentially the same, so that positions computed on any WGS coordinates can be plotted directly on the others without correction.
The WGS system is not based on a single point, but many points, fixed with extreme precision by satellite fixes and statistical methods. The result is an ellipsoid which fits the real surface of the earth, or geoid, far more accurately than any other. The WGS system is applicable worldwide. All regional datums can be referenced to WGS once a survey tie has been made.

210. The New North American Datum Of 1983
The Coast And Geodetic Survey of the National Ocean Service (NOS), NOAA, is responsible for charting United States waters. From 1927 to 1987, U.S. charts were based on NAD 27, using the Clarke 1866 ellipsoid. In 1989, the U.S. officially switched to NAD 83 (navigationally equivalent to WGS 84 and other WGS systems) for all mapping and charting purposes, and all new NOS chart production is based on this new standard.
The grid of interconnected surveys which criss-crosses the United States consists of some 250,000 control points, each consisting of the latitude and longitude of the point, plus additional data such as elevation. Converting the NAD 27 coordinates to NAD 83 involved recomputing the position of each point based on the new NAD 83 datum. In addition to the 250,000 U.S. control points, several thousand more were added to tie in surveys from Canada, Mexico, and Central America.
Conversion of new edition charts to the new datums, either WGS 84 or NAD 83, involves converting reference points on each chart from the old datum to the new, and adjusting the latitude and longitude grid (known as the graticule) so that it reflects the newly plotted positions. This adjustment of the graticule is the only difference between charts which differ only in datum. All charted features remain in exactly the same relative positions.

IMPACTS ON NAVIGATION

211. Datum Shifts
One impact of different datums on navigation appears when a navigation system provides a fix based on a datum different from that used for the nautical chart. The resulting plotted position may be different from the actual location on that chart. This difference is known as a datum shift.
Another effect on navigation occurs when shifting between charts that have been made using different datums. If any position is replotted on a chart of another datum using only latitude and longitude for locating that position, the newly plotted position will not match with respect to other charted features. This datum shift may be avoided by replotting using bearings and ranges to common points. If datum shift conversion notes for the applicable datums are given on the charts, positions defined by latitude and longitude may be replotted after applying the noted correction.
The positions given for chart corrections in the Notice to Mariners reflect the proper datum for each specific chart and edition number. Due to conversion of charts based on old datums to more modern ones, and the use of many different datums throughout the world, chart corrections intended for one edition of a chart may not be safely plotted on any other.
These datum shifts are not constant throughout a given area, but vary according to how the differing datums fit to-

gether. For example, the NAD 27 to NAD 83 conversion results in changes in latitude of 40 meters in Miami, 11 meters in New York, and 20 meters in Seattle. Longitude changes for this conversion are about 22 meters in Miami, 35 meters in New York, and 93 meters in Seattle.
Most charts produced by DMA and NOS show a “datum note.” This note is usually found in the title block or in the upper left margin of the chart. According to the year of the chart edition, the scale, and policy at the time of production, the note may say “World Geodetic System 1972 (WGS-72)”, “World Geodetic System 1984 (WGS-84)”, or “World Geodetic System (WGS).” A datum note for a chart for which satellite positions can be plotted without correction will read: “Positions obtained from satellite navigation systems referred to (REFERENCE DATUM) can be plotted directly on this chart.”
DMA reproductions of foreign chart‘s will usually be in the datum or reference system of the producing country. In these cases a conversion factor is given in the following format: “Positions obtained from satellite navigation systems referred to the (Reference Datum) must be moved X.XX minutes (Northward/Southward) and X.XX minutes (Eastward/ Westward) to agree with this chart.”
Some charts cannot be tied in to WGS because of lack of recent surveys. Currently issued charts of some areas are based on surveys or use data obtained in the age of sailing

ships. The lack of surveyed control points means that they cannot be properly referenced to modern geodetic systems. In this case there may be a note that says: “Adjustments to WGS cannot be determined for this chart.”
A few charts may have no datum note at all, but may carry a note which says: “From various sources to (year).” In these cases there is no way for the navigator to determine the mathematical difference between the local datum and WGS positions. However, if a radar or visual fix can be very accurately determined, the difference between this fix and a satellite fix can determine an approximate correction factor which will be reasonably consistent for that local area.
212. Minimizing Errors Caused By Differing Datums
To minimize problems caused by differing datums:
• Plot chart corrections only on the specific charts and editions for which they are intended. Each chart correction is specific to only one edition of a chart. When the same correction is made on two charts based on different datums, the positions for the same feature may differ slightly. This difference is equal to the datum shift between the two datums for that area.
• Try to determine the source and datum of positions of temporary features, such as drill rigs. In general they are given in the datum used in the area in question. Since these are usually positioned using satellites, WGS is the normal datum. A datum correction, if needed, might be found on a chart of the area.
• Remember that if the datum of a plotted feature is not known, position inaccuracies may result. It is wise to allow a margin of error if there is any doubt about the datum.
• Know how the datum of the positioning system you are using (Loran, GPS, etc.) relates to your chart. GPS and other modern positioning systems use the WGS datum. If your chart is on any other datum, you must apply a datum correction when plotting the GPS position of the chart.
Modern geodesy can support the goal of producing all the world’s charts on the same datum. Coupling an electronic chart with satellite positioning will eliminate the problem of differing datums because electronically derived positions and the video charts on which they are displayed are derived from one of the new worldwide datums.

CHAPTER 3

NAUTICAL CHARTS

CHART FUNDAMENTALS

300. Definitions

302. Selecting A Projection

A nautical chart represents part of the spherical earth on a plane surface. It shows water depth, the shoreline of adjacent land, topographic features, aids to navigation, and other navigational information. It is a work area on which the navigator plots courses, ascertains positions, and views the relationship of the ship to the surrounding area. It assists the navigator in avoiding dangers and arriving safely at his destination.
The actual form of a chart may vary. Traditional nautical charts have been printed on paper. Electronic charts consisting of a digital data base and a display system are in use and will eventually replace paper charts for operational use. An electronic chart is not simply a digital version of a paper chart; it introduces a new navigation methodology with capabilities and limitations very different from paper charts. The electronic chart will eventually become the legal equivalent of the paper chart when approved by the International Maritime Organization and the various governmental agencies which regulate navigation. Currently, however, mariners must maintain a paper chart on the bridge. See Chapter 14, The Integrated Bridge, for a discussion of electronic charts.
Should a marine accident occur, the nautical chart in use at the time takes on legal significance. In cases of grounding, collision, and other accidents, charts become critical records for reconstructing the event and assigning liability. Charts used in reconstructing the incident can also have tremendous training value.
301. Projections
Because a cartographer cannot transfer a sphere to a flat surface without distortion, he must project the surface of a sphere onto a developable surface. A developable surface is one that can be flattened to form a plane. This process is known as chart projection. If points on the surface of the sphere are projected from a single point, the projection is said to be perspective or geometric.
As the use of electronic charts becomes increasingly widespread, it is important to remember that the same cartographic principles that apply to paper charts apply to their depiction on video screens.

Each projection has certain preferable features. However, as the area covered by the chart becomes smaller, the differences between various projections become less noticeable. On the largest scale chart, such as of a harbor, all projections are practically identical. Some desirable properties of a projection are:
1. True shape of physical features. 2. Correct angular relationship. A projection with this
characteristic is conformal or orthomorphic. 3. Equal area, or the representation of areas in their
correct relative proportions. 4. Constant scale values for measuring distances. 5. Great circles represented as straight lines. 6. Rhumb lines represented as straight lines.
Some of these properties are mutually exclusive. For example, a single projection cannot be both conformal and equal area. Similarly, both great circles and rhumb lines cannot be represented on a single projection as straight lines.
303. Types Of Projections
The type of developable surface to which the spherical surface is transferred determines the projection’s classification. Further classification depends on whether the projection is centered on the equator (equatorial), a pole (polar), or some point or line between (oblique). The name of a projection indicates its type and its principal features.
Mariners most frequently use a Mercator projection, classified as a cylindrical projection upon a plane, the cylinder tangent along the equator. Similarly, a projection based upon a cylinder tangent along a meridian is called transverse (or inverse) Mercator or transverse (or inverse) orthomorphic. The Mercator is the most common projection used in maritime navigation, primarily because rhumb lines plot as straight lines.
In a simple conic projection, points on the surface of the earth are transferred to a tangent cone. In the Lambert conformal projection, the cone intersects the earth (a secant cone) at two small circles. In a polyconic projection, a series of tangent cones is used.
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In an azimuthal or zenithal projection, points on the earth are transferred directly to a plane. If the origin of the projecting rays is the center of the earth, a gnomonic projection results; if it is the point opposite the plane’s point of tangency, a stereographic projection; and if at infinity (the projecting lines being parallel to each other), an orthographic projection. The gnomonic, stereographic, and orthographic are perspective projections. In an azimuthal equidistant projection, which is not perspective, the scale of distances is constant along any radial line from the point of tangency. See Figure 303.

cide. These projections are classified as oblique or transverse projections.

Figure 303. Azimuthal projections: A, gnomonic; B, stereographic; C, (at infinity) orthographic.
Cylindrical and plane projections are special conical projections, using heights infinity and zero, respectively.
A graticule is the network of latitude and longitude lines laid out in accordance with the principles of any projection.
304. Cylindrical Projections
If a cylinder is placed around the earth, tangent along the equator, and the planes of the meridians are extended, they intersect the cylinder in a number of vertical lines. See Figure 304. These parallel lines of projection are equidistant from each other, unlike the terrestrial meridians from which they are derived which converge as the latitude increases. On the earth, parallels of latitude are perpendicular to the meridians, forming circles of progressively smaller diameter as the latitude increases. On the cylinder they are shown perpendicular to the projected meridians, but because a cylinder is everywhere of the same diameter, the projected parallels are all the same size.
If the cylinder is cut along a vertical line (a meridian) and spread out flat, the meridians appear as equally spaced vertical lines; and the parallels appear as horizontal lines. The parallels’ relative spacing differs in the various types of cylindrical projections.
If the cylinder is tangent along some great circle other than the equator, the projected pattern of latitude and longitude lines appears quite different from that described above, since the line of tangency and the equator no longer coin-

Figure 304. A cylindrical projection.
305. Mercator Projection
Navigators most often use the plane conformal projection known as the Mercator projection. The Mercator projection is not perspective, and its parallels can be derived mathematically as well as projected geometrically. Its distinguishing feature is that both the meridians and parallels are expanded at the same ratio with increased latitude. The expansion is equal to the secant of the latitude, with a small correction for the ellipticity of the earth. Since the secant of 90° is infinity, the projection cannot include the poles. Since the projection is conformal, expansion is the same in all directions and angles are correctly shown. Rhumb lines appear as straight lines, the directions of which can be measured directly on the chart. Distances can also be measured directly if the spread of latitude is small. Great circles, except meridians and the equator, appear as curved lines concave to the equator. Small areas appear in their correct shape but of increased size unless they are near the equator.
306. Meridional Parts
At the equator a degree of longitude is approximately

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Figure 306. A Mercator map of the world.

equal in length to a degree of latitude. As the distance from the equator increases, degrees of latitude remain approximately the same, while degrees of longitude become progressively shorter. Since degrees of longitude appear everywhere the same length in the Mercator projection, it is necessary to increase the length of the meridians if the expansion is to be equal in all directions. Thus, to maintain the correct proportions between degrees of latitude and degrees of longitude, the degrees of latitude must be progressively longer as the distance from the equator increases. This is illustrated in figure 306.
The length of a meridian, increased between the equator and any given latitude, expressed in minutes of arc at the equator as a unit, constitutes the number of meridional parts (M) corresponding to that latitude. Meridional parts, given in Table 6 for every minute of latitude from the equator to the pole, make it possible to construct a Mercator chart and to solve problems in Mercator sailing. These values are for the WGS ellipsoid of 1984.
307. Transverse Mercator Projections
Constructing a chart using Mercator principles, but

with the cylinder tangent along a meridian, results in a transverse Mercator or transverse orthomorphic projection. The word “inverse” is used interchangeably with “transverse.” These projections use a fictitious graticule similar to, but offset from, the familiar network of meridians and parallels. The tangent great circle is the fictitious equator. Ninety degrees from it are two fictitious poles. A group of great circles through these poles and perpendicular to the tangent great circle are the fictitious meridians, while a series of circles parallel to the plane of the tangent great circle form the fictitious parallels. The actual meridians and parallels appear as curved lines.
A straight line on the transverse or oblique Mercator projection makes the same angle with all fictitious meridians, but not with the terrestrial meridians. It is therefore a fictitious rhumb line. Near the tangent great circle, a straight line closely approximates a great circle. The projection is most useful in this area. Since the area of minimum distortion is near a meridian, this projection is useful for charts covering a large band of latitude and extending a relatively short distance on each side of the tangent meridian. It is sometimes used for star charts showing the evening sky at various seasons of the year. See Figure 307.

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Figure 309a. An oblique Mercator projection.

Figure 307. A transverse Mercator map of the Western Hemisphere.
308. Universal Transverse Mercator (UTM) Grid
The Universal Transverse Mercator (UTM) grid is a military grid superimposed upon a transverse Mercator graticule, or the representation of these grid lines upon any graticule. This grid system and these projections are often used for large-scale (harbor) nautical charts and military charts.
309. Oblique Mercator Projections
A Mercator projection in which the cylinder is tangent along a great circle other than the equator or a meridian is called an oblique Mercator or oblique orthomorphic projection. This projection is used principally to depict an area in the near vicinity of an oblique great circle. Figure 309c, for example, shows the great circle joining Washington and Moscow. Figure 309d shows an oblique Mercator map with the great circle between these two centers as the tangent great circle or fictitious equator. The limits of the chart of Figure 309c are indicated in Figure 309d. Note the large variation in scale as the latitude changes.

Figure 309b. The fictitious graticle of an oblique Mercator projection.

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Figure 309c. The great circle between Washington and Moscow as it appears on a Mercator map.

Figure 309d. An oblique Mercator map based upon a cylinder tangent along the great circle through Washington and Moscow. The map includes an area 500 miles on each side of the great circle. The limits of this map are indicated on the
Mercator map of Figure 309c

310. Rectangular Projection
A cylindrical projection similar to the Mercator, but with uniform spacing of the parallels, is called a rectangular projection. It is convenient for graphically depicting information where distortion is not important. The principal navigational use of this projection is for the star chart of the Air Almanac, where positions of stars are plotted by rectangular coordinates representing declination (ordinate) and sidereal hour angle (abscissa). Since the meridians are parallel, the parallels of latitude (including the equator and the poles) are all represented by lines of equal length.

converging toward the nearer pole. Limiting the area covered to that part of the cone near the surface of the earth limits distortion. A parallel along which there is no distortion is called a standard parallel. Neither the transverse conic projection, in which the axis of the cone is in the equatorial plane, nor the oblique conic projection, in which the axis of the cone is oblique to the plane of the equator, is ordinarily used for navigation. They are typically used for illustrative maps.
Using cones tangent at various parallels, a secant (intersecting) cone, or a series of cones varies the appearance and features of a conic projection.

311. Conic Projections

312. Simple Conic Projection

A conic projection is produced by transferring points from the surface of the earth to a cone or series of cones. This cone is then cut along an element and spread out flat to form the chart. When the axis of the cone coincides with the axis of the earth, then the parallels appear as arcs of circles, and the meridians appear as either straight or curved lines

A conic projection using a single tangent cone is a simple conic projection (Figure 312a). The height of the cone increases as the latitude of the tangent parallel decreases. At the equator, the height reaches infinity and the cone becomes a cylinder. At the pole, its height is zero, and the cone becomes a plane. Similar to the Mercator projection,

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the simple conic projection is not perspective since only the meridians are projected geometrically, each becoming an element of the cone. When this projection is spread out flat to form a map, the meridians appear as straight lines converging at the apex of the cone. The standard parallel, where the cone is tangent to the earth, appears as the arc of a circle with its center at the apex of the cone. The other

parallels are concentric circles. The distance along any meridian between consecutive parallels is in correct relation to the distance on the earth, and, therefore, can be derived mathematically. The pole is represented by a circle (Figure 312b). The scale is correct along any meridian and along the standard parallel. All other parallels are too great in length, with the error increasing with increased distance from the standard parallel. Since the scale is not the same in all directions about every point, the projection is neither a conformal nor equal-area projection. Its non-conformal nature is its principal disadvantage for navigation.
Since the scale is correct along the standard parallel and varies uniformly on each side, with comparatively little distortion near the standard parallel, this projection is useful for mapping an area covering a large spread of longitude and a comparatively narrow band of latitude. It was developed by Claudius Ptolemy in the second century A.D. to map just such an area: the Mediterranean Sea.

313. Lambert Conformal Projection

Figure 312a. A simple conic projection.

The useful latitude range of the simple conic projection can be increased by using a secant cone intersecting the earth at two standard parallels. See Figure 313. The area between the two standard parallels is compressed, and that beyond is expanded. Such a projection is called either a secant conic or conic projection with two standard parallels.

Figure 312b. A simple conic map of the Northern Hemisphere.

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The polyconic projection is widely used in atlases, particularly for areas of large range in latitude and reasonably large range in longitude, such as continents. However, since it is not conformal, this projection is not customarily used in navigation.

315. Azimuthal Projections

Figure 313. A secant cone for a conic projection with two standard parallels.
If in such a projection the spacing of the parallels is altered, such that the distortion is the same along them as along the meridians, the projection becomes conformal. This modification produces the Lambert conformal projection. If the chart is not carried far beyond the standard parallels, and if these are not a great distance apart, the distortion over the entire chart is small.
A straight line on this projection so nearly approximates a great circle that the two are nearly identical. Radio beacon signals travel great circles; thus, they can be plotted on this projection without correction. This feature, gained without sacrificing conformality, has made this projection popular for aeronautical charts because aircraft make wide use of radio aids to navigation. Except in high latitudes, where a slightly modified form of this projection has been used for polar charts, it has not replaced the Mercator projection for marine navigation.

If points on the earth are projected directly to a plane surface, a map is formed at once, without cutting and flattening, or “developing.” This can be considered a special case of a conic projection in which the cone has zero height.
The simplest case of the azimuthal projection is one in which the plane is tangent at one of the poles. The meridians are straight lines intersecting at the pole, and the parallels are concentric circles with their common center at the pole. Their spacing depends upon the method used to transfer points from the earth to the plane.
If the plane is tangent at some point other than a pole, straight lines through the point of tangency are great circles, and concentric circles with their common center at the point of tangency connect points of equal distance from that point. Distortion, which is zero at the point of tangency, increases along any great circle through this point. Along any circle whose center is the point of tangency, the distortion is constant. The bearing of any point from the point of tangency is correctly represented. It is for this reason that these projections are called azimuthal. They are also called zenithal. Several of the common azimuthal projections are perspective.
316. Gnomonic Projection
If a plane is tangent to the earth, and points are projected geometrically from the center of the earth, the result is a gnomonic projection. See Figure 316a. Since the projection is perspective, it can be demonstrated by placing a light at the center of a transparent terrestrial globe and holding a

314. Polyconic Projection

The latitude limitations of the secant conic projection can be minimized by using a series of cones. This results in a polyconic projection. In this projection, each parallel is the base of a tangent cone . At the edges of the chart, the area between parallels is expanded to eliminate gaps. The scale is correct along any parallel and along the central meridian of the projection. Along other meridians the scale increases with increased difference of longitude from the central meridian. Parallels appear as nonconcentric circles; meridians appear as curved lines converging toward the pole and concave to the central meridian.

Figure 316a. An oblique gnomonic projection.

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flat surface tangent to the sphere.
In an oblique gnomonic projection the meridians appear as straight lines converging toward the nearer pole. The parallels, except the equator, appear as curves (Figure 316b). As in all azimuthal projections, bearings from the point of tangency are correctly represented. The distance scale, however, changes rapidly. The projection is neither conformal nor equal area. Distortion is so great that shapes, as well as distances and areas, are very poorly represented, except near the point of tangency.

Figure 317a. An equatorial stereographic projection.

Figure 316b. An oblique gnomonic map with point of tangency at latitude 30°N, longitude 90°W.
The usefulness of this projection rests upon the fact that any great circle appears on the map as a straight line, giving charts made on this projection the common name great-circle charts.
Gnomonic charts are most often used for planning the great-circle track between points. Points along the determined track are then transferred to a Mercator projection. The great circle is then followed by following the rhumb lines from one point to the next. Computer programs which automatically calculate great circle routes between points and provide latitude and longitude of corresponding rhumb line endpoints are quickly making this use of the gnomonic chart obsolete.

317. Stereographic Projection
A stereographic projection results from projecting points on the surface of the earth onto a tangent plane, from a point on the surface of the earth opposite the point of tangency (Figure 317a). This projection is also called an azimuthal orthomorphic projection.
The scale of the stereographic projection increases with distance from the point of tangency, but it increases more slowly than in the gnomonic projection. The stereographic projection can show an entire hemisphere without excessive distortion (Figure 317b). As in other azimuthal projections,

Figure 317b. A stereographic map of the Western Hemisphere.
great circles through the point of tangency appear as straight lines. Other circles such as meridians and parallels appear as either circles or arcs of circles.
The principal navigational use of the stereographic projection is for charts of the polar regions and devices for mechanical or graphical solution of the navigational triangle. A Universal Polar Stereographic (UPS) grid, mathematically adjusted to the graticule, is used as a reference system.

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31

318. Orthographic Projection
If terrestrial points are projected geometrically from infinity to a tangent plane, an orthographic projection results (Figure 318a). This projection is not conformal; nor does it result in an equal area representation. Its principal use is in navigational astronomy because it is useful for illustrating and solving the navigational triangle. It is also useful for illustrating celestial coordinates. If the plane is tangent at a point on the equator, the parallels (including the equator) appear as straight lines. The meridians would appear as ellipses, except that the meridian through the point of tangency would appear as a straight line and the one 90° away would appear as a circle (Figure 318b).
319. Azimuthal Equidistant Projection
An azimuthal equidistant projection is an azimuthal projection in which the distance scale along any great circle through the point of tangency is constant. If a pole is the point of tangency, the meridians appear as straight radial

lines and the parallels as equally spaced concentric circles. If the plane is tangent at some point other than a pole, the concentric circles represent distances from the point of tangency. In this case, meridians and parallels appear as curves.
The projection can be used to portray the entire earth, the point 180° from the point of tangency appearing as the largest of the concentric circles. The projection is not conformal, equal area, or perspective. Near the point of tangency distortion is small, increasing with distance until shapes near the opposite side of the earth are unrecognizable (Figure 319).
The projection is useful because it combines the three features of being azimuthal, having a constant distance scale from the point of tangency, and permitting the entire earth to be shown on one map. Thus, if an important harbor or airport is selected as the point of tangency, the great-circle course, distance, and track from that point to any other point on the earth are quickly and accurately determined. For communication work with the station at the point of tangency, the path of an incoming signal is at once apparent if the direction of arrival has been determined and the direction to train a directional antenna can be determined easily. The projection is also used for polar charts and for the star finder, No. 2102D.

Figure 318a. An equatorial orthographic projection.

Figure 318b. An orthographic map of the Western Hemisphere.

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Figure 319. An azimuthal equidistant map of the world with the point of tangency latitude 40°N, longitude 100°W.

POLAR CHARTS

320. Polar Projections
Special consideration is given to the selection of projections for polar charts because the familiar projections become special cases with unique features.
In the case of cylindrical projections in which the axis of the cylinder is parallel to the polar axis of the earth, distortion becomes excessive and the scale changes rapidly. Such projections cannot be carried to the poles. However, both the transverse and oblique Mercator projections are used.
Conic projections with their axes parallel to the earth’s polar axis are limited in their usefulness for polar charts because parallels of latitude extending through a full 360° of longitude appear as arcs of circles rather than full circles. This is because a cone, when cut along an element and flattened, does not extend

through a full 360° without stretching or resuming its former conical shape. The usefulness of such projections is also limited by the fact that the pole appears as an arc of a circle instead of a point. However, by using a parallel very near the pole as the higher standard parallel, a conic projection with two standard parallels can be made. This requires little stretching to complete the circles of the parallels and eliminate that of the pole. Such a projection, called a modified Lambert conformal or Ney’s projection, is useful for polar charts. It is particularly familiar to those accustomed to using the ordinary Lambert conformal charts in lower latitudes.
Azimuthal projections are in their simplest form when tangent at a pole. This is because the meridians are straight lines intersecting at the pole, and parallels are concentric circles with their common center at the pole. Within a few

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33

degrees of latitude of the pole they all look similar; however, as the distance becomes greater, the spacing of the parallels becomes distinctive in each projection. In the polar azimuthal equidistant it is uniform; in the polar stereographic it increases with distance from the pole until the equator is shown at a distance from the pole equal to twice the length of the radius of the earth; in the polar gnomonic the increase is considerably greater, becoming infinity at the equator; in the polar orthographic it decreases with distance from the pole (Figure 320). All of these but the last are used for polar charts.

The projections commonly used for polar charts are the modified Lambert conformal, gnomonic, stereographic, and azimuthal equidistant. All of these projections are similar near the pole. All are essentially conformal, and a great circle on each is nearly a straight line.
As the distance from the pole increases, however, the distinctive features of each projection become important. The modified Lambert conformal projection is virtually conformal over its entire extent. The amount of its scale distortion is comparatively little if it is carried only to about 25° or 30° from the pole. Beyond this, the distortion increases rapidly. A great circle is very nearly a straight line anywhere on the chart. Distances and directions can be measured directly on the chart in the same manner as on a Lambert conformal chart. However, because this projection is not strictly conformal, and on it great circles are not exactly represented by straight lines, it is not suited for highly accurate work.
The polar gnomonic projection is the one polar projection on which great circles are exactly straight lines. However, a complete hemisphere cannot be represented upon a plane because the radius of 90° from the center would become infinity.
The polar stereographic projection is conformal over its entire extent, and a straight line closely approximates a great circle. See Figure 321. The scale distortion is not excessive for a considerable distance from the pole, but it is greater than that of the modified Lambert conformal projection.

Figure 320. Expansion of polar azimuthal projections. 321. Selection Of A Polar Projection

The principal considerations in the choice of a suitable projection for polar navigation are:

1. Conformality: When the projection represents angles correctly, the navigator can plot directly on the chart.
2. Great circle representation: Because great circles are more useful than rhumb lines at high altitudes, the projection should represent great circles as straight lines.
3. Scale variation: The projection should have a constant scale over the entire chart.
4. Meridian representation: The projection should show straight meridians to facilitate plotting and grid navigation
5. Limits: Wide limits reduce the number of projections needed to a minimum.

Figure 321. Polar stereographic projection.
The polar azimuthal equidistant projection is useful for showing a large area such as a hemisphere because there is

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no expansion along the meridians. However, the projection is not conformal and distances cannot be measured accurately in any but a north-south direction. Great circles other than the meridians differ somewhat from straight lines. The equator is a circle centered at the pole.
The two projections most commonly used for polar charts are the modified Lambert conformal and the polar stereographic. When a directional gyro is used as a directional reference, the track of the craft is approximately a great circle. A desirable chart is one on which a great circle is represented as a straight line with a constant scale and with angles correctly represented. These requirements are not met entirely by any single projection, but they are approximated by both the modified Lambert conformal and the polar stereographic. The scale is more nearly constant on the former, but the projection is not strictly conformal. The polar stereographic is conformal, and its maximum

scale variation can be reduced by using a plane which intersects the earth at some parallel intermediate between the pole and the lowest parallel. The portion within this standard parallel is compressed, and that portion outside is expanded.
The selection of a suitable projection for use in polar regions depends upon mission requirements. These requirements establish the relative importance of various features. For a relatively small area, any of several projections is suitable. For a large area, however, the choice is more difficult. If grid directions are to be used, it is important that all units in related operations use charts on the same projection, with the same standard parallels, so that a single grid direction exists between any two points. Nuclear powered submarine operations under the polar icecap have increased the need for grid directions in marine navigation.

SPECIAL CHARTS

322. Plotting Sheets
Position plotting sheets are “charts” designed primarily for open ocean navigation, where land, visual aids to navigation, and depth of water are not factors in navigation. They have a latitude and longitude graticule, and they may have one or more compass roses. The meridians are usually unlabeled, so a plotting sheet can be used for any longitude. Plotting sheets on Mercator projection are specific to latitude, and the navigator should have enough aboard for all latitudes for his voyage. Plotting sheets are less expensive than charts.
One use of a plotting sheet may occur in the event of an emergency when all charts have been lost or are otherwise unavailable. Directions on how to construct plotting sheets suitable for emergency purposes are given in Chapter 26, Emergency Navigation.
323. Grids
No system exists for showing the surface of the earth

on a plane without distortion. Moreover, the appearance of the surface varies with the projection and with the relation of that surface area to the point of tangency. One may want to identify a location or area simply by alpha-numeric rectangular coordinates. This is accomplished with a grid. In its usual form this consists of two series of lines drawn perpendicularly on the chart, marked by suitable alpha-numeric designations.
A grid may use the rectangular graticule of the Mercator projection or a set of arbitrary lines on a particular projection. The World Geodetic Reference System (GEOREF) is a method of designating latitude and longitude by a system of letters and numbers instead of by angular measure. It is not, therefore, strictly a grid. It is useful for operations extending over a wide area. Examples of the second type of grid are the Universal Transverse Mercator (UTM) grid, the Universal Polar Stereographic (UPS) grid, and the Temporary Geographic Grid (TGG). Since these systems are used primarily by military forces, they are sometimes called military grids.

CHART SCALES

324. Types Of Scales
The scale of a chart is the ratio of a given distance on the chart to the actual distance which it represents on the earth. It may be expressed in various ways. The most common are:
1. A simple ratio or fraction, known as the representative fraction. For example, 1:80,000 or 1/80,000 means that one unit (such as a meter) on the chart

represents 80,000 of the same unit on the surface of the earth. This scale is sometimes called the natural or fractional scale. 2. A statement that a given distance on the earth equals a given measure on the chart, or vice versa. For example, “30 miles to the inch” means that 1 inch on the chart represents 30 miles of the earth’s surface. Similarly, “2 inches to a mile” indicates that 2 inches on the chart represent 1 mile on the earth. This is some-

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times called the numerical scale. 3. A line or bar called a graphic scale may be drawn at
a convenient place on the chart and subdivided into nautical miles, meters, etc. All charts vary somewhat in scale from point to point, and in some projections the scale is not the same in all directions about a single point. A single subdivided line or bar for use over an entire chart is shown only when the chart is of such scale and projection that the scale varies a negligible amount over the chart, usually one of about 1:75,000 or larger. Since 1 minute of latitude is very nearly equal to 1 nautical mile, the latitude scale serves as an approximate graphic scale. On most nautical charts the east and west borders are subdivided to facilitate distance measurements.
On a Mercator chart the scale varies with the latitude. This is noticeable on a chart covering a relatively large distance in a north-south direction. On such a chart the border scale near the latitude in question should be used for measuring distances.
Of the various methods of indicating scale, the graphical method is normally available in some form on the chart. In addition, the scale is customarily stated on charts on which the scale does not change appreciably over the chart.
The ways of expressing the scale of a chart are readily interchangeable. For instance, in a nautical mile there are about 72,913.39 inches. If the natural scale of a chart is 1:80,000, one inch of the chart represents 80,000 inches of the earth, or a little more than a mile. To find the exact amount, divide the scale by the number of inches in a mile, or 80,000/72,913.39 = 1.097. Thus, a scale of 1:80,000 is the same as a scale of 1.097 (or approximately 1.1) miles to an inch. Stated another way, there are: 72,913.39/80,000 = 0.911 (approximately 0.9) inch to a mile. Similarly, if the scale is 60 nautical miles to an inch, the representative fraction is 1:(60 x 72,913.39) = 1:4,374,803.
A chart covering a relatively large area is called a small-scale chart and one covering a relatively small area is called a large-scale chart. Since the terms are relative, there is no sharp division between the two. Thus, a chart of scale 1:100,000 is large scale when compared with a chart of 1:1,000,000 but small scale when compared with one of 1:25,000.

As scale decreases, the amount of detail which can be shown decreases also. Cartographers selectively decrease the detail in a process called generalization when producing small scale charts using large scale charts as sources. The amount of detail shown depends on several factors, among them the coverage of the area at larger scales and the intended use of the chart.
325. Chart Classification By Scale
Charts are constructed on many different scales, ranging from about 1:2,500 to 1:14,000,000. Small-scale charts covering large areas are used for route planning and for offshore navigation. Charts of larger scale, covering smaller areas, are used as the vessel approaches land. Several methods of classifying charts according to scale are used in various nations. The following classifications of nautical charts are used by the National Ocean Service.
Sailing charts are the smallest scale charts used for planning, fixing position at sea, and for plotting the dead reckoning while proceeding on a long voyage. The scale is generally smaller than 1:600,000. The shoreline and topography are generalized and only offshore soundings, the principal navigational lights, outer buoys, and landmarks visible at considerable distances are shown.
General charts are intended for coastwise navigation outside of outlying reefs and shoals. The scales range from about 1:150,000 to 1:600,000.
Coastal charts are intended for inshore coastwise navigation, for entering or leaving bays and harbors of considerable width, and for navigating large inland waterways. The scales range from about 1:50,000 to 1:150,000.
Harbor charts are intended for navigation and anchorage in harbors and small waterways. The scale is generally larger than 1:50,000.
In the classification system used by the Defense Mapping Agency Hydrographic/Topographic Center, the sailing charts are incorporated in the general charts classification (smaller than about 1:150,000); those coast charts especially useful for approaching more confined waters (bays, harbors) are classified as approach charts. There is considerable overlap in these designations, and the classification of a chart is best determined by its use and by its relationship to other charts of the area. The use of insets complicates the placement of charts into rigid classifications.

CHART ACCURACY

326. Factors Relating To Accuracy
The accuracy of a chart depends upon the accuracy of the hydrographic surveys used to compile it and the suitability of its scale for its intended use.
Estimate the accuracy of a chart’s surveys from the

source notes given in the title of the chart. If the chart is based upon very old surveys, use it with caution. Many early surveys were inaccurate because of the technological limitations of the surveyor.
The number of soundings and their spacing indicates the completeness of the survey. Only a small fraction of the

36

NAUTICAL CHARTS

Figure 326a. Part of a “boat sheet,” showing the soundings obtained in a survey.

Figure 326b. Part of a nautical chart made from the boat sheet of Figure 326a. Compare the number of soundings in the two figures.

NAUTICAL CHARTS

37

soundings taken in a thorough survey are shown on the chart, but sparse or unevenly distributed soundings indicate that the survey was probably not made in detail. See Figure 326a and Figure 326b Large blank areas or absence of depth contours generally indicate lack of soundings in the area. Operate in an area with sparse sounding data only if operationally required and then only with the most extreme caution. Run the echo sounder continuously and operate at a reduced speed. Sparse sounding information does not necessarily indicate an incomplete survey. Relatively few soundings are shown when there is a large number of depth contours, or where the bottom is flat, or gently and evenly sloping. Additional soundings are shown when they are helpful in indicating the uneven character of a rough bottom.
Even a detailed survey may fail to locate every rock or pinnacle. In waters where they might be located, the best method for finding them is a wire drag survey. Areas that have been dragged may be indicated on the chart by limiting lines and green or purple tint and a note added to show the effective depth at which the drag was operated.
Changes in bottom contours are relatively rapid in areas such as entrances to harbors where there are strong currents or heavy surf. Similarly, there is sometimes a ten-

dency for dredged channels to shoal, especially if they are surrounded by sand or mud, and cross currents exist. Charts often contain notes indicating the bottom contours are known to change rapidly.
The same detail cannot be shown on a small-scale chart as on a large scale chart. On small-scale charts, detailed information is omitted or “generalized” in the areas covered by larger scale charts. The navigator should use the largest scale chart available for the area in which he is operating, especially when operating in the vicinity of hazards.
Charting agencies continually evaluate both the detail and the presentation of data appearing on a chart. Development of a new navigational aid may render previous charts inadequate. The development of radar, for example, required upgrading charts which lacked the detail required for reliable identification of radar targets.
After receiving a chart, the user is responsible for keeping it updated. Mariners reports of errors, changes, and suggestions are useful to charting agencies. Even with modern automated data collection techniques, there is no substitute for on-sight observation of hydrographic conditions by experienced mariners. This holds true especially in less frequently traveled areas of the world.

CHART READING

327. Chart Dates
NOS charts have two dates. At the top center of the chart is the date of the first edition of the chart. In the lower left corner of the chart is the current edition number and date. This date shows the latest date through which Notice to Mariners were applied to the chart. Any subsequent change will be printed in the Notice to Mariners. Any notices which accumulate between the chart date and the announcement date in the Notice to Mariners will be given with the announcement. Comparing the dates of the first and current editions gives an indication of how often thechart is updated. Charts of busy areas are updated more frequently than those of less traveled areas. This interval may vary from 6 months to more than ten years for NOS charts. This update interval may be much longer for certain DMAHTC charts in remote areas.
New editions of charts are both demand and source driven. Receiving significant new information may or may not initiate a new edition of a chart, depending on the demand for that chart. If it is in a sparsely-traveled area, other priorities may delay a new edition for several years. Conversely, a new edition may be printed without the receipt of significant new data if demand for the chart is high and stock levels are low. Notice to Mariners corrections are always included on new editions.
DMAHTC charts have the same two dates as the NOS

charts; the current chart edition number and date is given in the lower left corner. Certain DMAHTC charts are reproductions of foreign charts produced under joint agreements with a number of other countries. These charts, even though of recent date, may be based on foreign charts of considerably earlier date. Further, new editions of the foreign chart will not necessarily result in a new edition of the DMAHTC reproduction. In these cases, the foreign chart is the better chart to use.
A revised or corrected print contains corrections which have been published in Notice to Mariners. These corrected prints do not supersede a current edition. The date of the revision is given, along with the latest Notice to Mariners to which the chart has been corrected.
328. Title Block
See Figure 328. The chart title block should be the first thing a navigator looks at when receiving a new edition chart. The title itself tells what area the chart covers. The chart’s scale and projection appear below the title. The chart will give both vertical and horizontal datums and, if necessary, a datum conversion note. Source notes or diagrams will list the date of surveys and other charts used in compilation.

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NAUTICAL CHARTS

BALTIC SEA
GERMANY—NORTH COAST
DAHMESHÖVED TO WISMAR
From German Surveys SOUNDINGS IN METERS reduced to the approximate level of Mean Sea Level HEIGHTS IN METERS ABOVE MEAN SEA LEVEL
MERCATOR PROJECTION EUROPEAN DATUM SCALE 1:50,000
Figure 328. A chart title block.

329. Shoreline
The shoreline shown on nautical charts represents the line of contact between the land and water at a selected vertical datum. In areas affected by tidal fluctuations, this is usually the mean high-water line. In confined coastal waters of diminished tidal influence, a mean water level line may be used. The shoreline of interior waters (rivers, lakes) is usually a line representing a specified elevation above a selected datum. A shoreline is symbolized by a heavy line. A broken line indicates that the charted position is approximate only. The nature of the shore may be indicated.
If the low water line differs considerably from the high water line, then a dotted line represents the low water line. If the bottom in this area is composed of mud, sand, gravel or stones, the type of material will be indicated. If the bottom is composed of coral or rock, then the appropriate symbol will be used. The area alternately covered and uncovered may be shown by a tint which is usually a combination of the land and water tint.
The apparent shoreline shows the outer edge of marine vegetation where that limit would appear as shoreline to the mariner. It is also used to indicate where marine vegetation prevents the mariner from defining the shoreline. A light line symbolizes this shoreline. A broken line marks the inner edge when no other symbol (such as a cliff or levee) furnishes such a limit. The combined land-water tint or the land tint marks the area between inner and outer limits.
330. Chart Symbols
Much of the information contained on charts is shown by symbols. These symbols are not shown to scale, but they

indicate the correct position of the feature to which they refer. The standard symbols and abbreviations used on charts published by the United States of America are shown in Chart No. 1, Nautical Chart Symbols and Abbreviations. See Figure 330.
Electronic chart symbols are, within programming and display limits, much the same as printed ones. The less expensive electronic charts have less extensive symbol libraries, and the screen’s resolution may affect the presentation detail.
Most of the symbols and abbreviations shown in U.S. Chart No. 1 agree with recommendations of the International Hydrographic Organization (IHO). The layout is explained in the general remarks section of Chart No. 1.
The symbols and abbreviations on any given chart may differ somewhat from those shown in Chart No. 1. In addition, foreign charts may use different symbology. When using a foreign chart, the navigator should have available the Chart No. 1 from the country which produced the chart.
Chart No. 1 is organized according to subject matter, with each specific subject given a letter designator. The general subject areas are General, Topography, Hydrography, Aids and Services, and Indexes. Under each heading, letter designators further define subject areas, and individual numbers refer to specific symbols.
Information in Chart No. 1 is arranged in columns. The first column contains the IHO number code for the symbol in question. The next two columns show the symbol itself, in NOS and DMA formats. If the formats are the same, the two columns are combined into one. The next column is a text description of the symbol, term, or abbreviation. The next column contains the IHO standard symbol. The last column shows certain symbols used on foreign reproduction charts produced by DMA.

NAUTICAL CHARTS

39

Figure 330. Contents of U.S. Chart No. 1.

40

NAUTICAL CHARTS

331. Lettering
Except on some modified reproductions of foreign charts, cartographers have adopted certain lettering standards. Vertical type is used for features which are dry at high water and not affected by movement of the water; slanting type is used for underwater and floating features.
There are two important exceptions to the two general rules listed above. Vertical type is not used to represent heights above the waterline, and slanting type is not used to indicate soundings, except on metric charts. Section 332 below discusses the conventions for indicating soundings.
Evaluating the type of lettering used to denote a feature, one can determine whether a feature is visible at high tide. For instance, a rock might bear the title “ Rock” whether or not it extends above the surface. If the name is given in vertical letters, the rock constitutes a small islet; if in slanting type, the rock constitutes a reef, covered at high water.
332. Soundings
Charts show soundings in several ways. Numbers denote individual soundings. These numbers may be either vertical or slanting; both may be used on the same chart, distinguishing between data based upon different U.S. and foreign surveys, different datums, or smaller scale charts.
Large block letters at the top and bottom of the chart indicate the unit of measurement used for soundings. SOUNDINGS IN FATHOMS indicates soundings are in fathoms or fathoms and fractions. SOUNDINGS IN FATHOMS AND FEET indicates the soundings are in fathoms and feet. A similar convention is followed when the soundings are in meters or meters and tenths.
A depth conversion scale is placed outside the neatline on the chart for use in converting charted depths to feet, meters, or fathoms. “No bottom” soundings are indicated by a number with a line over the top and a dot over the line. This indicates that the spot was sounded to the depth indicated without reaching the bottom. Areas which have been wire dragged are shown by a broken limiting line, and the clear effective depth is indicated, with a characteristic symbol under the numbers. On DMAHTC charts a purple or green tint is shown within the swept area.
Soundings are supplemented by depth contours, lines connecting points of equal depth. These lines present a picture of the bottom. The types of lines used for various depths are shown in Section I of Chart No. 1. On some charts depth contours are shown in solid lines; the depth represented by each line is shown by numbers placed in breaks in the lines, as with land contours. Solid line depth contours are derived from intensively developed hydrographic surveys. A broken or indefinite contour is substituted for a solid depth contour whenever the reliability of the contour is questionable.
Depth contours are labeled with numerals in the unit of measurement of the soundings. A chart presenting a more detailed indication of the bottom configuration with fewer

numerical soundings is useful when bottom contour navigating. Such a chart can be made only for areas which have undergone a detailed survey
Shoal areas often are given a blue tint. Charts designed to give maximum emphasis to the configuration of the bottom show depths beyond the 100-fathom curve over the entire chart by depth contours similar to the contours shown on land areas to indicate graduations in height. These are called bottom contour or bathymetric charts.
On electronic charts, a variety of other color schemes may be used, according to the manufacturer of the system. Color perception studies are being used to determine the best presentation.
The side limits of dredged channels are indicated by broken lines. The project depth and the date of dredging, if known, are shown by a statement in or along the channel. The possibility of silting is always present. Local authorities should be consulted for the controlling depth. NOS Charts frequently show controlling depths in a table, which is kept current by the Notice to Mariners.
The chart scale is generally too small to permit all soundings to be shown. In the selection of soundings, least depths are shown first. This conservative sounding pattern provides safety and ensures an uncluttered chart appearance. Steep changes in depth may be indicated by more dense soundings in the area. The limits of shoal water indicated on the chart may be in error, and nearby areas of undetected shallow water may not be included on the chart. Given this possibility, areas where shoal water is known to exist should be avoided. If the navigator must enter an area containing shoals, he must exercise extreme caution in avoiding shallow areas which may have escaped detection. By constructing a “safety range” around known shoals and ensuring his vessel does not approach the shoal any closer than the safety range, the navigator can increase his chances of successfully navigating through shoal water. Constant use of the echo sounder is also important.
333. Bottom Description
Abbreviations listed in Section J of Chart No. 1 are used to indicate what substance forms the bottom. The meaning of these terms can be found in the Glossary of Marine Navigation. Knowing the characteristic of the bottom is most important when anchoring.
334. Depths And Datums
Depths are indicated by soundings or explanatory notes. Only a small percentage of the soundings obtained in a hydrographic survey can be shown on a nautical chart. The least depths are generally selected first, and a pattern built around them to provide a representative indication of bottom relief. In shallow water, soundings may be spaced 0.2 to 0.4 inch apart. The spacing is gradually increased as water deepens, until a spacing of 0.8 to 1.0 inch is reached in deeper waters offshore. Where a sufficient number of soundings are available to permit adequate interpretation,

NAUTICAL CHARTS

41

depth curves are drawn in at selected intervals. All depths indicated on charts are reckoned from a se-
lected level of the water, called the chart sounding datum. The various chart datums are explained in Chapter 9, Tides and Tidal Currents. On charts made from surveys conducted by the United States, the chart datum is selected with regard to the tides of the region. Depths shown are the least depths to be expected under average conditions. On charts based on foreign charts and surveys the datum is that of the original authority. When it is known, the datum used is stated on the chart. In some cases where the chart is based upon old surveys, particularly in areas where the range of tide is not great, the sounding datum may not be known.
For most National Ocean Service charts of the United States and Puerto Rico, the chart datum is mean lower low water. Most Defense Mapping Agency Hydrographic/Topographic Center charts are based upon mean low water, mean lower low water, or mean low water springs. The chart datum for charts published by other countries varies greatly, but is usually lower than mean low water. On charts of the Baltic Sea, Black Sea, the Great Lakes, and other areas where tidal effects are small or without significance, the datum adopted is an arbitrary height approximating the mean water level.
The chart datum of the largest scale chart of an area is generally the same as the reference level from which height of tide is tabulated in the tide tables.
The chart datum is usually only an approximation of the actual mean value, because determination of the actual mean height usually requires a longer series of tidal observations than is usually available to the cartographer. In addition, the heights of the tide vary as a function of time.
Since the chart datum is generally a computed mean or average height at some state of the tide, the depth of water at any particular moment may be less than shown on the chart. For example, if the chart datum is mean lower low water, the depth of water at lower low water will be less than the charted depth about as often as it is greater. A lower depth is indicated in the tide tables by a minus sign (–).

ed in Section K of Chart No. 1. A rock uncovered at mean high water may be shown as
an islet. If an isolated, offlying rock is known to uncover at the sounding datum but to be covered at high water, the chart shows the appropriate symbol for a rock and gives the height above the sounding datum. The chart can give this height one of two ways. It can use a statement such as “Uncov 2 ft.,” or it can indicate the number of feet the rock protrudes above the sounding datum, underline this value, and enclose it in parentheses (i.e. (2)). A rock which does not uncover is shown by an enclosed figure approximating its dimensions and filled with land tint. It may be enclosed by a dotted depth curve for emphasis.
A tinted, irregular-line figure of approximately true dimensions is used to show a detached coral reef which uncovers at the chart datum. For a coral or rocky reef which is submerged at chart datum, the sunken rock symbol or an appropriate statement is used, enclosed by a dotted or broken line if the limits have been determined.
Several different symbols mark wrecks. The nature of the wreck or scale of the chart determines the correct symbol. A sunken wreck with less than 11 fathoms of water over it is considered dangerous and its symbol is surrounded by a dotted curve. The curve is omitted if the wreck is deeper than 11 fathoms. The safe clearance over a wreck, if known, is indicated by a standard sounding number placed at the wreck. If this depth was determined by a wire drag, the sounding is underscored by the wire drag symbol. An unsurveyed wreck over which the exact depth is unknown but a safe clearance depth is known is depicted with a solid line above the symbol.
Tide rips, eddies, and kelp are shown by symbol or legend.
Piles, dolphins (clusters of piles), snags, and stumps are shown by small circles and a label identifying the type of obstruction. If such dangers are submerged, the letters “Subm” precede the label.
Fish stakes and traps are shown when known to be permanent or hazardous to navigation.

335. Heights

337. Aids To Navigation

The shoreline shown on charts is generally mean high water. A light’s height is usually reckoned from mean sea level. The heights of overhanging obstructions (bridges, power cables, etc.) are usually reckoned from mean high water. A high water reference gives the mariner the minimum clearance expected.
Since heights are usually reckoned from high water and depths from some form of low water, the reference levels are seldom the same. Except where the range of tide is very large, this is of little practical significance.
336. Dangers
Dangers are shown by appropriate symbols, as indicat-

Aids to navigation are shown by symbols listed in Sections P through S of Chart No. 1. Abbreviations and additional descriptive text supplement these symbols. In order to make the symbols conspicuous, the chart shows them in size greatly exaggerated relative to the scale of the chart. “Position approximate” circles are used on floating aids to indicate that they have no exact position because they move around their moorings. For most floating aids, the position circle in the symbol marks the approximate location of the anchor or sinker. The actual aid may be displaced from this location by the scope of its mooring.
The type and number of aids to navigation shown on a chart and the amount of information given in their legends varies with the scale of the chart. Smaller scale charts may have fewer aids indicated and less information than larger

42

NAUTICAL CHARTS

scale charts of the same area. Lighthouses and other navigation lights are shown as
black dots with purple disks or as black dots with purple flare symbols. The center of the dot is the position of the light. Some modified facsimile foreign charts use a small star instead of a dot.
On large-scale charts the legend elements of lights are shown in the following order:

Legend

Example

Meaning

Characteristic F1(2)

group flashing; 2 flashes

Color

R

red

Period

10s

2 flashes in 10 seconds

Height

80m

80 meters

Range

19M

19 nautical miles

Designation

“6”

light number 6

The legend for this light would appear on the chart:

Fl(2) R 10s 80m 19M “6”

As chart scale decreases, information in the legend is selectively deleted to avoid clutter. The order of deletion is usually height first, followed by period, group repetition interval (e.g. (2)), designation, and range. Characteristic and color will almost always be shown.
Small triangles mark red daybeacons; small squares mark all others. On DMAHTC charts, pictorial beacons are used when the IALA buoyage system has been implemented. The center of the triangle marks the position of the aid. Except on Intracoastal Waterway charts and charts of state waterways, the abbreviation “Bn” is shown beside the symbol, along with the appropriate abbreviation for color if known. For black beacons the triangle is solid black and there is no color abbreviation. All beacon abbreviations are in vertical lettering.
Radiobeacons are indicated on the chart by a purple circle accompanied by the appropriate abbreviation indicating an ordinary radiobeacon (R Bn) or a radar beacon (Ramark or Racon, for example).
A variety of symbols, determined by both the charting agency and the types of buoys, indicate navigation buoys. IALA buoys (see Chapter 5, Short Range Aids to Navigation) in foreign areas are depicted by various styles of symbols with proper topmarks and colors; the position circle which shows the approximate location of the sinker is at the base of the symbol.
A mooring buoy is shown by one of several symbols as indicated in Chart No. 1. It may be labeled with a berth number or other information.

A buoy symbol with a horizontal line indicates the buoy has horizontal bands. A vertical line indicates vertical stripes; crossed lines indicate a checked pattern. There is no significance to the angle at which the buoy symbol appears on the chart. The symbol is placed so as to avoid interference with other features.
Lighted buoys are indicated by a purple flare from the buoy symbol or by a small purple disk centered on the position circle.
Abbreviations for light legends, type and color of buoy, designation, and any other pertinent information given near the symbol are in slanted type. The letter C, N, or S indicates a can, nun, or spar, respectively. Other buoys are assumed to be pillar buoys, except for special buoys such as spherical, barrel, etc. The number or letter designation of the buoy is given in quotation marks on NOS charts. On other charts they may be given without quotation marks or other punctuation.
Aeronautical lights included in the light lists are shown by the lighthouse symbol, accompanied by the abbreviation “AERO.” The characteristics shown depend principally upon the effective range of other navigational lights in the vicinity and the usefulness of the light for marine navigation.
Directional ranges are indicated by a broken or solid line. The solid line, indicating that part of the range intended for navigation, may be broken at irregular intervals to avoid being drawn through soundings. That part of the range line drawn only to guide the eye to the objects to be kept in range is broken at regular intervals. The direction, if given, is expressed in degrees, clockwise from true north.
Sound signals are indicated by the appropriate word in capital letters (HORN, BELL, GONG, or WHIS) or an abbreviation indicating the type of sound. Sound signals of any type except submarine sound signals may be represented by three purple 45° arcs of concentric circles near the top of the aid. These are not shown if the type of signal is listed. The location of a sound signal which does not accompany a visual aid, either lighted or unlighted, is shown by a small circle and the appropriate word in vertical block letters.
Private aids, when shown, are marked “Priv” on NOS charts. Some privately maintained unlighted fixed aids are indicated by a small circle accompanied by the word “Marker,” or a larger circle with a dot in the center and the word “MARKER.” A privately maintained lighted aid has a light symbol and is accompanied by the characteristics and the usual indication of its private nature. Private aids should be used with caution.
A light sector is the sector or area bounded by two radii and the arc of a circle in which a light is visible or in which it has a distinctive color different from that of adjoining sectors. The limiting radii are indicated on the chart by dotted or dashed lines. Sector colors are indicated by words spelled out if space permits, or by abbreviations (W, R, etc.) if it does not. Limits of light sectors and arcs of visibility as observed from a vessel are given in the light lists, in clockwise order.

NAUTICAL CHARTS

43

338. Land Areas
The amount of detail shown on the land areas of nautical charts depends upon the scale and the intended purpose of the chart. Contours, form lines, and shading indicate relief.
Contours are lines connecting points of equal elevation. Heights are usually expressed in feet (or in meters with means for conversion to feet). The interval between contours is uniform over any one chart, except that certain intermediate contours are sometimes shown by broken line. When contours are broken, their locations are approximate.
Form lines are approximations of contours used for the purpose of indicating relative elevations. They are used in areas where accurate information is not available in sufficient detail to permit exact location of contours. Elevations of individual form lines are not indicated on the chart.
Spot elevations are generally given only for summits or for tops of conspicuous landmarks. The heights of spot elevations and contours are given with reference to mean high water when this information is available.
When there is insufficient space to show the heights of islets or rocks, they are indicated by slanting figures enclosed in parentheses in the water area nearby.
339. Cities And Roads
Cities are shown in a generalized pattern that approximates their extent and shape. Street names are generally not charted except those along the waterfront on the largest scale charts. In general, only the main arteries and thoroughfares or major coastal highways are shown on smaller scale charts. Occasionally, highway numbers are given. When shown, trails are indicated by a light broken line. Buildings along the waterfront or individual ones back from the waterfront but of special interest to the mariner are shown on large-scale charts. Special symbols from Chart No. 1 are used for certain kinds of buildings. A single line with cross marks indicates both single and double track railroads. City electric railways are usually not charted. Airports are shown on small-scale charts by symbol and on large-scale charts by the shape of runways. The scale of the chart determines if single or double lines show breakwaters and jetties; broken lines show the submerged portion of these features.
340. Landmarks
Landmarks are shown by symbols in Chart No. 1. A large circle with a dot at its center is used to indicate that the position is precise and may be used without reservation for plotting bearings. A small circle without a dot is used for landmarks not accurately located. Capital and lower case letters are used to identify an approximate landmark: “Mon,” “Cup,” or “Dome.” The abbreviation “PA” (position approximate) may also appear. An accurate landmark is identified by all capital type ( “MON,” “CUP,” “DOME”).

When only one object of a group is charted, its name is followed by a descriptive legend in parenthesis, including the number of objects in the group, for example “(TALLEST OF FOUR)”or “(NORTHEAST OF THREE).”
341. Miscellaneous Chart Features
A measured nautical mile indicated on a chart is accurate to within 6 feet of the correct length. Most measured miles in the United States were made before 1959, when the United States adopted the International Nautical Mile. The new value is within 6 feet of the previous standard length of 6,080.20 feet. If the measured distance differs from the standard value by more than 6 feet, the actual measured distance is stated and the words “measured mile” are omitted.
Periods after abbreviations in water areas are omitted because these might be mistaken for rocks. However, a lower case i or j is dotted.
Commercial radio broadcasting stations are shown on charts when they are of value to the mariner either as landmarks or sources of direction-finding bearings.
Lines of demarcation between the areas in which international and inland navigation rules apply are shown only when they cannot be adequately described in notes on the chart.
Compass roses are placed at convenient locations on Mercator charts to facilitate the plotting of bearings and courses. The outer circle is graduated in degrees with zero at true north. The inner circle indicates magnetic north.
On many DMAHTC charts magnetic variation is given to the nearest 1' by notes in the centers of compass roses; the annual change is given to the nearest 1' to permit correction of the given value at a later date. On NOS charts, variation is to the nearest 15', updated at each new edition if over three years old. The current practice of DMAHTC is to give the magnetic variation to the nearest 1', but the magnetic information on new editions is only updated to conform with the latest five year epoch. Whenever a chart is reprinted, the magnetic information is updated to the latest epoch. On other charts, the variation is given by a series of isogonic lines connecting points of equal variation; usually a separate line represents each degree of variation. The line of zero variation is called the agonic line. Many plans and insets show neither compass roses nor isogonic lines, but indicate magnetic information by note. A local magnetic disturbance of sufficient force to cause noticeable deflection of the magnetic compass, called local attraction, is indicated by a note on the chart.
Currents are sometimes shown on charts with arrows giving the directions and figures showing speeds. The information refers to the usual or average conditions. According to tides and weather, conditions at any given time may differ considerably from those shown.
Review chart notes carefully because they provide important information. Several types of notes are used. Those in the margin give such information as chart number, pub-

44

NAUTICAL CHARTS

lication notes, and identification of adjoining charts. Notes in connection with the chart title include information on scale, sources of data, tidal information, soundings, and cautions. Another class of notes covers such topics as local magnetic disturbance, controlling depths of channels, hazards to navigation, and anchorages.
A datum note will show the datum of the chart (See Chapter 2, Geodesy and Datums in Navigation). It may also contain instructions on plotting positions from the WGS 84 or NAD 83 datums on the chart if such a conversion is needed.
Anchorage areas are labeled with a variety of magenta, black, or green lines depending on the status of the area. Anchorage berths are shown as purple circles, with the number or letter assigned to the berth inscribed within the circle. Caution notes are sometimes shown when there are specific anchoring regulations.
Spoil areas are shown within short broken black lines. Spoil areas are tinted blue on NOS charts and labeled. These areas contain no soundings and should be avoided.
Firing and bombing practice areas in the United States territorial and adjacent waters are shown on NOS and DMAHTC charts of the same area and comparable scale.

Danger areas established for short periods of time are not charted but are announced locally. Most military commands charged with supervision of gunnery and missile firing areas promulgate a weekly schedule listing activated danger areas. This schedule is subjected to frequent change; the mariner should always ensure he has the latest schedule prior to proceeding into a gunnery or missile firing area. Danger areas in effect for longer periods are published in the Notice to Mariners. Any aid to navigation established to mark a danger area or a fixed or floating target is shown on charts.
Traffic separation schemes are shown on standard nautical charts of scale 1:600,000 and larger and are printed in magenta.
A logarithmic time-speed-distance nomogram with an explanation of its application is shown on harbor charts.
Tidal information boxes are shown on charts of scales 1:200,000 and larger for NOS charts, and various scales on DMA charts, according to the source. See Figure 341a.
Tabulations of controlling depths are shown on some National Ocean Service harbor and coastal charts. See Figure 341b.
Study Chart No. 1 thoroughly to become familiar with all the symbols used to depict the wide variety of features on nautical charts.

Place Olongapo . . . . . .

TIDAL INFORMATION

Position

Height above datum of soundings

Mean High Water

Mean Low Water

N. Lat.

E. Long.

Higher

Lower

Lower

Higher

meters

meters

meters

meters

14°49'

120°17' . . . 0.9 . . . . . . 0.4 . . . . . . 0.0 . . . . . . 0.3 . . .

Figure 341a. Tidal box.

NANTUCKET HARBOR
Tabulated from surveys by the Corps of Engineers - report of June 1972 and surveys of Nov. 1971

Controlling depths in channels entering from seaward in feet at Mean Low Water

Project Dimensions

Name of Channel Entrance Channel

Left outside quarter
11.1

Middle half of channel
15.0

Right outside quarter
15.0

Date of
Survey
11 - 71

Width (feet)
300

Length (naut. miles)
1.2

Depth M. L. W.
(feet).
15

Note.-The Corps of Engineers should be consulted for changing conditions subsequent to the above. Figure 341b. Tabulations of controlling depths.

NAUTICAL CHARTS

45

REPRODUCTIONS OF FOREIGN CHARTS

342. Modified Facsimiles
Modified facsimile charts are modified reproductions of foreign charts produced in accordance with bilateral international agreements. These reproductions provide the mariner with up-to-date charts of foreign waters. Modified facsimile charts published by DMAHTC are, in general, reproduced with minimal changes, as listed below:
1. The original name of the chart may be removed and replaced by an anglicized version.
2. English language equivalents of names and terms on the original chart are printed in a suitable glossary on the reproduction, as appropriate.
3. All hydrographic information, except bottom characteristics, is shown as depicted on the original chart.
4. Bottom characteristics are as depicted in Chart No. 1, or as on the original with a glossary.
5. The unit of measurement used for soundings is shown in block letters outside the upper and lower

neatlines. 6. A scale for converting charted depth to feet, meters,
or fathoms is added.
7. Blue tint is shown from a significant depth curve to the shoreline.
8. Blue tint is added to all dangers enclosed by a dotted danger curve, dangerous wrecks, foul areas, obstructions, rocks awash, sunken rocks, and swept wrecks.
9. Caution notes are shown in purple and enclosed in a box.
10. Restricted, danger, and prohibited areas are usually outlined in purple and labeled appropriately.
11. Traffic separation schemes are shown in purple.
12. A note on traffic separation schemes, printed in black, is added to the chart.
13. Wire dragged (swept) areas are shown in purple or green.
14. Corrections are provided to shift the horizontal datum to the World Geodetic System (1984).

INTERNATIONAL CHARTS

343. International Chart Standards
The need for mariners and chart makers to understand and use nautical charts of different nations became increasingly apparent as the maritime nations of the world developed their own establishments for the compilation and publication of nautical charts from hydrographic surveys. Representatives of twenty-two nations formed a Hydrographic Conference in London in 1919. That conference resulted in the establishment of the International Hydrographic Bureau (IHB) in Monaco in 1921. Today, the IHB’s successor, the International Hydrographic Organization (IHO) continues to provide international standards for the cartographers of its member nations. (See Chapter 1, Introduction to Marine Navigation, for a description of the IHO.)

Recognizing the considerable duplication of effort by member states, the IHO in 1967 moved to introduce the first international chart. It formed a committee of six member states to formulate specifications for two series of international charts. Eighty-three small-scale charts were approved; responsibility for compiling these charts has subsequently been accepted by the member states’ Hydrographic Offices.
Once a Member State publishes an international chart, reproduction material is made available to any other Member State which may wish to print the chart for its own purposes.
International charts can be identified by the letters INT before the chart number and the International Hydrographic Organization seal in addition to other national seals which may appear.

CHART NUMBERING SYSTEM

344. Description Of The Numbering System

the chart system which are not actually charts.

DMAHTC and NOS use a system in which numbers are assigned in accordance with both the scale and geographical area of coverage of a chart. With the exception of certain charts produced for military use only, one- to five-digit numbers are used. With the exception of one-digit numbers, the first digit identifies the area; the number of digits establishes the scale range. The one-digit numbers are used for certain products in

Number of Digits
1 2 3 4 5

Scale
No Scale 1:9 million and smaller 1:2 million to 1:9 million Special Purpose 1:2 million and larger

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NAUTICAL CHARTS

Figure 344a. Ocean basins with region numbers.

Two- and three-digit numbers are assigned to those small-scale charts which depict a major portion of an ocean basin or a large area. The first digit identifies the applicable ocean basin. See Figure 344a. Two-digit numbers are used for charts of scale 1:9,000,000 and smaller. Three-digit numbers are used for charts of scale 1:2,000,000 to 1:9,000,000.
Due to the limited sizes of certain ocean basins, no charts for navigational use at scales of 1:9,000,000 and smaller are published to cover these basins. The otherwise unused twodigit numbers (30 to 49 and 70 to 79) are assigned to special world charts such as chart 33, Horizontal Intensity of the Earth’s Magnetic Field, chart 42, Magnetic Variation, and chart 76, Standard Time Zone Chart of the World.
One exception to the scale range criteria for three-digit numbers is the use of three-digit numbers for a series of position plotting sheets. They are of larger scale than 1:2,000,000 because they have application in ocean basins and can be used in all longitudes.
Four-digit numbers are used for non-navigational and special purpose charts, such as chart 5090, Maneuvering Board; chart 5101, Gnomonic Plotting Chart North Atlantic; and chart 7707, Omega Plotting Chart.
Five-digit numbers are assigned to those charts of scale 1:2,000,000 and larger that cover portions of the coastline rather than significant portions of ocean basins. These charts are based on the regions of the nautical chart index. See Figure 344b.
The first of the five digits indicates the region; the second digit indicates the subregion; the last three digits

indicate the geographical sequence of the chart within the subregion. Many numbers have been left unused so that any future charts may be placed in their proper geographical sequence.
In order to establish a logical numbering system within the geographical subregions (for the 1:2,000,000 and larger-scale charts), a worldwide skeleton framework of coastal charts was laid out at a scale 1:250,000. This series was used as basic coverage except in areas where a coordinated series at about this scale already existed (such as the coast of Norway where a coordinated series of 1:200,000 charts was available). Within each region, the geographical subregions are numbered counterclockwise around the continents, and within each subregion the basic series also is numbered counterclockwise around the continents. The basic coverage is assigned generally every 20th digit, except that the first 40 numbers in each subregion are reserved for smaller-scale coverage. Charts with scales larger than the basic coverage are assigned one of the 19 numbers following the number assigned to the sheet within which it falls. Figure 344c shows the numbering sequence in Iceland. Note the sequence of numbers around the coast, the direction of numbering, and the numbering of larger scale charts within the limits of smaller scales.
Five-digit numbers are also assigned to the charts produced by other hydrographic offices. This numbering system is applied to foreign charts so that they can be filed in logical sequence with the charts produced by the Defense Mapping Agency Hydrographic/Topographic Center and the National Ocean Service.

NAUTICAL CHARTS

Figure 344b. Regions and subregions of the nautical chart index.

47

48

NAUTICAL CHARTS

Figure 344c. Chart coverage of Iceland, illustrating the sequence and direction of the U.S. chart numbering system.

NAUTICAL CHARTS

49

345. Exceptions To The System
Exceptions to the numbering system for military needs are as follows:
1. Bottom contour charts are not intended for surface navigation, and do not portray portions of a coastline. They chart parts of the ocean basins. They are identified with a letter plus four digits and are not available to civilian navigators.
2. Combat charts have 6-digit numbers beginning with an “8.” They are not available to civilian navigators.
346. Chart Catalogs
Chart catalogs provide information regarding not only chart coverage, but also a variety of special purpose charts and publications of interest. Keep a corrected chart catalog aboard ship for review by the navigator. The DMAHTC catalog is available to military navigators. It contains operating

area charts and other special products not available for civilian use, but it does not contain any classified listings. The NOS catalogs contain all unclassified civilian-use NOS and DMAHTC charts. Military navigators receive their nautical charts and publications directly from DMAHTC; civilian navigators purchase them from NOS sales agents.
347. Stock Numbers
The stock number and bar code are generally found in the lower left corner of a DMA chart, and in the lower right corner of an NOS chart. The first two digits of the stock number refer to the region and subregion. These are followed by three letters, the first of which refers to the portfolio to which the chart belongs; the second two denote the type of chart: CO for coastal, HA for harbor and approach, and OA for military operating area charts. The last five digits are the actual chart number.

USING CHARTS

348. Preliminary Steps
Upon receiving a new paper chart, verify its announcement in the Notice to Mariners and correct it with all applicable corrections. Read all the chart’s notes; there should be no question about the meanings of symbols or the units in which depths are given. Since the latitude and longitude scales differ considerably on various charts, carefully note those on the chart to be used.
Prepare piloting charts as discussed in Chapter 8 and open ocean transit charts as discussed in Chapter 25.
Place additional information on the chart as required. Arcs of circles might be drawn around navigational lights to indicate the limit of visibility at the height of eye of an observer on the bridge. Notes regarding other information from the light lists, tide tables, tidal current tables, and sailing directions might prove helpful.
The preparation of electronic charts for use is determined by the operator’s manual for the system. If the electronic chart system in use is not IMO-approved, the navigator is required to maintain a concurrent plot on paper charts.
349. Maintaining Paper Charts
A mariner navigating on an uncorrected chart is courting disaster. The chart’s print date reflects the latest Notice to Mariners used to update the chart; responsibility for maintaining it after this date lies with the user. The weekly Notice to Mariners contains information needed for maintaining charts. Radio broadcasts give advance notice of urgent corrections. Local Notice to Mariners should be consulted for inshore areas. The navigator must develop a system to keep track of chart corrections and to ensure that the chart he is us-

ing is updated with the latest correction. A convenient way of keeping this record is with a Chart/Publication Correction Record Card system. Using this system, the navigator does not immediately update every chart in his portfolio when he receives the Notice to Mariners. Instead, he constructs a card for every chart in his portfolio and notes the correction on this card. When the time comes to use the chart, he pulls the chart and chart’s card, and he makes the indicated corrections on the chart. This system ensures that every chart is properly corrected prior to use.
A Summary of Corrections, containing a cumulative listing of previously published Notice to Mariners corrections, is published annually in 5 volumes by DMAHTC. Thus, to fully correct a chart whose edition date is several years old, the navigator needs only the Summary of Corrections for that region and the notices from that Summary forward; he does not need to obtain notices all the way back to the edition date. See Chapter 4, Nautical Publications, for a description of the Summaries and Notice to Mariners.
When a new edition of a chart is published, it is normally furnished automatically to U.S. Government vessels. It should not be used until it is announced as ready for use in the Notice to Mariners. Until that time, corrections in the Notice apply to the old edition and should not be applied to the new one. When it is announced, a new edition of a chart replaces an older one.
Commercial users and others who don’t automatically receive new editions should obtain new editions from their sales agent. Occasionally, charts may be received or purchased several weeks in advance of their announcement in the Notice to Mariners. This is usually due to extensive rescheming of a chart region and the need to announce groups of charts together to avoid lapses in coverage. The mariner bears the responsibility for ensuring that his charts are the

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current edition. The very fact that a new edition has been prepared indicates that there have been changes that cannot adequately be shown by hand corrections.
350. Use And Stowage Of Charts
Use and stow charts carefully. This is especially true with digital charts contained on electronic media. Keep optical and magnetic media containing chart data out of the sun, inside dust covers, and away from magnetic influences. Placing a disk in an inhospitable environment will destroy important data.
Make permanent corrections to paper charts in ink so that they will not be inadvertently erased. Pencil in all other markings so that they can be easily erased without damaging the chart. Lay out and label tracks on charts of frequently-traveled ports in ink. Draw lines and labels no larger than necessary. Do not obscure sounding data or other information when labeling a chart. When a voyage is completed, carefully erase the charts unless there has been a grounding or collision. In this case, preserve the charts without change because they will play a critical role in the investigation.
When not in use, stow charts flat in their proper portfolio. Minimize their folding and properly index them for easy retrieval.

351. Chart Lighting
Mariners often work in a red light environment because red light is least disturbing to night adapted vision. Such lighting seriously affects the appearance of a chart. Before using a chart in red light, test the effect red light has on its markings. Do not outline or otherwise indicate navigational hazards in red pencil because red markings disappear under red light.
The above point cannot be overemphasized; do not highlight danger areas on charts with red markers. Several ships have grounded on charted hazards simply because their conning officers were operating in a red light environment that obscured dangers highlighted on their charts in red pen. Always highlight danger areas on charts with a color that will not disappear in red light.
352. Small-Craft Charts
Although the small-craft charts published by the National Ocean Service are designed primarily for boatmen, these charts at scales of 1:80,000 and larger are in some cases the only charts available of inland waters transited by large vessels. In other cases the small-craft charts may provide a better presentation of navigational hazards than the standard nautical chart because of scale and detail. Therefore, navigators should use these charts in areas where they provide the best coverage.

CHAPTER 4

NAUTICAL PUBLICATIONS

INTRODUCTION

400. Definitions

navigate his ship safely.

The navigator uses many information sources when planning and conducting a voyage. These sources include notices to mariners, sailing directions, light lists, tide tables, sight reduction tables, and almanacs. Historically, this information has been found in printed publications; increasingly, it is being integrated into computer-based electronic systems. The navigator must know what information he needs to navigate his ship safely and how to obtain it.
This chapter will refer only to printed publications. If the navigator has access to this data on an electronic database, only his method of access will differ. The publications discussed here form a basic navigation library; the navigator must also obtain all supplementary materials required to

401. Types And Sources Of Publications
While voyage planning and navigating, a mariner must refer to both texts and tables. Examples of text include sailing directions, coast pilots, and notices to mariners. Examples of tables include light lists and sight reduction tables.
Navigational publications are available from many sources. Military customers automatically receive or requisition most required publications. The civilian navigator obtains his publications from a publisher’s agent. Larger agents representing many publishers can completely supply a ship’s chart and publication library.

NAUTICAL TEXTS

402. Sailing Directions
Defense Mapping Agency Hydrographic/Topographic Center Sailing Directions consist of 37 Enroutes and 10 Planning Guides. Planning Guides describe general features of ocean basins; Enroutes describe features of coastlines, ports, and harbors.
Sailing Directions are updated when new data requires extensive revision of an existing text. These data are obtained from several sources, including pilots and foreign Sailing Directions.
One book comprises the Planning Guide and Enroute for Antarctica. This consolidation allows for a more effective presentation of material on this unique area.
The Planning Guides are relatively permanent; by contrast, Sailing Directions (Enroute) are frequently updated. Between updates, both are corrected by the Notice to Mariners.
403. Sailing Directions (Planning Guide)
Planning Guides assist the navigator in planning an extensive oceanic voyage. Each of the Guides covers an area determined by an arbitrary division of the world’s seas into eight “ocean basins.” This division is shown in Figure 403.

A Planning Guide’s first chapter contains information about the countries adjacent to the applicable ocean basin. It also covers pratique, pilotage, signals, and shipping regulations. Search and Rescue topics include the location of all lifesaving stations.
The second chapter contains information on the physical environment of an ocean basin. It consists of Ocean Summaries and descriptions of local coastal phenomena. This gives the mariner meteorological and oceanographic information to be considered in planning a route.
The third chapter lists foreign firing danger areas not shown in other DMAHTC publications. A graphic key identifies Submarine Operating Areas. This chapter also identifies publications listing danger areas and gives pertinent navigation cautions.
The fourth chapter describes recommended steamship routes. To facilitate planning, the publication shows entire routes to foreign ports originating from all major U.S. ports. This chapter also includes all applicable Traffic Separation Schemes.
The fifth and final chapter describes available radionavigation systems and the area’s system of lights, beacons, and buoys.
Appendices contain information on buoyage systems, route charts, and area meteorological conditions.
51

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Figure 403. The 8 ocean basins as organized for Sailing Directions (Planning Guides).

404. Sailing Directions (Enroute)

405. Coast Pilots

Each volume of the Sailing Directions (Enroute) contains numbered sections along a coast or through a strait. Figure 404a illustrates this division. Each sector is discussed in turn. A preface with detailed information about authorities, references, and conventions used in each book precedes the sector discussions. Finally, each book provides conversions between feet, fathoms, and meters.
The Chart Information Graphic, the first item in each chapter, is a graphic key for charts pertaining to a sector. See Figure 404b. The graduation of the border scale of the chartlet enables navigators to identify the largest scale chart for a location and to find a feature listed in the Index-Gazetteer. These graphics are not maintained by Notice to Mariners; one should refer to the chart catalog for updated chart listings.
Other graphics may contain special information on local winds and weather, anchorages, significant coastal features, and navigation dangers.
A foreign terms glossary, an appendix of anchorages, and a comprehensive Index-Gazetteer follow the sector discussions. The Index-Gazetteer is an alphabetical listing of described and charted features. The Index lists each feature by geographic coordinates and sector number for use with the graphic key. Features mentioned in the text are listed by page number.

The National Ocean Service publishes nine United States Coast Pilots to supplement nautical charts of U.S. waters. Information comes from field inspections, survey vessels, and various harbor authorities. Maritime officials and pilotage associations provide additional information. Coast Pilots provide more detailed information than Sailing Directions because Sailing Directions are intended exclusively for the oceangoing mariner. The Notice to Mariners updates Coast Pilots.
Each volume contains comprehensive sections on local operational considerations and navigation regulations. Following chapters contain detailed discussions of coastal navigation. An appendix provides information on obtaining additional weather information, communications services, and other data. An index and additional tables complete the volume.
406. Other Nautical Texts
The government publishes several other nautical texts. The Defense Mapping Agency, for example, publishes the Maneuvering Board Manual (Pub. 217), The Radar Navigation Manual (Pub.1310) and the American Practical Navigator (Pub. 9).
The U.S. Coast Guard publishes navigation rules for international and inland waters. This publication, officially known as Commandant Instruction M16672.2b, contains

NAUTICAL PUBLICATIONS

53

Figure 404a. Sector Limits graphic.
Additional chart coverage may be found in CATP2 Catalog of Nautical Charts.
Figure 404b. Chart Information graphic.

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the Inland Navigation Rules enacted in December 1980 and effective on all inland waters of the United States including the Great Lakes, as well as the International Regulations for the Prevention of Collisions at Sea, enacted in 1972 (1972 COLREGS). Mariners should ensure that they have the updated issue. The Coast Guard also publishes comprehensive user’s manuals for the Omega, Loran, and GPS navigation systems; Navigation and Vessel Inspection Circulars; and the Chemical Data Guide for Bulk Shipment by Water.
The Government Printing Office provides several publications on navigation, safety at sea, communications, weather, and related topics. Additionally, it publishes pro-

visions of the Code of Federal Regulations (CFR) relating to maritime matters. A number of private publishers also provide maritime publications.
The International Maritime Organization, International Hydrographic Organization, and other governing international organizations provide information on international navigation regulations. Chapter 1 gives these organizations’ addresses. Regulations for various Vessel Traffic Services (VTS), canals, lock systems, and other regulated waterways are published by the authorities which operate them.

USING THE LIGHT LISTS

407. Light Lists
The United States publishes two different light lists. The U.S. Coast Guard publishes the Light List for lights in U.S. territorial waters; DMAHTC publishes the List of Lights for lights in foreign waters.
Light lists furnish complete information about navigation lights and other navigation aids. They supplement, but do not replace, charts and sailing directions. Consult the chart for the location and light characteristics of all navigation aids; consult the light lists to determine their detailed description.
The Notice to Mariners corrects both lists. Corrections which have accumulated since the print date are included in the Notice to Mariners as a Summary of Corrections. All of these summary corrections, and any corrections published subsequently, should be noted in the “Record of Corrections.”
A navigator needs to know both the identity of a light and when he can expect to see it; he often plans the ship’s track to pass within a light’s range. If lights are not sighted when predicted, the vessel may be significantly off course and standing into danger.
A circle with a radius equal to the visible range of the light usually defines the area in which a light can be seen. On some bearings, however, obstructions may reduce the range. In this case, the obstructed arc might differ with height of eye and distance. Also, lights of different colors may be seen at different distances. Consider these facts both when identifying a light and predicting the range at which it can be seen.
Atmospheric conditions have a major effect on a light’s range. Fog, haze, dust, smoke, or precipitation can obscure a light. Additionally, a light can be extinguished. Always report an extinguished light so maritime authorities can issue a warning.
On a dark, clear night, the visual range is limited by either: (1) luminous intensity, or (2) curvature of the earth. Regardless of the height of eye, one cannot see a weak light beyond a certain luminous range. Assuming light travels lin-

early, an observer located below the light’s visible horizon cannot see it. The Distance to the Horizon table gives the distance to the horizon for various heights of eye. The light lists contain a condensed version of this table. Abnormal refraction patterns might change this range; therefore, one cannot exactly predict the range at which a light will be seen.
408. Determining Range And Bearing Of A Light At Initial Sighting
A light’s luminous range is the maximum range at which an observer can see a light under existing visibility conditions. This luminous range ignores the elevation of the light, the observer’s height of eye, the curvature of the earth, and interference from background lighting. It is determined from the known nominal range and the existing visibility conditions. The nominal range is the maximum distance at which a light can be seen in weather conditions where visibility is 10 nautical miles.
The U.S. Coast Guard Light List usually lists a light’s nominal range. Use the Luminous Range Diagram shown in the Light List and Figure 408a to convert this nominal range to luminous range. Remember that the luminous ranges obtained are approximate because of atmospheric or background lighting conditions. Estimate the meteorological visibility by the Meteorological Optical Range Table, Figure 408b. Next, enter the Luminous Range Diagram with the nominal range on the horizontal nominal range scale. Follow a vertical line until it intersects the curve or reaches the region on the diagram representing the meteorological visibility. Finally, follow a horizontal line from this point or region until it intersects the vertical luminous range scale.
Example 1: The nominal range of a light as extracted from the Light List is 15 nautical miles.
Required: The luminous range when the meteorological visibility is (1) 11 nautical miles and (2) 1 nautical mile.
Solution: To find the luminous range when the meteo-

NAUTICAL PUBLICATIONS

55

Figure 408a. Luminous Range Diagram.

rological visibility is 11 nautical miles, enter the Luminous Range Diagram with nominal range 15 nautical miles on the horizontal nominal range scale; follow a vertical line upward until it intersects the curve on the diagram representing a meteorological visibility of 11 nautical miles; from this point follow a horizontal line to the right until it intersects the vertical luminous range scale at 16 nautical miles. A similar procedure is followed to find the luminous range when the meteorological visibility is 1 nautical mile. Answers: (1) 16 nautical miles; (2) 3 nautical miles.
A light’s geographic range depends upon the height of both the light and the observer. Sum the observer’s distance to the horizon based on his height of eye and the light’s distance

to the horizon based on its height to calculate a light’s geographic range. See Figure 408c. This illustration uses a light 150 feet above the water. Table 12, Distance of the Horizon, yields a value of 14.3 nautical miles for a height of 150 feet. Within this range, the light, if powerful enough and atmospheric conditions permit, is visible regardless of the height of eye of the observer. Beyond 14.3 nautical miles, the geographic range depends upon the observer’s height of eye. Thus, by the Distance of the Horizon table mentioned above, an observer with height of eye of 5 feet can see the light on his horizon if he is 2.6 miles beyond the horizon of the light. The geographic range of the light is therefore 16.9 miles. For a height of 30 feet the distance is 14.3 + 6.4 = 20.7 miles. If the height of eye is 70 feet, the geographic range is 14.3 + 9.8 = 24.1 miles. A height of eye of 15 feet is often assumed when tabulating lights’ geographic ranges.

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Code No.

Yards

Weather

0 Dense fog . . . . . . . . . . . . . . . . . Less than 50

1 Thick fog . . . . . . . . . . . . . . . . . . . . . .50-200

2 Moderate fog . . . . . . . . . . . . . . . . . .200-500

3 Light fog . . . . . . . . . . . . . . . . . . . .500-1000

Nautical Miles

4 Thin fog . . . . . . . . . . . . . . . . . . . . . . . . 1/2-1

5 Haze . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-2

6 Light Haze . . . . . . . . . . . . . . . . . . . . 2-5 1/2

7 Clear . . . . . . . . . . . . . . . . . . . . . . . . 5 1/2-11

8 Very Clear . . . . . . . . . . . . . . . . . . 11.0-27.0.

9 Exceptionally Clear . . . . . . . . . . . Over 27.0

From the International Visibility Code.

Figure 408b. Meteorlogical Optical Range Table

To predict the bearing and range at which a vessel will initially sight a light first determine the light’s geographic range. Compare the geographic range with the light’s luminous range. The lesser of the two ranges is the range at which the light will first be sighted. Plot a visibility arc centered on the

light and with a radius equal to the lesser of the geographic or luminous ranges. Extend the vessel’s track until it intersects the visibility arc. The bearing from the intersection point to the light is the light’s predicted bearing at first sighting.
If the extended track crosses the visibility arc at a small angle, a small lateral track error may result in large bearing and time prediction errors. This is particularly apparent if the vessel is farther from the light than predicted; the vessel may pass the light without sighting it. However, not sighting a light when predicted does not always indicate the vessel is farther from the light than expected. It could also mean that atmospheric conditions are affecting visibility.
Example 2: The nominal range of a navigational light 120 feet above the chart datum is 20 nautical miles. The meteorological visibility is 27 nautical miles.
Required: The distance at which an observer at a height of eye of 50 feet can expect to see the light.
Solution: The maximum range at which the light may be seen is the lesser of the luminous or geographic ranges. At 120 feet the distance to the horizon, by table or formula, is 12.8 miles. Add 8.3 miles, the distance to the horizon for a height of eye of 50 feet to determine the geographic range. The geographic range, 21.1 miles, is less than the luminous range, 40 miles.
Answer: 21 nautical miles. Because of various uncertainties, the range is rounded off to the nearest whole mile.

Figure 408c. Geographic Range of a light.

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57

When first sighting a light, an observer can determine if it is on the horizon by immediately reducing his height of eye. If the light disappears and then reappears when the observer returns to his original height, the light is on the horizon. This process is called bobbing a light.
If a vessel has considerable vertical motion due to rough seas, a light sighted on the horizon may alternately appear and disappear. Wave tops may also obstruct the light periodically. This may cause the characteristic to appear different than expected. The light’s true characteristics can be observed either by closing the range to the light or by the observer’s increasing his height of eye.
If a light’s range given in a foreign publication approximates the light’s geographic range for a 15-foot observer’s height of eye, assume that the printed range is the light’s geographic range. Also assume that publication has listed the lesser of the geographic and nominal ranges. Therefore, if the light’s listed range approximates the geographic range for an observer with a height of eye of 15 feet, then assume that the light’s limiting range is the geographic range. Then, calculate the light’s true geographic range using the actual observer’s height of eye, not the assumed height of eye of 15 feet. This calculated true geographic range is the range at which the light will first be sighted.
Example 3: The range of a light as printed on a foreign chart is 17 miles. The light is 120 feet above chart datum. The meteorological visibility is 10 nautical miles.
Required: The distance at which an observer at a height of eye of 50 feet can expect to see the light.
Solution: Calculate the geographic range of the light assuming a 15 foot observer’s height of eye. At 120 feet the distance to the horizon is 12.8 miles. Add 4.5 miles (the distance to the horizon at a height of 15 feet) to 12.8 miles; this range is 17.3 miles. This approximates the range listed on the chart. Then assuming that the charted range is the geographic range for a 15-foot observer height of eye and that the nominal range is the greater than this charted range, the predicted range is found by calculating the true geographic range with a 50 foot height of eye for the observer.
Answer: The predicted range = 12.8 mi. + 8.3 mi. = 21.1 mi.. The distance in excess of the charted range depends on the luminous intensity of the

light and the meteorological visibility.
409. USCG Light Lists
The U.S. Coast Guard Light List (7 volumes) gives information on lighted navigation aids, unlighted buoys, radiobeacons, radio direction finder calibration stations, daybeacons, racons, and Loran stations.
Each volume of the Light List contains aids to navigation in geographic order from north to south along the Atlantic coast, from east to west along the Gulf coast, and from south to north along the Pacific coast. It lists seacoast aids first, followed by entrance and harbor aids listed from seaward. Intracoastal Waterway aids are listed last in geographic order in the direction from New Jersey to Florida to the Texas/ Mexico border.
The listings are preceded by a description of the aids to navigation system in the United States, luminous range diagram, geographic range tables, and other information.
410. DMAHTC List of Lights, Radio Aids, and Fog Signals
The Defense Mapping Agency Hydrographic/Topographic Center publishes the List of Lights, Radio Aids, and Fog Signals (usually referred to as the List of Lights, not to be confused with the Coast Guard’s Light List). In addition to information on lighted aids to navigation and sound signals in foreign waters, the DMAHTC List of Lights provides information on storm signals, signal stations, racons, radiobeacons, and radio direction finder calibration stations located at or near lights. For more details on radio navigational aids, consult Pub. 117, Radio Navigational Aids.
The DMAHTC List of Lights does not include information on lighted buoys inside harbors. It does include certain aeronautical lights situated near the coast; however, these lights are not designed for marine navigation and are subject to unreported changes.
Foreign notices to mariners are the main correctional information source for the DMAHTC Lists of Lights; other sources, such as ship reports, are also used. Many aids to navigation in less developed countries may not be well maintained. They are subject to damage by storms and vandalism, and repairs may be delayed for long periods.

MISCELLANEOUS NAUTICAL PUBLICATIONS

411. DMAHTC Radio Navigational Aids (Pub. 117)
This publication is a selected list of worldwide radio stations which perform services to the mariner. Topics covered include radio direction finder and radar stations, radio time signals, radio navigation warnings, distress and safety

communications, medical advice via radio, long-range navigation aids, the AMVER system, and interim procedures for U.S. vessels in the event of an outbreak of hostilities. Pub. 117 is corrected via the Notice to Mariners and is updated periodically with a new edition.
Though Pub. 117 is essentially a list of radio stations

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providing vital maritime communication and navigation services, it also contains information which explains the capabilities and limitations of the various systems.
412. Chart No. 1
Chart No. 1 is not actually a chart but a book containing a key to chart symbols. Most countries which produce charts also produce such a list. The U.S. Chart No. 1 contains a listing of chart symbols in four categories:
• Chart symbols used by the National Ocean Service • Chart symbols used by the Defense Mapping
Agency • Chart symbols recommended by the International
Hydrographic Organization • Chart symbols used on foreign charts reproduced by
DMAHTC
Subjects covered include general features of charts, topography, hydrography, and aids to navigation. There is also a complete index of abbreviations and an explanation of the IALA buoyage system.
413. DMAHTC World Port Index (Pub. 150)
The World Port Index contains a tabular listing of thousands of ports throughout the world, describing their locations, characteristics, facilities, and services available. Information is arranged geographically; the index is arranged alphabetically.
Coded information is presented in columns and rows. This information supplements information in the Sailing Directions. The applicable volume of Sailing Directions and the number of the harbor chart are given in the World Port Index. The Notice to Mariners corrects this book.
414. DMAHTC Distances Between Ports (Pub. 151)
This publication lists the distances between major ports. Reciprocal distances between two ports may differ due to different routes chosen because of currents and climatic conditions. To reduce the number of listings needed, junction points along major routes are used to consolidate routes converging from different directions.
This book can be most effectively used for voyage planning in conjunction with the proper volume(s) of the Sailing Directions (Planning Guide). It is corrected via the Notice to Mariners.
415. DMAHTC International Code Of Signals (Pub. 102)
This book lists the signals to be employed by vessels at sea to communicate a variety of information relating to safety, distress, medical, and operational information. This

publication became effective in 1969. According to this code, each signal has a unique and
complete meaning. The signals can be transmitted via Morse light and sound, flag, radio-telegraphy and -telephony, and semaphore. Since these methods of signaling are internationally recognized, differences in language between sender and receiver are immaterial; the message will be understood when decoded in the language of the receiver, regardless of the language of the sender. The Notice to Mariners corrects Pub. 102.
416. Almanacs
For celestial sight reduction, the navigator needs an almanac for ephemeris data. The Nautical Almanac, produced jointly by H.M. Nautical Almanac Office and the U.S. Naval Observatory, is the most common almanac used for celestial navigation. It also contains information on sunrise, sunset, moonrise, and moonset, as well as compact sight reduction tables. The Nautical Almanac is published annually.
The Air Almanac contains slightly less accurate ephemeris data for air navigation. It can be used for marine navigation if slightly reduced accuracy is acceptable.
Chapter 19 provides more detailed information on using the Nautical Almanac.
417. Sight Reduction Tables
Without a calculator or computer programmed for sight reduction, the navigator needs sight reduction tables to solve the celestial triangle. Two different sets of tables are commonly used at sea.
Sight Reduction Tables for Marine Navigation, Pub. 229, consists of six volumes of tables designed for use with the Nautical Almanac for solution of the celestial triangle by the Marcq Saint Hilaire or intercept method. The tabular data are the solutions of the navigational triangle of which two sides and the included angle are known and it is necessary to find the third side and adjacent angle.
Each volume of Pub. 229 includes two 8 degree zones, comprising 15 degree bands from 0 to 90 degrees, with a 1° degree overlap between volumes. Pub. 229 is a joint publication produced by the Defense Mapping Agency, the U.S. Naval Observatory, and the Royal Greenwich Observatory.
Sight Reduction Tables for Air Navigation, Pub. 249, is also a joint production of the three organizations above. It is issued in three volumes. Volume 1 contains the values of the altitude and true azimuth of seven selected stars chosen to provide, for any given position and time, the best observations. A new edition is issued every 5 years for the upcoming astronomical epoch. Volumes 2 (0° to 40°) and 3 (39° to 89°) provide for sights of the sun, moon, and planets.

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418. Catalogs
A chart catalog is a valuable reference to the navigator for voyage planning, inventory control, and ordering. There are two major types of catalogs, one for the military and one for the civilian market.
The military navigator will see the DMA nautical chart catalog as part of a larger suite of catalogs including aeronautical (Part 1), hydrographic (Part 2), and topographic (Part 3) products. Each Part consists of one or more volumes. Unclassified DMA nautical charts are listed in Part 2, Volume 1. This is available only to U.S. military users, DoD contractors, and those who support them.
This catalog contains comprehensive ordering instructions and information about the products listed. Also listed are addresses of all Combat Support Center field offices, information on crisis support, and other special situations. The catalog is organized by geographic region corresponding to

the chart regions 1 through 9. A special section of miscellaneous charts and publications is included. This section also lists products produced by NOS, the U.S. Army Corps of Engineers, U.S. Coast Guard, U.S. Naval Oceanographic Office, and some foreign publications from the United Kingdom and Canada.
The civilian navigator should refer to catalogs produced by the National Ocean Service. For U.S. waters, NOS charts are listed in a series of single sheet “charts” showing a major region of the U.S. with individual chart graphics shown. These catalogs also list charts showing titles and scales. Finally, it lists sales agents from whom the products may be purchased.
DMA products for the civilian navigator are listed by NOS in a series of regionalized catalogs similar to Part 2 Volume 1. These catalogs are also available through authorized NOS chart agents.

MARITIME SAFETY INFORMATION

419. Notice To Mariners
The Notice to Mariners is published weekly by the Defense Mapping Agency Hydrographic/Topographic Center (DMAHTC), prepared jointly with the National Ocean Service (NOS) and the U.S. Coast Guard. It advises mariners of important matters affecting navigational safety, including new hydrographic information, changes in channels and aids to navigation, and other important data. The information in the Notice to Mariners is formatted to simplify the correction of paper charts, sailing directions, light lists, and other publications produced by DMAHTC, NOS, and the U.S. Coast Guard.
It is the responsibility of users to decide which of their charts and publications require correction. Suitable records of Notice to Mariners should be maintained to facilitate the updating of charts and publications prior to use.
Information for the Notice to Mariners is contributed by: the Defense Mapping Agency Hydrographic/Topographic Center (Department of Defense) for waters outside the territorial limits of the United States; National Ocean Service (National Oceanic and Atmospheric Administration, Department of Commerce), which is charged with surveying and charting the coasts and harbors of the United States and its territories; the U.S. Coast Guard (Department of Transportation) which is responsible for the safety of life at sea and the establishment and operation of aids to navigation; and the Army Corps of Engineers (Department of Defense), which is charged with the improvement of rivers and harbors of the United States. In addition, important contributions are made by foreign hydrographic offices and cooperating observers of all nationalities.
Over 60 countries which produce nautical charts also produce a notice to mariners. About one third of these are

weekly, another third are bi-monthly or monthly, and the rest irregularly issued according to need. Much of the data in the U.S. Notice to Mariners is obtained from these foreign notices.
Correct U.S. charts with the U.S. Notice to Mariners. Similarly, correct foreign charts using the foreign notice because chart datums often vary according to region and geographic positions are not the same for different datums.
The Notice consists of a page of Hydrograms listing important items in the notice, a chart correction section organized by ascending chart number, a publications correction section, and a summary of broadcast navigation warnings and miscellaneous information.
Mariners are requested to cooperate in the correction of charts and publications by reporting all discrepancies between published information and conditions actually observed and by recommending appropriate improvements. A convenient reporting form is provided in the back of each Notice to Mariners.
Notice to Mariners No. 1 of each year contains important information on a variety of subjects which supplements information not usually found on charts and in navigational publications. This information is published as Special Notice to Mariners Paragraphs. Additional items considered of interest to the mariner are also included in this Notice.
420. Summary Of Corrections
A close companion to the Notice to Mariners is the Summary of Corrections. The Summary is published in five volumes. Each volume covers a major portion of the earth including several chart regions and many subregions. Volume 5 also includes special charts and publications corrected by the Notice to Mariners. Since the Summaries

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contain cumulative corrections, any chart, regardless of its print date, can be corrected with the proper volume of the Summary and all subsequent Notice to Mariners.
421. The Navigation Information Network
Most of the weekly Notice to Mariners production is computerized. This system is known as the Automated Notice to Mariners System (ANMS). Design work on this system began in 1975, and the first Notice produced with it was issued in 1980. This system’s software allows remote query via modem. This remote access system is known as the Navigation Information Network (NAVINFONET).
Data available through NAVINFONET includes chart corrections, DMA List of Lights corrections, Coast Guard Light List corrections, radio warnings, MARAD Advisories, DMA hydrographic product catalog corrections, drill rig locations, ship hostile action report (SHAR) files, and GPS navigation system status reports. Messages can also be left for DMAHTC staff regarding suggestions, changes, corrections or comments on any navigation products.
The system does not have the capability to send graphics files, which prevents the transfer of chartlets. However, navigators can access most other significant information contained in the Notice to Mariners. Information is updated daily or weekly according to the Notice to Mariners production schedule. The system supports most internationally recognized telephone protocols and can presently transfer data at a maximum rate of 9600 baud.
NAVINFONET is not a replacement for the weekly Notice to Mariners, and in certain respects the accuracy of information cannot be verified by DMA. Certain files, for example, are entered directly into the data base without editing by DMA staff. Also, drill rig locations are furnished by the companies which operate them. They are not required to provide these positions, and they cannot be verified. However, within these limitations, the system can provide information 2 to 3 weeks sooner than the printed Notice to Mariners, because the paper Notice must be compiled, edited, printed, and mailed after the digital version is completed.
NAVINFONET access is free, but the user must pay telephone charges. All users must register and receive a password by writing or calling DMAHTC, Attn.: MCCNAVINFONET, Mail Stop D-44, 4600 Sangamore Rd., Bethesda, MD, 20816-5003; telephone (301) 227-3296.
The U.S. Coast and Geodetic Survey operates a similar free computerized marine information bulletin board containing a list of wrecks and obstructions, a nautical chart

locator, a list of marine sediments samples, a datum conversion program for NAD 27 to NAD 83 datum conversions, and a list of aerial photographs available from NOAA. The modem phone number is (301) 713-4573, the voice line (301) 713-2653, and FAX (301) 713-4581. The address of the office is NOAA, NOS, C&GS, (N/CG211), 1315 EastWest Highway, Silver Spring, MD, 20910
422. Local Notice To Mariners
The Local Notice to Mariners is issued by each U.S. Coast Guard District to disseminate important information affecting navigational safety within that District. This Notice reports changes and deficiencies in aids to navigation maintained by the Coast Guard. Other marine information such as new charts, channel depths, naval operations, and regattas is included. Since temporary information of short duration is not included in the weekly Notice to Mariners, the Local Notice to Mariners may be the only source of such information. Small craft using the Intracoastal Waterway and small harbors not normally used by oceangoing vessels need it to keep charts and publications up-to-date. Since correcting information for U.S. charts in the DMAHTC Notice is obtained from the Coast Guard Local Notices, it is normal to expect a lag of 1 or 2 weeks for the DMAHTC Notice to publish a correction from this source.
The Local Notice to Mariners may be obtained free of charge by contacting the appropriate Coast Guard District Commander. Vessels operating in ports and waterways in several districts must obtain the Local Notice to Mariners from each district. See Figure 422 for a complete list of U.S. Coast Guard Districts.
423. Electronic Notice To Mariners
Electronic chart development is proceeding rapidly. The correction of these charts will become a major issue. In the near future, the quality standards of digital charts will permit the replacement of traditional paper charts. Neither paper nor electronic charts should be used unless corrected through the latest Notice to Mariners. Chapter 14 discusses potential methods for correcting electronic charts.
Until the electronic chart is recognized as being the legal equivalent of the paper chart, however, it cannot replace the paper chart on the bridge. Presently, therefore, the mariner must continue to use traditional paper charts. Their use, in turn, necessitates the continued use of the Notice to Mariners correction system.

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COMMANDER, FIRST COAST GUARD DISTRICT 408 ATLANTIC AVENUE BOSTON, MA 02110-3350 PHONE: DAY 617-223-8338, NIGHT 617-223-8558
COMMANDER, SECOND COAST GUARD DISTRICT 1222 SPRUCE STREET ST. LOUIS, MO 63103-2832 PHONE: DAY 314-539-3714, NIGHT 314-539-3709
COMMANDER, FIFTH COAST GUARD DISTRICT FEDERAL BUILDING 431 CRAWFORD STREET PORTSMOUTH, VA 23704-5004 PHONE: DAY 804-398-6486, NIGHT 804-398-6231
COMMANDER, SEVENTH COAST GUARD DISTRICT BRICKELL PLAZA FEDERAL BUILDING 909 SE 1ST AVENUE, RM: 406 MIAMI, FL 33131-3050 PHONE: DAY 305-536-5621, NIGHT 305-536-5611
COMMANDER GREATER ANTILLES SECTION U.S. COAST GUARD P.O. BOX S-2029 SAN JUAN, PR 00903-2029 PHONE: 809-729-6870
COMMANDER, EIGHTH COAST GUARD DISTRICT HALE BOGGS FEDERAL BUILDING 501 MAGAZINE STREET NEW ORLEANS, LA 70130-3396 PHONE: DAY 504-589-6234, NIGHT 504-589-6225

COMMANDER, NINTH COAST GUARD DISTRICT 1240 EAST 9TH STREET CLEVELAND, OH 44199-2060 PHONE: DAY 216-522-3991, NIGHT 216-522-3984
COMMANDER, ELEVENTH COAST GUARD DISTRICT FEDERAL BUILDING 501 W. OCEAN BLVD. LONG BEACH, CA 90822-5399 PHONE: DAY 310-980-4300, NIGHT 310-980-4400
COMMANDER, THIRTEENTH COAST GUARD DISTRICT FEDERAL BUILDING 915 SECOND AVENUE SEATTLE, WA 98174-1067 PHONE: DAY 206-220-7280, NIGHT 206-220-7004
COMMANDER, FOURTEENTH COAST GUARD DISTRICT PRINCE KALANIANAOLE FEDERAL BLDG. 9TH FLOOR, ROOM 9139 300 ALA MOANA BLVD. HONOLULU, HI 96850-4982 PHONE: DAY 808-541-2317, NIGHT 808-541-2500
COMMANDER, SEVENTEENTH COAST GUARD DISTRICT P.O. BOX 25517 JUNEAU, AK 99802-5517 PHONE: DAY 907-463-2245, NIGHT 907-463-2000

Figure 422. U.S. Coast Guard Districts.

CHAPTER 5

SHORT RANGE AIDS TO NAVIGATION

DEFINING SHORT RANGE AIDS TO NAVIGATION

500. Terms And Definitions
The term “short range aids to navigation” encompasses lighted and unlighted beacons, ranges, leading lights, buoys, and their associated sound signals. Each short range aid to navigation, commonly referred to as a NAVAID, fits within a system designed to warn the mariner of dangers and direct him toward safe water. An aid’s function determines its color, shape, light characteristic, and sound. This chapter explains the U.S. Aids to Navigation System as well as the international IALA Maritime Buoyage System.
The placement and maintenance of marine aids to navigation in U.S. waters is the responsibility of the United States Coast Guard. The Coast Guard maintains lighthous-

es, radiobeacons, racons, Loran C, sound signals, buoys, and daybeacons on the navigable waters of the United States, its territories, and possessions. Additionally, the Coast Guard exercises control over privately owned navigation aid systems.
A beacon is a stationary, visual navigation aid. Large lighthouses and small single-pile structures are both beacons. Lighted beacons are called lights; unlighted beacons are daybeacons. All beacons exhibit a daymark of some sort. In the case of a lighthouse, the color and type of structure are the daymarks. On small structures, these daymarks, consisting of colored geometric shapes called dayboards, often have lateral significance. Conversely, the markings on lighthouses and towers convey no lateral significance.

FIXED LIGHTS

501. Major And Minor Lights
Lights vary from tall, high intensity coastal lights to battery-powered lanterns on single wooden piles. Immovable, highly visible, and accurately charted, fixed lights provide navigators with an excellent source for bearings. The structures are often distinctively colored to aid in identification. See Figure 501a.
A major light is a high-intensity light exhibited from a fixed structure or a marine site. Major lights include primary seacoast lights and secondary lights. Primary seacoast lights are those major lights established for making landfall from sea and coastwise passages from headland to headland. Secondary lights are those major lights established at harbor entrances and other locations where high intensity and reliability are required.
A minor light usually displays a light of low to moderate intensity. Minor lights are established in harbors, along channels, rivers, and in isolated locations. They usually have numbering, coloring, and light and sound characteristics that are part of the lateral system of buoyage.
Lighthouses are placed where they will be of most use: on prominent headlands, at harbor and port entrances, on isolated dangers, or at other points where mariners can best use them to fix their position. The lighthouse’s principal purpose is to support a light at a considerable height above the water, thereby increasing its geographic range. Support equipment is often housed near the tower.

With few exceptions, all major lights are operated automatically. There are also many automatic lights on smaller structures maintained by the Coast Guard or other attendants. Unmanned major lights may have emergency generators and automatic monitoring equipment to increase the light’s reliability.
Light structures’ appearances vary. Lights in low-lying areas usually are supported by tall towers; conversely, light structures on high cliffs may be relatively short. However its support tower is constructed, almost all lights are similarly generated, focused, colored, and characterized.
Some major lights use modern rotating or flashing lights, but many older lights use Fresnel lenses. These lenses consist of intricately patterned pieces of glass in a heavy brass framework. Modern Fresnel-type lenses are cast from high-grade plastic; they are much smaller and lighter than their glass counterparts.
A buoyant beacon provides nearly the positional accuracy of a light in a place where a buoy would normally be used. See Figure 501b. The buoyant beacon consists of a heavy sinker to which a pipe structure is tightly moored. A buoyancy chamber near the surface supports the pipe. The light, radar reflector, and other devices are located atop the pipe above the surface of the water. The pipe with its buoyancy chamber tends to remain upright even in severe weather and heavy currents, providing a smaller watch circle than a buoy. The buoyant beacon is most useful along narrow ship channels in relatively sheltered water.
63

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Figure 501a. Typical offshore light station.

Figure 501b. Typical design for a buoyant beacon.

502. Range Lights
Range lights are light pairs that indicate a specific line of position when they are in line. The higher rear light is placed behind the front light. When the mariner sees the lights vertically in line, he is on the range line. If the front light appears left of the rear light, the observer is to the right of the rangeline; if the front appears to the right of the rear, the observer is left of the rangeline. Range lights are sometimes equipped with high intensity lights for daylight use. These are effective for long channels in hazy conditions when dayboards might not be seen. The range light structures are usually also equipped with dayboards for ordinary daytime use. Some smaller ranges, primarily in the Intracoastal Waterway and other inland waters, have just the dayboards with no lights. See Figure 502.
To enhance the visibility of range lights, the Coast Guard has developed 15-foot long lighted tubes called light pipes. They are mounted vertically, and the mariner sees them as vertical bars of light distinct from background

lighting. Installation of light pipes is proceeding on several range markers throughout the country. The Coast Guard is also experimenting with long range sodium lights for areas requiring visibility greater than the light pipes can provide.
The output from a low pressure sodium light is almost entirely at one wavelength. This allows the use of an inexpensive band-pass filter to make the light visible even during the daytime. This arrangement eliminates the need for high intensity lights with their large power requirements.
Range lights are usually white, red, or green. They display various characteristics differentiating them from surrounding lights.
A directional light is a single light that projects a high intensity, special characteristic beam in a given direction. It is used in cases where a two-light range may not be practicable. A directional sector light is a directional light that emits two or more colored beams. The beams have a precisely oriented boundary between them. A normal application of a sector light would show three colored sections: red, white, and green. The white sector would indicate that the vessel is on the

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65

Figure 502. Range lights.

channel centerline; the green sector would indicate that the vessel is off the channel centerline in the direction of deep water; and the red sector would indicate that the vessel is off the centerline in the direction of shoal water.

ulations and the applicable Coast Pilot. Certain bridges may also be equipped with sound signals and radar reflectors.
505. Shore Lights

503. Aeronautical Lights
Aeronautical lights may be the first lights observed at night when approaching the coast. Those situated near the coast and visible from sea are listed in the List of Lights. These lights are not listed in the Coast Guard Light List. They usually flash alternating white and green.
Aeronautical lights are sequenced geographically in the List of Lights along with marine navigation lights. However, since they are not maintained for marine navigation, they are subject to changes of which maritime authorities may not be informed. These changes will be published in Notice to Airmen but perhaps not in Notice To Mariners.
504. Bridge Lights
Red, green, and white lights mark bridges across navigable waters of the United States. Red lights mark piers and other parts of the bridge. Red lights are also used on drawbridges to show when they are in the closed position. Green lights mark open drawbridges and mark the centerline of navigable channels through fixed bridges. The position will vary according to the type of structure. Navigational lights on bridges in the U.S. are prescribed by Coast Guard regulations.
Infrequently-used bridges may be unlighted. In foreign waters, the type and method of lighting may be different from those normally found in the United States. Drawbridges which must be opened to allow passage operate upon sound and light signals given by the vessel and acknowledged by the bridge. These required signals are detailed in the Code of Federal Reg-

Shore lights usually have a shore-based power supply. Lights on pilings, such as those found in the Intracoastal Waterway, are battery powered. Solar panels may be installed to enhance the light’s power supply. The lights consist of a power source, a flasher to determine the characteristic, a lamp changer to replace burned-out lamps, and a focusing lens.
Various types of rotating lights are in use. They do not have flashers but remain continuously lit while a lens or reflector rotates around the horizon.
The whole light system is carefully engineered to provide the maximum amount of light to the mariner for the least power use. Specially designed filaments and special grades of materials are used in the light to withstand the harsh marine environment.
The flasher electronically determines the characteristic by selectively interrupting the light’s power supply according to the chosen cycle.
The lamp changer consists of several sockets arranged around a central hub. When the circuit is broken by a burned-out filament, a new lamp is rotated into position. Almost all lights have daylight switches which turn the light off at sunrise and on at dusk.
The lens for small lights may be one of several types. The common ones in use are omni-directional lenses of 155mm, 250mm, and 300mm. In addition, lights using parabolic mirrors or focused-beam lenses are used in leading lights and ranges. The lamp filaments must be carefully aligned with the plane of the lens or mirror to provide the maximum output of light. The lens’ size is chosen according to the type of platform, power source, and lamp characteris-

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tics. Additionally, environmental characteristics of the location are considered. Various types of light-condensing panels, reflex reflectors, or colored sector panels may be in-

stalled inside the lens to provide the proper characteristic. A special heavy 200mm lantern is used in locations
where ice and breaking water are a hazard.

LIGHT CHARACTERISTICS

506. Characteristics
A light has distinctive characteristics which distinguish it from other lights or convey specific information. A light may show a distinctive sequence of light and dark intervals. Additionally, a light may display a distinctive color or color sequence. In the Light Lists, the dark intervals are referred to as eclipses. An occulting light is a light totally eclipsed at regular intervals, the duration of light always being greater than the duration of darkness. A flashing light is a light which flashes at regular intervals, the duration of light always being less than the duration of darkness. An isophase light flashes at regular intervals, the duration of light being equal to the duration of darkness.
Light phase characteristics (Figure 506a and Figure 506b) are the distinctive sequences of light and dark intervals or sequences in the variations of the luminous intensity of a light. The light phase characteristics of lights which change color do not differ from those of lights which do not change color. A light showing different colors alternately is described as an alternating light. The alternating characteristic may be used with other light phase characteristics.
Light-sensitive switches extinguish most lighted navigation aids during daylight hours. However, owing to the various sensitivity of the light switches, all lights do not come on or go off at the same time. Mariners should account for this when identifying aids to navigation during twilight periods when some lighted aids are on while others are not.
507. Light Sectors
Sectors of colored glass or plastic are sometimes placed in the lanterns of certain lights to indicate dangerous waters. Lights so equipped show different colors when observed from different bearings. A sector changes the color of a light, but not its characteristic, when viewed from certain directions. For example, a four second flashing white light having a red sector will appear as a four second flashing red light when viewed from within the red sector.
Sectors may be only a few degrees in width or extend in a wide arc from deep water toward shore. Bearings referring to sectors are expressed in degrees true as observed from a vessel.
In most cases, areas covered by red sectors should be avoided. The nature of the danger can be determined from the chart. In some cases a narrow sector may mark the best water across a shoal, or a turning point in a channel.
Sectors generated by shadow-casting filters do not have precise boundaries as directional sector lights do.

Therefore, the transition from one color to another is not abrupt. The colors change through an arc of uncertainty of 2° or greater, depending on the optical design of the light. Therefore determining bearings by observing the color change is less accurate than obtaining a bearing with an azimuth circle.
508. Factors Affecting Range And Characteristics
The condition of the atmosphere has a considerable effect upon a light’s range. Sometimes lights are obscured by fog, haze, dust, smoke, or precipitation. On the other hand, refraction may cause a light to be seen farther than under ordinary circumstances. A light of low intensity will be easily obscured by unfavorable conditions of the atmosphere. For this reason, the intensity of a light should always be considered when looking for it in thick weather. Haze and distance may reduce the apparent duration of a light’s flash. In some conditions of the atmosphere, white lights may have a reddish hue. In clear weather green lights may have a more whitish hue.
Lights placed at great elevations are more frequently obscured by clouds, mist, and fog than those near sea level. In regions where ice conditions prevail, an unattended light’s lantern panes may become covered with ice or snow This may reduce the light’s luminous range and change the light’s observed color.
The distance from a light cannot be estimated by its apparent brightness. There are too many factors which can change the perceived intensity. Also, a powerful, distant light may sometimes be confused with a smaller, closer one with similar characteristics. Every light sighted should be carefully evaluated to determine if it is the one expected.
The presence of bright shore lights may make it difficult to distinguish navigational lights from background lighting. Lights may also be obscured by various shore obstructions, natural and man-made. The Coast Guard requests mariners to report these cases to the nearest Coast Guard station.
A light’s loom is seen through haze or the reflection from low-lying clouds when the light is beyond its geographic range. Only the most powerful lights can generate a loom. The loom may sometimes be sufficiently defined to obtain a bearing. If not, an accurate bearing on a light beyond geographic range may sometimes be obtained by ascending to a higher level where the light can be seen, and noting a star directly over the light. The bearing of the star can then be obtained from the navigating bridge and the bearing to the light plotted indirectly.
At short distances, some of the brighter flashing lights may show a faint continuous light, or faint flashes, between

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67

Figure 506a. Light phase characteristics. = = = = = = THIS FIGURE HAS TO BE REPAIRED!! = = = = = =

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Figure 506b. Light phase characteristics.

SHORT RANGE AIDS TO NAVIGATION

69

regular flashes. This is due to reflections of a rotating lens on panes of glass in the lighthouse.
If a light is not sighted within a reasonable time after prediction, a dangerous situation may exist. Conversely, the light may simply be obscured or extinguished. The ship’s position should immediately be fixed by other means to determine any possibility of danger.
The apparent characteristic of a complex light may change with the distance of the observer. For example, a light with a characteristic of fixed white and alternating flashing white and red may initially show as a simple flashing white light. As the vessel draws nearer, the red flash will become visible and the characteristic will apparently be alternating flashing white and red. Later, the fainter fixed white light will be seen between the flashes and the true characteristic of the light finally recognized as fixed white,

alternating flashing white and red (F W Al W R). This is because for a given candlepower, white is the most visible color, green less so, and red least of the three. This fact also accounts for the different ranges given in the Light Lists for some multi-color sector lights. The same lamp has different ranges according to the color imparted by the sector glass.
A light may be extinguished due to weather, battery failure, vandalism, or other causes. In the case of unattended lights, this condition might not be immediately corrected. The mariner should report this condition to the nearest Coast Guard station. During periods of armed conflict, certain lights may be deliberately extinguished without notice.
Offshore light stations should always be left well off the course whenever searoom permits.

BUOYS

509. Definitions And Types

Buoys are floating aids to navigation. They mark channels, indicate shoals and obstructions, and warn the mariner of dangers. Buoys are used where fixed aids would be uneconomical or impractical due to the depth of water. By their color, shape, topmark, number, and light characteristics, buoys indicate to the mariner how to avoid hazards and stay in safe water. The federal buoyage system in the U.S. is maintained by the Coast Guard.
There are many different sizes and types of buoys designed to meet a wide range of environmental conditions and user requirements. The size of a buoy is determined primarily by its location. In general, the smallest buoy which will stand up to local weather and current conditions is chosen.
There are five types of buoys maintained by the Coast Guard. They are:

1. Lateral marks. 2. Isolated danger marks. 3. Safe water marks. 4. Special marks. 5. Information/regulatory marks.
These conform in general to the specifications of the International Association of Lighthouse Authorities (IALA) buoyage system.
A lighted buoy is a floating hull with a tower on which a light is mounted. Batteries for the light are in watertight pockets in the buoy hull or in watertight boxes mounted on the buoy hull. To keep the buoy in an upright position, a counterweight is attached to the hull below the water surface. A radar reflector is built into the buoy tower.
The largest of the typical U.S. Coast Guard buoys can be moored in up to 190 feet of water, limited by the weight of chain the hull can support. The focal plane of the light is

Figure 509. Buoy showing counterweight.
15 to 20 feet high. The designed nominal visual range is 3.8 miles, and the radar range 4 miles. Actual conditions will cause these range figures to vary considerably.
The smallest buoys are designed for protected water. Some are made of plastic and weigh only 40 pounds. Specially designed buoys are used for fast current, ice, and other environmental conditions.
A variety of special purpose buoys are owned by other governmental organizations. Examples of these organizations include the Panama Canal Commission, the St. Lawrence Seaway Development Corporation, NOAA, and the Department of Defense. These buoys are usually navigational marks or data collection buoys with traditional round, boat-shaped, or discus-shaped hulls.
A special class of buoy, the Ocean Data Acquisition

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System (ODAS) buoy, is moored or floats free in offshore waters. Positions are promulgated through radio warnings. These buoys are generally not large enough to cause damage in a collision, but should be given a wide berth regardless, as any loss would almost certainly result in the interruption of valuable scientific experiments. They are generally bright orange or yellow in color, with vertical stripes on moored buoys and horizontal bands on free-floating ones, and have a strobe light for night visibility.
Even in clear weather, the danger of collision with a buoy exists. If struck head-on, a large buoy can inflict severe damage to a large ship; it can sink a smaller one. Reduced visibility or heavy background lighting can contribute to the problem. The Coast Guard sometimes receives reports of buoys missing from station that were actually run down and sunk. Tugboats and towboats towing or pushing barges are particularly dangerous to buoys because of poor over-the-bow visibility when pushing or yawing during towing. The professional mariner must report any collision with a buoy to the nearest Coast Guard unit. Failure to do so may cause the next vessel to miss the channel or hit the obstruction marked by the buoy; it can also lead to fines and legal liability.
Routine on-station buoy maintenance consists of inspecting the mooring, cleaning the hull and superstructure, replacing the batteries, flasher, and lamps, checking wiring and venting systems, and verifying the buoy’s exact position. Every few years, each buoy is replaced by a similar aid and returned to a Coast Guard maintenance facility for complete refurbishment.
The placement of a buoy depends on its purpose and its position on the chart. Most buoys are placed on charted position as accurately as conditions allow. However, if a buoy’s purpose is to mark a shoal and the shoal is found to be in a different position than the chart shows, the buoy will be placed to properly mark the shoal, and not on its charted position.

Under normal conditions, the lenses used on buoys are 155mm in diameter at the base. 200 mm lenses are used where breaking waves or swells call for the larger lens. They are colored according to the charted characteristic of the buoy. As in shore lights, the lamp must be carefully focused so that the filament is directly in line with the focal plane of the lens. This ensures that the majority of the light produced is focused in a 360° horizontal fan beam A buoy light has a relatively narrow vertical profile. Because the buoy rocks in the sea, the focal plane may only be visible for fractions of a second at great ranges. A realistic range for sighting buoy lights is 4-6 miles in good visibility.
511. Sound Signals On Buoys
Lighted sound buoys have the same general configuration as lighted buoys but are equipped with either a bell, gong, whistle, or horn. Bells and gongs are sounded by tappers hanging from the tower that swing as the buoys roll in the sea. Bell buoys produce only one tone; gong buoys produce several tones. The tone-producing device is mounted between the legs of the pillar or tower.
Whistle buoys make a loud moaning sound caused by the rising and falling motions of the buoy in the sea. A sound buoy equipped with an electronic horn will produce a pure tone at regular intervals regardless of the sea state. Unlighted sound buoys have the same general appearance as lighted buoys, but their underwater shape is designed to make them lively in all sea states.
512. Buoy Moorings
Buoys require moorings to hold them in position. Typically the mooring consists of chain and a large concrete

510. Lights On Buoys

Buoy light systems consist of a battery pack, a flasher which determines the characteristic, a lamp changer which automatically replaces burned-out bulbs, a lens to focus the light, and a housing which supports the lens and protects the electrical equipment.
The batteries consist of 12-volt lead/acid type batteries electrically connected to provide sufficient power to run the proper flash characteristic and lamp size. These battery packs are contained in pockets in the buoy hull, accessible through water-tight bolted hatches or externally mounted boxes. Careful calculations based on light characteristics determine how much battery power to install.
The flasher determines the characteristic of the lamp. It is installed in the housing supporting the lens.
The lamp changer consists of several sockets arranged around a central hub. A new lamp rotates into position if the active one burns out.

Figure 512. A sinker used to anchor a buoy.

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or cast iron sinker. See Figure 512. Because buoys are subjected to waves, wind, and tides, the moorings must be deployed with chain lengths much greater than the water depth. The scope of chain will normally be about 3 times the water depth. The length of the mooring chain defines a watch circle within which the buoy can be expected to swing. It is for this reason that the charted buoy symbol has a “position approximate” circle to indicate its charted position, whereas a light position is shown by a dot at the exact location. Actual watch circles do not necessarily coincide with the “position approximate” circles which represent them.
Over several years, the chain gradually wears out and must be replaced with new. The worn chain is often cast into the concrete of new sinkers.
513. Large Navigational Buoys
Large navigational buoys are moored in open water at approaches to major seacoast ports. These 40-foot diameter buoys (Figure 513) show lights from heights of about

36 feet above the water. Emergency lights automatically energize if the main light is extinguished. These buoys may also have a radiobeacon and sound signals. Their condition is monitored by radio from shore.
514. Wreck Buoys
A wreck buoy usually cannot be placed directly over the wreck it is intended to mark because the buoy tender may not want to pass over a shallow wreck or risk fouling the buoy mooring. For this reason, a wreck buoy is usually placed as closely as possible on the seaward or channelward side of a wreck. In some situations, two buoys may be used to mark the wreck, one lying off each end. The wreck may lie directly between them or inshore of a line between them, depending on the local situation. The Local Notice To Mariners should be consulted concerning details of the placement of wreck buoys on individual wrecks. Often it will also give particulars of the wreck and what activities may be in progress to clear it.

Figure 513. Large navigational buoy.

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The charted position of a wreck buoy will usually be offset from the actual geographic position so that the wreck and buoy symbols do not coincide. Only on the largest scale chart will the actual and charted positions of both wreck and buoy be the same. Where they might overlap, it is the wreck symbol which occupies the exact charted position and the buoy symbol which is offset.
Wreck buoys are required to be placed by the owner of the wreck, but they may be placed by the Coast Guard if the owner is unable to comply with this requirement. In general, privately placed aids are not as reliable as Coast Guard aids.
Sunken wrecks are sometimes moved away from their buoys by storms, currents, freshets, or other causes. Just as shoals may shift away from the buoys placed to mark them, wrecks may shift away from wreck buoys.
515. Fallibility Of Buoys
Buoys cannot be relied on to maintain their charted positions consistently. They are subject to a variety of hazards including severe weather, collision, mooring casualties, and electrical failure. Report any discrepancy noted in a buoy to the U.S. Coast Guard.
The buoy symbol shown on charts indicates the ap-

proximate position of the sinker which secures the buoy to the seabed. The approximate position is used because of practical limitations in placing and keeping buoys and their sinkers in precise geographical locations. These limitations include prevailing atmospheric and sea conditions, the slope and type of material making up the seabed, the scope of the mooring chain, and the fact that the positions of the buoys and the sinkers are not under continuous surveillance. The position of the buoy shifts around the area shown by the chart symbol due to the forces of wind and current.
A buoy may not be in its charted position because of changes in the feature it marks. For example, a buoy meant to mark a shoal whose boundaries are shifting might frequently be moved to mark the shoal accurately. A Local Notice To Mariners will report the change, and a Notice To Mariners chart correction may also be written. In some small channels which change often, buoys are not charted even when considered permanent; local knowledge is advised in such areas.
For these reasons, a mariner must not rely completely upon the position or operation of buoys, but should navigate using bearings of charted features, structures, and aids to navigation on shore. Further, a vessel attempting to pass too close aboard a buoy risks a collision with the buoy or the obstruction it marks.

BUOYAGE SYSTEMS

516. Lateral And Cardinal Systems
There are two major types of buoyage systems: the lateral system and the cardinal system. The lateral system is best suited for well-defined channels. The description of each buoy indicates the direction of danger relative to the course which is normally followed. In principle, the positions of marks in the lateral system are determined by the general direction taken by the mariner when approaching port from seaward. These positions may also be determined with reference to the main stream of flood current. The United States Aids to Navigation System is a lateral system.
The cardinal system is best suited for coasts with numerous isolated rocks, shoals, and islands, and for dangers in the open sea. The characteristic of each buoy indicates the approximate true bearing of the danger it marks. Thus, an eastern quadrant buoy marks a danger which lies to the west of the buoy. The following pages diagram the cardinal and lateral buoyage systems as found outside the United States.
517. The IALA Maritime Buoyage System
Although most of the major maritime nations have used either the lateral or the cardinal system for many years, details such as the buoy shapes and colors have varied from country to country. With the increase in maritime com-

merce between countries, the need for a uniform system of buoyage became apparent.
In 1889, an International Marine Conference held in Washington, D.C., recommended that in the lateral system, starboard hand buoys be painted red and port hand buoys black. Unfortunately, when lights for buoys were introduced some years later, some European countries placed red lights on the black port hand buoys to conform with the red lights marking the port side of harbor entrances, while in North America red lights were placed on red starboard hand buoys. In 1936, a League of Nations subcommittee recommended a coloring system opposite to the 1889 proposal.
The International Association of Lighthouse Authorities (IALA) is a non-governmental organization which consists of representatives of the worldwide community of aids to navigation services to promote information exchange and recommend improvements based on new technologies. In 1980, with the assistance of IMO and the IHO, the lighthouse authorities from 50 countries and representatives of 9 international organizations concerned with aids to navigation met and adopted the IALA Maritime Buoyage System. They established two regions, Region A and Region B, for the entire world. Region A roughly corresponds to the 1936 League of Nations system, and Region B to the older 1889 system.
Lateral marks differ between Regions A and B. Lateral marks in Region A use red and green colors by day and night

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to indicate port and starboard sides of channels, respectively. In Region B, these colors are reversed with red to starboard and green to port. In both systems, the conventional direction of buoyage is considered to be returning from sea, hence the phrase “red right returning” in IALA region B.
518. Types Of Marks
The IALA Maritime Buoyage System applies to all fixed and floating marks, other than lighthouses, sector lights, leading lights and daymarks, lightships and large navigational buoys, and indicates:
1. The side and center-lines of navigable channels. 2. Natural dangers, wrecks, and other obstructions. 3. Regulated navigation areas. 4. Other important features.
Most lighted and unlighted beacons other than leading marks are included in the system. In general, beacon topmarks will have the same shape and colors as those used on buoys. The system provides five types of marks which may be used in any combination:

3. Sphere. 4. Pillar. 5. Spar.
In the case of can, conical, and spherical, the shapes have lateral significance because the shape indicates the correct side to pass. With pillar and spar buoys, the shape has no special significance.
The term “pillar” is used to describe any buoy which is smaller than a “large navigation buoy (LNB)” and which has a tall, central structure on a broad base; it includes beacon buoys, high focal plane buoys, and others (except spar buoys) whose body shape does not indicate the correct side to pass.
Topmarks
The IALA System makes use of can, conical, spherical, and X-shaped topmarks only. Topmarks on pillar and spar buoys are particularly important and will be used wherever practicable, but ice or other severe conditions may occasionally prevent their use.
Colors Of Lights

1. Lateral marks indicate port and starboard sides of channels.
2. Cardinal marks, named according to the four points of the compass, indicate that the navigable water lies to the named side of the mark.
3. Isolated danger marks erected on, or moored directly on or over, dangers of limited extent.
4. Safe water marks, such as midchannel buoys. 5. Special marks, the purpose of which is apparent from
reference to the chart or other nautical documents.
Characteristics Of Marks
The significance of a mark depends on one or more features:
1. By day—color, shape, and topmark. 2. By night—light color and phase characteristics.
Colors Of Marks

Where marks are lighted, red and green lights are reserved for lateral marks, and yellow for special marks. The other types of marks have a white light, distinguished one from another by phase characteristic.
Phase Characteristics Of Lights
Red and green lights may have any phase characteristic, as the color alone is sufficient to show on which side they should be passed. Special marks, when lighted, have a yellow light with any phase characteristic not reserved for white lights of the system. The other types of marks have clearly specified phase characteristics of white light: various quick-flashing phase characteristics for cardinal marks, group flashing (2) for isolated danger marks, and relatively long periods of light for safe water marks.
Some shore lights specifically excluded from the IALA System may coincidentally have characteristics corresponding to those approved for use with the new marks. Care is needed to ensure that such lights are not misinterpreted.

The colors red and green are reserved for lateral marks, and yellow for special marks. The other types of marks have black and yellow or black and red horizontal bands, or red and white vertical stripes.
Shapes Of Marks
There are five basic buoy shapes:
1. Can. 2. Cone.

519. IALA Lateral Marks
Lateral marks are generally used for well-defined channels; they indicate the port and starboard hand sides of the route to be followed, and are used in conjunction with a conventional direction of buoyage.
This direction is defined in one of two ways:
1. Local direction of buoyage is the direction taken by the mariner when approaching a harbor, river estuary, or other waterway from seaward.

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2. General direction of buoyage is determined by the buoyage authorities, following a clockwise direction around continental land-masses, given in sailing directions, and, if necessary, indicated on charts by a large open arrow symbol.

Colors
Black and yellow horizontal bands are used to color a cardinal mark. The position of the black band, or bands, is related to the points of the black topmarks.

In some places, particularly straits open at both ends, the local direction of buoyage may be overridden by the general direction.
Along the coasts of the United States, the characteristics assume that proceeding “from seaward” constitutes a clockwise direction: a southerly direction along the Atlantic coast, a westerly direction along the Gulf of Mexico coast, and a northerly direction along the Pacific coast. On the Great Lakes, a westerly and northerly direction is taken as being “from seaward” (except on Lake Michigan, where a southerly direction is used). On the Mississippi and Ohio Rivers and their tributaries, the characteristics of aids to navigation are determined as proceeding from sea toward the head of navigation. On the Intracoastal Waterway, proceeding in a generally southerly direction along the Atlantic coast, and in a generally westerly direction along the gulf coast, is considered as proceeding “from seaward.”
520. IALA Cardinal Marks
A cardinal mark is used in conjunction with the compass to indicate where the mariner may find the best navigable water. It is placed in one of the four quadrants (north, east, south, and west), bounded by the true bearings NW-NE, NE-SE, SE-SW, and SW-NW, taken from the point of interest. A cardinal mark takes its name from the quadrant in which it is placed.
The mariner is safe if he passes north of a north mark, east of an east mark, south of a south mark, and west of a west mark.
A cardinal mark may be used to:
1. Indicate that the deepest water in an area is on the named side of the mark.
2. Indicate the safe side on which to pass a danger. 3. Emphasize a feature in a channel, such as a bend,
junction, bifurcation, or end of a shoal.
Topmarks
Black double-cone topmarks are the most important feature, by day, of cardinal marks. The cones are vertically placed, one over the other. The arrangement of the cones is very logical: North is two cones with their points up (as in “north-up”). South is two cones, points down. East is two cones with bases together, and west is two cones with points together, which gives a wineglass shape. “West is a Wineglass” is a memory aid.
Cardinal marks carry topmarks whenever practicable, with the cones as large as possible and clearly separated.

N Points up S Points down W Points together E Points apart

Black above yellow. Black below yellow. Black, yellow above and below. Yellow, black above and below.

Shape

The shape of a cardinal mark is not significant, but buoys must be pillars or spars.

Lights

When lighted, a cardinal mark exhibits a white light; its characteristics are based on a group of quick or very quick flashes which distinguish it as a cardinal mark and indicate its quadrant. The distinguishing quick or very quick flashes are:
North—Uninterrupted East—three flashes in a group South—six flashes in a group followed by a long flash West—nine flashes in a group

As a memory aid, the number of flashes in each group can be associated with a clock face as follows:
(3 o’clock—E, 6 o’clock—S, and 9 o’clock—W).

The long flash (of not less than 2 seconds duration), immediately following the group of flashes of a south cardinal mark, is to ensure that its six flashes cannot be mistaken for three or nine.
The periods of the east, south, and west lights are, respectively, 10, 15, and 15 seconds if quick flashing; and 5, 10, and 10 seconds if very quick flashing.
Quick flashing lights flash at a rate between 50 and 79 flashes per minute, usually either 50 or 60. Very quick flashing lights flash at a rate between 80 and 159 flashes per minute, usually either 100 or 120.
It is necessary to have a choice of quick flashing or very quick flashing lights in order to avoid confusion if, for example, two north buoys are placed near enough to each other for one to be mistaken for the other.

521. IALA Isolated Danger Marks

An isolated danger mark is erected on, or moored on or above, an isolated danger of limited extent which has navigable water all around it. The extent of the surrounding navigable water is immaterial; such a mark can, for example, indicate either a shoal which is well offshore or an islet separated by a narrow channel from the coast.

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Position

Lights

On a chart, the position of a danger is the center of the symbol or sounding indicating that danger; an isolated danger buoy may therefore be slightly displaced from its geographic position to avoid overprinting the two symbols. The smaller the scale, the greater this offset will be. At very large scales the symbol may be correctly charted.

When lighted, safe water marks exhibit a white light. This light can be occulting, isophase, a single long flash, or Morse “A.” If a long flash (i.e. a flash of not less than 2 seconds) is used, the period of the light will be 10 seconds. As a memory aid, remember a single flash and a single sphere topmark.

Topmark
A black double-sphere topmark is, by day, the most important feature of an isolated danger mark. Whenever practicable, this topmark will be carried with the spheres as large as possible, disposed vertically, and clearly separated.
Color
Black with one or more red horizontal bands are the colors used for isolated danger marks.
Shape
The shape of an isolated danger mark is not significant, but a buoy will be a pillar or a spar.
Light
When lighted, a white flashing light showing a group of two flashes is used to denote an isolated danger mark. As a memory aid, associate two flashes with two balls in the topmark.
522. IALA Safe Water Marks
A safe water mark is used to indicate that there is navigable water all around the mark. Such a mark may be used as a center line, mid-channel, or landfall buoy.
Color
Red and white vertical stripes are used for safe water marks, and distinguish them from the black-banded, danger-marking marks.
Shape
Spherical, pillar, or spar buoys may be used as safe water marks.
Topmark

523. IALA Special Marks
A special mark may be used to indicate a special area or feature which is apparent by referring to a chart, sailing directions, or notices to mariners. Uses include:
1. Ocean Data Acquisition System (ODAS) buoys. 2. Traffic separation marks. 3. Spoil ground marks. 4. Military exercise zone marks. 5. Cable or pipeline marks, including outfall pipes. 6. Recreation zone marks.
Another function of a special mark is to define a channel within a channel. For example, a channel for deep draft vessels in a wide estuary, where the limits of the channel for normal navigation are marked by red and green lateral buoys, may have its boundaries or centerline marked by yellow buoys of the appropriate lateral shapes.
Color
Yellow is the color used for special marks.
Shape
The shape of a special mark is optional, but must not conflict with that used for a lateral or a safe water mark. For example, an outfall buoy on the port hand side of a channel could be can-shaped but not conical.
Topmark
When a topmark is carried it takes the form of a single yellow X.
Lights
When a light is exhibited it is yellow. It may show any phase characteristic except those used for the white lights of cardinal, isolated danger, and safe water marks, In the case of ODAS buoys, the phase characteristic used is groupflashing with a group of five flashes every 20 seconds.

A single red spherical topmark will be carried, whenever practicable, by a pillar or spar buoy used as a safe water mark.

524. IALA New Dangers A newly discovered hazard to navigation not yet shown

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on charts, included in sailing directions, or announced by a Notice To Mariners is termed a new danger. The term covers naturally occurring and man-made obstructions.
Marking
A new danger is marked by one or more cardinal or lateral marks in accordance with the IALA system rules. If the danger is especially grave, at least one of the marks will be duplicated as soon as practicable by an identical mark until the danger has been sufficiently identified.
Lights
If a lighted mark is used for a new danger, it must exhibit a quick flashing or very quick flashing light. If a cardinal mark is used, it must exhibit a white light; if a lateral mark, a red or green light.
Racons
The duplicate mark may carry a Racon, Morse coded D, showing a signal length of 1 nautical mile on a radar display.
525. Chart Symbols And Abbreviations
Spar buoys and spindle buoys are represented by the same symbol; it is slanted to distinguish them from upright beacon symbols. The abbreviated description of the color of a buoy is given under the symbol. Where a buoy is colored in bands, the colors are indicated in sequence from the top. If the sequence of the bands is not known, or if the buoy is striped, the colors are indicated with the darker color first.
Topmarks
Topmark symbols are solid black except when the topmark is red.
Lights
The period of the light of a cardinal mark is determined by its quadrant and its flash characteristic (either quickflashing or a very quick-flashing). The light’s period is less important than its phase characteristic. Where space on charts is limited, the period may be omitted.
Light flares
Magenta light-flares are normally slanted and inserted with their points adjacent to the position circles at the base of the symbols so the flare symbols do not obscure the topmark symbols.

Radar Reflectors
Radar reflectors are not affected by the IALA buoyage rules. They are not charted for several reasons. It can be assumed that most major buoys are fitted with radar reflectors. It is also necessary to reduce the size and complexity of buoy symbols and associated legends. Finally, it is understood that, in the case of cardinal buoys, buoyage authorities site the reflector so that it cannot be mistaken for a topmark. For these reasons, radar reflectors are not charted under IALA rules.
The symbols and abbreviations of the IALA Maritime Buoyage System may be found in U.S.. Chart No. 1, Nautical Chart Symbols and Abbreviations, and in foreign equivalents.
526. Description Of The U.S. Aids to Navigation System
In the United States, the U.S. Coast Guard has incorporated the major features of the IALA system with the existing infrastructure of buoys and lights as explained below.
Colors
Under this system, green buoys mark a channel’s port side and obstructions which must be passed by keeping the buoy on the port hand. Red buoys mark a channel’s starboard side and obstructions which must be passed by keeping the buoy on the starboard hand.
Red and green horizontally banded preferred channel buoys mark junctions or bifurcations in a channel or obstructions which may be passed on either side. If the topmost band is green, the preferred channel will be followed by keeping the buoy on the port hand. If the topmost band is red, the preferred channel will be followed by keeping the buoy on the starboard hand.
Red and white vertically striped safe water buoys mark a fairway or mid-channel.
Reflective material is placed on buoys to assist in their detection at night with a searchlight. The color of the reflective material agrees with the buoy color. Red or green reflective material may be placed on preferred channel (junction) buoys; red if topmost band is red or green if the topmost band is green. White reflective material is used on safe water buoys. Special purpose buoys display yellow reflective material. Warning or regulatory buoys display orange reflective horizontal bands and a warning symbol. Intracoastal Waterway buoys display a yellow reflective square, triangle, or horizontal strip along with the reflective material coincident with the buoy’s function.
Shapes
Certain unlighted buoys are differentiated by shape. Red buoys and red and green horizontally banded buoys with the topmost band red are cone-shaped buoys called nuns. Green buoys and green and red horizontally banded buoys with the topmost band green are cylinder-shaped buoys called cans.

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Unlighted red and white vertically striped buoys may be pillar shaped or spherical. Lighted buoys, sound buoys, and spar buoys are not differentiated by shape to indicate the side on which they should be passed. Their purpose is indicated not by shape but by the color, number, or light characteristics.
Numbers
All solid colored buoys are numbered, red buoys bearing even numbers and green buoys bearing odd numbers. (Note that this same rule applies in IALA System A also.) The numbers increase from seaward upstream or toward land. No other colored buoys are numbered; however, any buoy may have a letter for identification.
Light colors
Red lights are used only on red buoys or red and green horizontally banded buoys with the topmost band red. Green lights are used only on the green buoys or green and red horizontally banded buoys with the topmost band green. White lights are used on both “safe water” aids showing a Morse A characteristic and on Information and Regulatory aids.

obstructions, will flash not less than 50 flashes but not more than 80 flashes per minute (quick flashing, Q). Lights on preferred channel buoys will show a series of grouped flashes with successive groups in a period having different number of flashes—composite group flashing (or a quick light in which the sequence of flashes is interrupted by regularly repeated eclipses of constant and long duration). Lights on safe water buoys will always show a white Morse Code “A” (Short-Long) flash recurring at the rate of approximately eight times per minute.
Daylight Controls
Lighted buoys have a special device to energize the light when darkness falls and to de-energize the light when day breaks. These devices are not of equal sensitivity; therefore all lights do not come on or go off at the same time. Mariners should ensure correct identification of aids during twilight periods when some light aids to navigation are on while others are not.
Special Purpose Buoys

Light Characteristics
Lights on red buoys or green buoys, if not occulting or isophase, will generally be regularly flashing (Fl). For ordinary purposes, the frequency of flashes will be not more than 50 flashes per minute. Lights with a distinct cautionary significance, such as at sharp turns or marking dangerous

Buoys for special purposes are colored yellow. White buoys with orange bands are for information or regulatory purposes. The shape of special purpose buoys has no significance. They are not numbered, but they may be lettered. If lighted, special purpose buoys display a yellow light usually with fixed or slow flash characteristics. Information and regulatory buoys, if lighted, display white lights.

BEACONS
527. Definition And Description
Beacons are fixed aids to navigation placed on shore or on pilings in relatively shallow water. If unlighted, the beacon is referred to as a daybeacon. A daybeacon is identified by its color and the color, shape, and number of its dayboard. The simplest form of daybeacon consists of a single pile with a dayboard affixed at or near its top. See Figure 527. Daybeacons may be used to form an unlighted range.
Dayboards identify aids to navigation against daylight backgrounds. The size of the dayboard required to make the aid conspicuous depends upon the aid’s intended range.
Most dayboards also display numbers or letters for identification. The numbers, letters, and borders of most dayboards have reflective tape to make them visible at night.
The detection, recognition, and identification distances vary widely for any particular dayboard. They depend upon the luminance of the dayboard, the sun’s position, and the local visibility conditions.

Figure 527. Daybeacon.

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SOUND SIGNALS

528. Types Of Sound Signals
Most lighthouses and offshore light platforms, as well as some minor light structures and buoys, are equipped with sound-producing devices to help the mariner in periods of low visibility. Charts and Light Lists contain the information required for positive identification. Buoys fitted with bells, gongs, or whistles actuated by wave motion may produce no sound when the sea is calm. Sound signals are not designed to identify the buoy or beacon for navigation purposes. Rather, they allow the mariner to pass clear of the buoy or beacon during low visibility.
Sound signals vary. The navigator must use the Light List to determine the exact length of each blast and silent interval. The various types of sound signals also differ in tone, facilitating recognition of the respective stations.
Diaphones produce sound with a slotted piston moved back and forth by compressed air. Blasts may consist of a high and low tone. These alternate-pitch signals are called “two-tone.” Diaphones are not used by the Coast Guard, but the mariner may find them on some private navigation aids.
Horns produce sound by means of a disc diaphragm operated pneumatically or electrically. Duplex or triplex horn units of differing pitch produce a chime signal.
Sirens produce sound with either a disc or a cupshaped rotor actuated electrically or pneumatically. Sirens are not used on U.S. navigation aids.
Whistles use compressed air emitted through a circumferential slot into a cylindrical bell chamber.
Bells and gongs are sounded with a mechanically operated hammer.

2. Under certain atmospheric conditions, when a sound signal has a combination high and low tone, it is not unusual for one of the tones to be inaudible. In the case of sirens, which produce a varying tone, portions of the signal may not be heard.
3. When the sound is screened by an obstruction, there are areas where it is inaudible.
4. Operators may not activate a remotely controlled sound aid for a condition unobserved from the controlling station.
5. Some sound signals cannot be immediately started. 6. The status of the vessel’s engines and the location
of the observer both affect the effective range of the aid.
These considerations justify the utmost caution when navigating near land in a fog. A navigator can never rely on sound signals alone; he should continuously man both the radar and fathometer. He should place lookouts in positions where the noises in the ship are least likely to interfere with hearing a sound signal. The aid upon which a sound signal rests is usually a good radar target, but collision with the aid or the danger it marks is always a possibility.
Emergency signals are sounded at some of the light and fog signal stations when the main and stand-by sound signals are inoperative. Some of these emergency sound signals are of a different type and characteristic than the main sound signal. The characteristics of the emergency sound signals are listed in the Light List.
The mariner should never assume:

529. Limitations Of Sound Signals
As aids to navigation, sound signals have serious limitations because sound travels through the air in an unpredictable manner.
It has been clearly established that:
1. Sound signals are heard at greatly varying distances and that the distance at which a sound signal can be heard may vary with the bearing and timing of the signal.

1. That he is out of ordinary hearing distance because he fails to hear the sound signal.
2. That because he hears a sound signal faintly, he is far from it.
3. That because he hears it clearly, he is near it. 4. That the distance from and the intensity of a sound on
any one occasion is a guide for any future occasion. 5. That the sound signal is not sounding because he does
not hear it, even when in close proximity. 6. That the sound signal is in the direction the sound ap-
pears to come from.

MISCELLANEOUS U.S. SYSTEMS

530. Intracoastal Waterway Aids To Navigation
The Intracoastal Waterway (ICW) runs parallel to the Atlantic and Gulf of Mexico coasts from Manasquan Inlet on the New Jersey shore to the Texas/Mexican border. It follows

rivers, sloughs, estuaries, tidal channels, and other natural waterways, connected with dredged channels where necessary. Some of the aids marking these waters are marked with yellow; otherwise, the marking of buoys and beacons follows the same system as that in other U.S. waterways.

SHORT RANGE AIDS TO NAVIGATION

79

Yellow symbols indicate that an aid marks the Intracoastal Waterway. Yellow triangles indicate starboard hand aids, and yellow squares indicate port hand aids when following the ICW’s conventional direction of buoyage. Nonlateral aids such as safe water, isolated danger, and front range boards are marked with a horizontal yellow band. Rear range boards do not display the yellow band. At a junction with a federally-maintained waterway, the preferred channel mark will display a yellow triangle or square as appropriate. Junctions between the ICW and privately maintained waterways are not marked with preferred channel buoys.
531. Western Rivers System
Aids to navigation on the Mississippi River and its tributaries above Baton Rouge generally conform to the lateral system of buoyage in use in the rest of the U.S. The following differences are significant:
1. Buoys are not numbered. 2. The numbers on lights and daybeacons do not have
lateral significance; they indicate the mileage from a designated point, normally the river mouth. 3. Flashing lights on the left side proceeding upstream show single green or white flashes while those on the right side show group flashing red or white flashes. 4. Diamond shaped crossing daymarks are used to indicate where the channel crosses from one side of the river to the other.
532. The Uniform State Waterway Marking System (USWMS)

from more than one direction, aids to navigation having cardinal significance may be used. The aids conforming to the cardinal system consist of three distinctly colored buoys.
1. A white buoy with a red top must be passed to the south or west of the buoy.
2. A white buoy with a black top must be passed to the north or east of the buoy.
3. A buoy showing alternate vertical red and white stripes indicates that an obstruction to navigation extends from the nearest shore to the buoy and that he must not pass between the buoy and the nearest shore.
The shape of buoys has no significance under the USWMS.
Regulatory buoys are colored white with orange horizontal bands completely around them. One band is at the top of the buoy and a second band just above the waterline of the buoy so that both orange bands are clearly visible.
Geometric shapes colored orange are placed on the white portion of the buoy body. The authorized geometric shapes and meanings associated with them are as follows:
1. A vertical open faced diamond shape means danger.
2. A vertical open faced diamond shape with a cross centered in the diamond means that vessels are excluded from the marked area.
3. A circular shape means that vessels in the marked area are subject to certain operating restrictions.
4. A square or rectangular shape indicates that directions or information is written inside the shape.

This system was developed jointly by the U.S. Coast Guard and state boating administrators to assist the small craft operator in those state waters marked by participating states. The USWMS consists of two categories of aids to navigation. The first is a system of aids to navigation, generally compatible with the Federal lateral system of buoyage, supplementing the federal system in state waters. The other is a system of regulatory markers to warn small craft operator of dangers or to provide general information.
On a well-defined channel, red and black buoys are established in pairs called gates; the channel lies between the buoys. The buoy which marks the left side of the channel viewed looking upstream or toward the head of navigation is black; the buoy which marks the right side of the channel is red.
In an irregularly-defined channel, buoys may be staggered on alternate sides of the channel, but they are spaced at sufficiently close intervals to mark clearly the channel lying between them.
When there is no well-defined channel or when a body of water is obstructed by objects whose nature or location is such that the obstruction can be approached by a vessel

Regulatory markers consist of square and rectangular shaped signs displayed from fixed structures. Each sign is white with an orange border. Geometric shapes with the same meanings as those displayed on buoys are centered on the sign boards. The geometric shape displayed on a regulatory marker tells the mariner if he should stay well clear of the marker or if he may approach the marker in order to read directions.
533. Private Aids To Navigation
A private navigation aid is any aid established and maintained by entities other than the Coast Guard.
The Coast Guard must approve the placement of private navigation aids. In addition, the District Engineer, U.S. Army Corps of Engineers, must approve the placement of any structure, including aids to navigation, in the navigable waters of the U.S.
Private aids to navigation are similar to the aids established and maintained by the U.S. Coast Guard; they are specially designated on the chart and in the Light List. In

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SHORT RANGE AIDS TO NAVIGATION

some cases, particularly on large commercial structures, the aids are the same type of equipment used by the Coast Guard. Although the Coast Guard periodically inspects some private navigation aids, the mariner should exercise special caution when using them.
In addition to private aids to navigation, numerous types of construction and anchor buoys are used in various oil drilling operations and marine construction. These buoys are not charted, as they are temporary, and may not be lighted well or at all. Mariners should give a wide berth to drilling and construction sites to avoid the possibility of fouling moorings. This is a particular danger in offshore oil

fields, where large anchors are often used to stabilize the positions of drill rigs in deep water. Up to eight anchors may be placed at various positions as much as a mile from the drill ship. These may or may not be marked by buoys.
534. Protection By Law
It is unlawful to impair the usefulness of any navigation aid established and maintained by the United States. If any vessel collides with an navigation aid, it is the legal duty of the person in charge of the vessel to report the accident to the nearest U.S. Coast Guard station.

CHAPTER 6

MAGNETIC COMPASS ADJUSTMENT

GENERAL PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT

600. Introduction
This chapter presents information and procedures for magnetic compass adjustment. Sections 601 and 613 cover procedures designed to eliminate compass errors satisfactorily. Refer to Figure 607 for condensed information regarding the various compass errors and their correction.
The term compass adjustment refers to any change of permanent magnet or soft iron correctors to reduce normal compass errors. The term compass compensation refers to any change in the current slupplied to the compass compensating coils to reduce degaussing errors.
601. Adjustment Check-Off List
If the magnetic adjustment necessitates (a) movement of degaussing compensating coils, or (b) a change of Flinders bar length, check also the coil compensation per section 646.
Expeditious compass adjustment depends on the application of the various correctors in an optimum sequence designed to minimize the number of correction steps. Certain adjustments may be made conveniently at dockside, simplifying the at sea adjustment procedures.
Moving the wrong corrector wastes time and upsets all previous adjustments, so be careful to make the correct adjustments. Throughout an adjustment, special care should be taken to pair off spare magnets so that the resultant field about them will be negligible. To make doubly sure that the compass is not affected by a spare magnet’s stray field, keep them at an appropriate distance until they are actually inserted into the binnacle.
A. Dockside tests and adjustments.

f. Alignment of magnets in binnacle. g. Alignment of heeling magnet tube under pivot
point of compass. h. See that corrector magnets are available.
2. Physical checks of gyro, azimuth circle, and peloruses. a. Alignment of peloruses with fore-and-aft line of ship (section 610). b. Synchronize gyro repeaters with master gyro. c. Ensure azimuth circles and peloruses are in good condition.
3. Necessary data. a. Past history or log data which might establish length of Flinders bar (sections 610 and 623). b. Azimuths for date and observer’s position (section 633 and Chapter 17). c. Ranges or distant objects in vicinity if needed (local charts). d. Correct variation (local charts). e. Degaussing coil current settings for swing for residual deviations after adjustment and compensation (ship’s Degaussing Folder).
4. Precautions. a. Determine transient deviations of compass from gyro repeaters, doors, guns, etc. (sections 636 and 639). b. Secure all effective magnetic gear in normal seagoing position before beginning adjustments. c. Make sure degaussing coils are secured before beginning adjustments. Use reversal sequence, if necessary. d. Whenever possible, correctors should be placed symmetrically with respect to the compass.

1. Physical checks on the compass and binnacle. a. Remove any bubbles in compass bowl (section 610). b. Test for moment and sensibility of compass needles (section 610). c. Remove any slack in gimbal arrangement. d. Magnetization check of spheres and Flinders bar (section 610). e. Alignment of compass with fore-and-aft line of ship (section 610).

5. Adjustments. a. Place Flinders bar according to best available information (sections 610, 622 through 625). b. Set spheres at mid-position, or as indicated by last deviation table. c. Adjust heeling magnet, using balanced dip needle if available (section 637).
B. Adjustments at sea. Make these adjustments with the ship on an even keel and steady on each heading. When using
81

82

MAGNETIC COMPASS ADJUSTMENT

the gyro, swing slowly from heading to heading and check

Fore-and-aft and athwartship magnets

Deviation Magnets

Easterly on east and westerly on west.
(+B error)

Westerly on east and easterly on west.
(-B error)

Deviation Spheres

Quadrantial spheres
E. on NE, E. on SE, W. on SW,
and W. on NW. (+D error)

W. on NE, E. on SE, W. on SW, andE. on NW. (-D error)

Deviation change with latitude change
Bar

Flinders bar
E. on E. and W. on W W. on E. and E. on W when sailing toward when sailing toward equator from north equator from north latitude or away from latitude or away from equator to south equator to south latitude. latitude.

No fore and aft magnets in binnacle.

Place magnets red Place magnets red No spheres on

forward.

aft.

binnacle.

Place spheres athwartship.

Place spheres fore and aft.

No bar in holder.

Place required of bar Place required amount

forward.

of bar aft.

Fore and aft magnets red forward.

Raise magnets.

Lower magnets.

Spheres at athwartship position.

Move spheres toward Move spheres

Increase amount of bar Deacrease amount

compass or use

outwards or remove. Bar forward of binnacle. forward.

of bar forward.

larger spheres.

Fore and aft

Lower magnets.

magnets red aft.

Raise magnets.

Spheres at fore and Move spheres

Move spheres toward

aft position.

outward or remove. compass or use

larger spheres.

Decrease amount of Increase amount of

Bar aft of binnacle. bar aft.

bar aft.

Deviation Magnets

Easterly on north Westerly on north

and westerly on and easterly on

south.

south.

(+C error)

(-C error)

Deviation Spheres

E. on N, W. on E, E. on S,
and W. on W. (+E error)

W. on N, E. on E, W. on S,
and E. on W. (-E error)

Bar
Deviation change with latitude

W. on E. and E. on W. E. on E. and W. on W. when sailing toward when sailing toward equator from south equator from south latitude or away from latitude or away from equator to north equator to south latitude latitude.

change

No athwartship Place athwartship Place athwartship No spheres on Place spheres at port Place spheres at

Heeling magnet

magnets

in magnets

red magnets red port. binnacle.

forward and starboard starboard foreward

(Adjust with changes in magnetic latitude)

binnacle.

starboard.

Athwartship magnets starboard.

Raise magnets. red

Athwartship

Lower magnets.

magnets red port.

Lower magnets. Raise magnets.

aft

intercardinal and port aft If compass north is attracted to high side of ship when rolling, raise

positions.

intercardinal

the heeling magnet if red end is up and lower the heeling magnet if blue

positions.

end is up.

Spheres

at Slew

spheres Slew

spheres If compass north is attracted to low side of ship when rolling, lower

athwartship position.

clockwise through required angle.

counter-clockwise through required angle.

the heeling end is up.

magnet

if

red

end

is

up

and

raise

the

heeling

magnet

if

blue

NOTE: Any change in placement of the heeling magnet will affect the

Spheres at fore and Slew spheres counter- Slew spheres

deviations on all headings.

aft position.

clockwise through clockwise through

required angle.

required angle.

Figure 601. Mechanics of magnetic compass adjustment.

gyro error by sun’s azimuth or ranges on each heading to ensure a greater degree of accuracy (section 631). Be sure gyro is set for the mean speed and latitude of the vessel. Note all precautions in section A-4 above. Fly the “OSCAR QUEBEC” international code signal to indicate such work is in progress. Section 631 discusses methods for placing the ship on desired headings.
1. Adjust the heeling magnet while the ship is rolling on north and south magnetic headings until the oscillations of the compass card have been reduced to an average minimum. This step is not required if prior adjustment has been made using a dip needle to indicate proper placement of the heeling magnet.
2. Come to a cardinal magnetic heading, e.g., east (090°). Insert fore-and-aft B magnets, or move the existing B magnets, to remove all deviation.
3. Come to a south (180°) magnetic heading. Insert athwartship C magnets, or move the existing C magnets, to remove all deviation.
4. Come to a west (270°) magnetic heading. Correct half of any observed deviation by moving the B magnets.
5. Come to a north (000°) magnetic heading. Correct

half of any observed deviation by moving the C magnets. The cardinal heading adjustments should now be complete. 6. Come to any intercardinal magnetic heading, e.g., northeast (045°). Correct any observed deviation by moving the spheres in or out. 7. Come to the next intercardinal magnetic heading, e.g., southeast (135°). Correct half of any observed deviation by moving the spheres.
The intercardinal heading adjustments should now be complete, although more accurate results might be obtained by correcting the D error determined from the deviations on all four intercardinal headings, as discussed in section 615.
8. Secure all correctors before swinging for residual deviations.
9. Swing for residual undegaussed deviations on as many headings as desired, although the eight cardinal and intercardinal headings should be sufficient.
10. Should there still be any large deviations, analyze the deviation curve to determine the necessary

MAGNETIC COMPASS ADJUSTMENT

83

corrections and repeat as necessary steps 1 through 9 above.
11. Record deviations and the details of corrector positions on the deviation card to be posted near the compass.
12. Swing for residual degaussed deviations with the degaussing circuits properly energized.
13. Record deviations for degaussed conditions on the deviation card.
The above check-off list describes a simplified method of adjusting compasses, designed to serve as a workable outline for the novice who chooses to follow a step-by-step procedure. The dockside tests and adjustments are essential as a foundation for the adjustments at sea. Neglecting the dockside procedures may lead to spurious results or needless repetition of the procedures at sea. Give careful consideration to these dockside checks prior to making the final adjustment. This will allow time to repair or replace faulty compasses, anneal or replace magnetized spheres or Flinders bars, realign the binnacle, move a gyro repeater if it is affecting the compass, or to make any other necessary preliminary repairs.
Expeditious compass adjustment depends upon the application of the various correctors in a logical sequence so as to achieve the final adjustment with a minimum number of steps. The above check-off list accomplishes this purpose. Figure 607 presents the various compass errors and their correction in condensed form. Frequent, careful observations should be made to determine the constancy of deviations, and results should be systematically recorded. Significant changes in deviation will indicate the need for readjustment.
To avoid Gaussin error (section 636) when adjusting and swinging ship for residuals, the ship should be steady on the desired heading for at least 2 minutes prior to observing the deviation.

the unlike poles will attract each other. Magnetism can be either permanent or induced. A
bar having permanent magnetism will retain its magnetism when it is removed from the magnetizing field. A bar having induced magnetism will lose its magnetism when removed from the magnetizing field. Whether or not a bar will retain its magnetism on removal from the magnetizing field will depend on the strength of that field, the degree of hardness of the iron (retentivity), and also upon the amount of physical stress applied to the bar while in the magnetizing field. The harder the iron, the more permanent will be the magnetism acquired.
603. Terrestrial Magnetism
Consider the earth as a huge magnet surrounded by magnetic flux lines connecting its two magnetic poles. These magnetic poles are near, but not coincidental with, the earth’s geographic poles. Since the north seeking end of a compass needle is conventionally called the north pole, or positive pole, it must therefore be attracted to a south pole, or negative pole.
Figure 603a illustrates the earth and its surrounding magnetic field. The flux lines enter the surface of the earth at different angles to the horizontal, at different magnetic atitudes. This angle is called the angle of magnetic dip, θ, and

602. The Magnetic Compass And Magnetism

The principle of the present day magnetic compass is no different from that of the compasses used by ancient mariners. It consists of a magnetized needle, or an array of needles, allowed to rotate in the horizontal plane. The superiority of the present day compasses over ancient ones results from a better knowledge of the laws of magnetism which govern the behavior of the compass and from greater precision in construction.
Any piece of metal on becoming magnetized will develop regions of concentrated magnetism called poles. Any such magnet will have at least two poles of opposite polarity. Magnetic force (flux) lines connect one pole of such a magnet with the other pole. The number of such lines per unit area represents the intensity of the magnetic field in that area. If two such magnetic bars or magnets are placed close to each other, the like poles will repel each other and

Figure 603a. Terrestrial magnetism.

84

MAGNETIC COMPASS ADJUSTMENT

Figure 603b. Magnetic dip chart, a simplification of chart 30. Figure 603c. Magnetic variation chart, a simplification of chart 42.

MAGNETIC COMPASS ADJUSTMENT

85

increases from 0°, at the magnetic equator, to 90° at the magnetic poles. The total magnetic field is generally considered as having two components: H, the horizontal component; and Z, the vertical component. These components change as the angle θ, changes, such that H is maximum at the magnetic equator and decreases in the direction of either pole; Z is zero at the magnetic equator and increases in the direction of either pole. The values of magnetic dip may be found on Chart 30 (shown simplified in Figure 603b). The values of H and Z may be found on charts 33 and 36.
Since the magnetic poles of the earth do not coincide with the geographic poles, a compass needle in line with the earth’s magnetic field will not indicate true north, but magnetic north. The angular difference between the true meridian (great circle connecting the geographic poles) and the magnetic meridian (direction of the lines of magnetic flux) is called variation. This variation has different values at different locations on the earth. These values of magnetic variation may be found on Chart 42 (shown simplified in Figure 603c), on pilot charts, and, on the compass rose of navigational charts. The variation for most given areas undergoes an annual change, the amount of which is also noted on charts.
604. Ship’s Magnetism
A ship under construction or major repair will acquire permanent magnetism due to hammering and jarring while sitting stationary in the earth’s magnetic field. After launching, the ship will lose some of this original magnetism as a result of vibration and pounding in varying magnetic fields, and will eventually reach a more or less stable magnetic condition. The magnetism which remains is the permanent magnetism of the ship.
The fact that a ship has permanent magnetism does not mean that it cannot also acquire induced magnetism when placed in the earth’s magnetic field. The magnetism induced in any given piece of soft iron is a function of the field intensity, the alignment of the soft iron in that field, and the physical properties and dimensions of the iron. This induced magnetism may add to, or subtract from, the permanent magnetism already present in the ship, depending on how the ship is aligned in the magnetic field. The softer the iron, the more readily it will be magnetized by the earth’s magnetic field, and the more readily it will give up its magnetism when removed from that field.
The magnetism in the various structures of a ship, which tends to change as a result of cruising, vibration, or aging, but which does not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. This magnetism, at any instant, is part of the ship’s permanent magnetism, and consequently must be corrected by permanent magnet correctors. It is the principal cause of deviation changes on a magnetic compass. Subsequent reference to permanent magnetism will refer to the apparent permanent magnetism which includes the existing permanent and subpermanent magnetism.

A ship, then, has a combination of permanent, subpermanent, and induced magnetism. Therefore, the ship’s apparent permanent magnetic condition is subject to change from deperming, excessive shocks, welding, and vibration. The ship’s induced magnetism will vary with the earth’s magnetic field strength and with the alignment of the ship in that field.
605. Magnetic Adjustment
A rod of soft iron, in a plane parallel to the earth’s horizontal magnetic field, H, will have a north pole induced in the end toward the north geographic pole and a south pole induced in the end toward the south geographic pole. This same rod in a horizontal plane, but at right angles to the horizontal earth’s field, would have no magnetism induced in it, because its alignment in the magnetic field is such that there will be no tendency toward linear magnetization, and the rod is of negligible cross section. Should the rod be aligned in some horizontal direction between those headings which create maximum and zero induction, it would be induced by an amount which is a function of the angle of alignment. If a similar rod is placed in a vertical position in northern latitudes so as to be aligned with the vertical earth’s field Z, it will have a south pole induced at the upper end and a north pole induced at the lower end. These polarities of vertical induced magnetization will be reversed in southern latitudes.
The amount of horizontal or vertical induction in such rods, or in ships whose construction is equivalent to combinations of such rods, will vary with the intensity of H and Z, heading and heel of the ship.
The magnetic compass must be corrected for the vessel’s permanent and induced magnetism so that its operation approximates that of a completely nonmagnetic vessel. Ship’s magnetic conditions create magnetic compass deviations and sectors of sluggishness and unsteadiness. Deviation is defined as deflection right or left of the magnetic meridian. Adjusting the compass consists of arranging magnetic and soft iron correctors about the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship.
The total permanent magnetic field effect at the compass may be broken into three components, mutually 90° apart, as shown in Figure 605a.
The vertical permanent component tilts the compass card, and, when the ship rolls or pitches, causes oscillating deflections of the card. Oscillation effects which accompany roll are maximum on north and south compass headings, and those which accompany pitch are maximum on east and west compass headings.
The horizontal B and C components of permanent magnetism cause varying deviations of the compass as the ship swings in heading on an even keel. Plotting these deviations against compass heading yields the sine and cosine curves shown in Figure 605b. These deviation curves are called semicircular curves because they reverse direction by 180°.
A vector analysis is helpful in determining deviations or

86

MAGNETIC COMPASS ADJUSTMENT

Figure 605a. Components of permanent magnetic field.

the strength of deviating fields. For example, a ship as shown in Figure 605c on an east magnetic heading will subject its compass to a combination of magnetic effects; namely, the earth’s horizontal field H, and the deviating field B, at right angles to the field H. The compass needle will align itself in the resultant field which is represented by the vector sum of H and B, as shown. A similar analysis will reveal that the resulting directive force on the compass

would be maximum on a north heading and minimum on a south heading because the deviations for both conditions are zero.
The magnitude of the deviation caused by the permanent B magnetic field will vary with different values of H; hence, deviations resulting from permanent magnetic fields will vary with the magnetic latitude of the ship.

Figure 605b. Permanent magnetic deviation effects.

MAGNETIC COMPASS ADJUSTMENT

87

Figure 605c. General force diagram.
606. Induced Magnetism And Its Effects On The Compass
Induced magnetism varies with the strength of the sur-

rounding field, the mass of metal, and the alignment of the metal in the field. Since the intensity of the earth’s magnetic field varies over the earth’s surface, the induced magnetism in a ship will vary with latitude, heading, and heel of the ship.
With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, will create deviations which plot as a semicircular deviation curve. This is true because the vertical induction changes magnitude and polarity only with magnetic latitude and heel, and not with heading of the ship. Therefore, as long as the ship is in the same magnetic latitude, its vertical induced pole swinging about the compass will produce the same effect on the compass as a permanent pole swinging about the compass.
The earth’s field induction in certain other unsymmetrical arrangements of horizontal soft iron create a constant A deviation curve. In addition to this magnetic A error, there are constant A deviations resulting from: (1) physical misalignments of the compass, pelorus, or gyro; (2) errors in calculating the sun’s azimuth, observing time, or taking bearings.
The nature, magnitude, and polarity of all these induced effects are dependent upon the disposition of metal, the symmetry or asymmetry of the ship, the location of the binnacle, the strength of the earth’s magnetic field, and the angle of dip.

Coefficient

Type deviation curve

A

Constant.

Compass headings of maximum deviation
Same on all.

Causes of such errors
Human-error in calculations _ _ _ _ _ _ _ _ _ _ _ _ Physical-compass, gyro, pelorus alignment _ _ _ _ _ _ _ Magnetic-unsymmetrical arrangements of horiz. soft iron.

Correctors for such errors
Check methods and calculations _ _ _ Check alignments _ _ _ _ _ _ _ _ Rare arrangement of soft iron rods.

Magnetic or compass headings on which to apply correctors
Any.

B

Semicircular
sinφ .

090˚ 270˚

Fore-and-aft component of permanent magnetic field_ _ _ Fore-and-aft B magnets _ _ _ _ _ _ Induced magnetism in unsymmetrical vertical iron forward or Flinders bar (forward or aft) _ _ _ _ 090˚ or 270˚.
aft of compass.

C

Semicircular
cosφ .

000˚ 180˚

Athwartship component of permanent magnetic field- - - - - - - Athwartship C magnets _ _ _ _ _ _ Induced magnetism in unsymmetrical vertical iron port or Flinders bar (port or starboard) _ _ _ 000˚ or 180˚.
starboard of compass.

Quadrantral

045˚

D

sin 2φ .

135˚ 225˚

Induced magnetism in all symmetrical arrangements of horizontal soft iron.

315˚

Spheres on appropriate axis. (athwartship for +D) (fore and aft for -D) See sketch a

045˚, 135˚, 225˚, or 315˚.

E

Quadrantral

000˚ 090˚

cos 2φ . 180˚

Induced magnetism in all unsymmetrical arrangements of horizontal soft iron.

Spheres on appropriate axis. (port fwd.-stb’d for +E) (stb’d fwd.-port aft for -E)

270˚

See sketch b

000˚, 090˚, 180˚, or 270˚.

Heeling

Oscillations with roll or pitch. Deviations with constant list.

000˚
} 180˚

roll

} 090˚ pitc

270˚ h

Change in the horizontal component of the induced or permanent magnetic fields at the compass due to rolling or pitching of the ship.

Heeling magnet (must be readjusted for latitude changes).

090˚ or 270˚ with dip needle. 000˚ or 180˚ while rolling.

Deviation = A + B sinφ + C cosφ + D sin2φ + E cos2φ (φ = compass heading)

Figure 607. Summary of compass errors and adjustments.

88

MAGNETIC COMPASS ADJUSTMENT

Certain heeling errors, in addition to those resulting from permanent magnetism, are created by the presence of both horizontal and vertical soft iron which experience changing induction as the ship rolls in the earth’s magnetic field. This part of the heeling error will naturally change in magnitude with changes of magnetic latitude of the ship. Oscillation effects accompanying roll are maximum on north and south headings, just as with the permanent magnetic heeling errors.
607. Adjustments And Correctors
Since some magnetic effects are functions of the vessel’s magnetic latitude and others are not, each individual effect should be corrected independently. Furthermore, to make the corrections, use (1) permanent magnet correctors to compensate for permanent magnetic fields at the compass, and (2) soft iron correctors to compensate for induced magnetism. The compass binnacle provides support for both the compass and such correctors. Typical binnacles hold the following correctors:
1. Vertical permanent heeling magnet in the central

vertical tube. 2. Fore-and-aft B permanent magnets in their trays. 3. Athwartship C permanent magnets in their trays. 4. Vertical soft iron Flinders bar in its external tube. 5. Soft iron quadrantal spheres.
The heeling magnet is the only corrector which corrects for both permanent and induced effects. Therefore, it must be adjusted occasionally for changes in ship’s latitude. However, any movement of the heeling magnet will require readjustment of other correctors.
Figure 607 summarizes all the various magnetic conditions in a ship, the types of deviation curves they create, the correctors for each effect, and headings on which each corrector is adjusted. Apply the correctors symmetrically and as far away from the compass as possible. This preserves the uniformity of magnetic fields about the compass needle array.
Fortunately, each magnetic effect has a slightly different characteristic curve. This makes identification and correction convenient. Analyzing a complete deviation curve for its different components allows one to anticipate the necessary corrections.

COMPASS OPERATION

608. Effects Of Errors On The Compass
An uncorrected compass suffers large deviations and sluggish, unsteady operation. These conditions may be associated with the maximum and minimum directive force acting on the compass. The maximum deviation occurs at the point of average directive force; and the zero deviations occur at the points of maximum and minimum directive force.
Applying correctors to reduce compass deviation effects compass error correction. Applying correctors to equalize the directive forces across the compass position could also effect compass correction. The deviation method is most often used because it utilizes the compass itself as the correction indicator. Equalizing the directive forces would require an additional piece of test and calibration equipment.
Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the earth’s directive force, H. This condition will not only create sluggish and unsteady sectors, but may even freeze the compass to one reading or to one quadrant, regardless of the heading of the ship. Should the compass become so frozen, the polarity of the magnetism which must be attracting the compass needles is indicated; hence, correction may be effected simply by the application of permanent magnet correctors, in suitable quantity to neutralize this magnetism. Whenever such adjustments are made, it would be well to have the ship placed on a heading such that the unfreezing of the compass needles will be immediately evident. For exam-

ple, a ship whose compass is frozen to a north reading would require fore-and-aft B corrector magnets with the positive ends forward in order to neutralize the existing negative pole which attracted the compass. If made on an east heading, such an adjustment would be practically complete when the compass card was freed to indicate an east heading.
609. Reasons For Correcting Compass
There are several reasons for correcting the errors of the magnetic compass:
1. It is easier to use a magnetic compass if the deviations are small.
2. Even known and compensated for deviation introduces error because the compass operates sluggishly and unsteadily when deviation is present.
3. Even though the deviations are compensated for, they will be subject to appreciable change as a function of heel and magnetic latitude.
Once properly adjusted, the magnetic compass deviations should remain constant until there is some change in the magnetic condition of the vessel resulting from magnetic treatment, shock from gunfire, vibration, repair, or structural changes. Frequently, the movement of nearby guns, doors, gyro repeaters, or cargo affects the compass greatly.

MAGNETIC COMPASS ADJUSTMENT

89

DETAILED PROCEDURES FOR COMPASS ADJUSTMENT

610. Dockside Tests And Adjustments
Section 601, the Adjustment Checkoff List, gives the physical checks required before beginning an adjustment. The adjustment procedure assumes that these checks have been completed. The navigator will avoid much delay by making these checks before starting the magnet and soft iron corrector adjustments. The most important of these checks are discussed below.
Should the compass have a small bubble, add compass fluid through the filling plug on the compass bowl. If an appreciable amount of compass liquid has leaked out, check the sealing gasket and filling plug for leaks.
Take the compass to a place free from all magnetic influences except the earth’s magnetic field for tests of moment and sensibility. These tests involve measurements of the time of vibration and the ability of the compass card to return to a consistent reading after deflection. These tests will indicate the condition of the pivot, jewel, and magnetic strength of the compass needles.
Next, check the spheres and Flinders bar for residual magnetism. Move the spheres as close to the compass as possible and slowly rotate each sphere separately. Any appreciable deflection (2° or more) of the compass needles resulting from this rotation indicates residual magnetism in the spheres. The Flinders bar magnetization check is preferably made with the ship on an east or west compass heading. To make this check: (a) note the compass reading with the Flinders bar in the holder; (b) invert the Flinders bar in the holder and again note the compass reading. Any appreciable difference (2° or more) between these observed readings indicates residual magnetism in the Flinders bar. Spheres or Flinders bars which show signs of such residual magnetism should be annealed, i.e., heated to a dull red and allowed to cool slowly.
Correct alignment of the lubber’s line of the compass, gyro repeater, and pelorus with the fore-and-aft line of the ship is important. Any misalignment will produce a constant error in the deviation curve. All of these instruments may be aligned correctly with the fore-andaft line of the ship by using the azimuth circle and a metal tape measure. Should the instrument be located on the centerline of the ship, a sight is taken on a mast or other object on the centerline. If the instrument is not on the centerline, measure the distance from the centerline of the ship to the center of the instrument. Mark this distance off from the centerline forward or abaft the compass and place reference marks on the deck. Take sights on these marks.
Align the compass so that the compass’ lubber’s line is parallel to the fore-and-aft line of the ship. Steering compasses may occasionally be deliberately misaligned in order to correct for any magnetic A error present, as discussed in

section 611. Adjust the Flinders bar first because it is subject to
induction from several of the correctors and its adjustment is not dependent on any single observation. To adjust the Flinders bar, use one of the following methods:
1. Use deviation data obtained at two different magnetic latitudes to calculate the proper length of Flinders bar for any particular compass location. Sections 622 through 624 contain details on acquiring the data and making the required calculations.
2. If the above method is impractical, set the Flinders bar length by:
a. Using a Flinders bar length determined by other ships of similar structure.
b. Studying the arrangement of masts, stacks, and other vertical structures and estimating the Flinders bar length required.
If these methods are not suitable, omit the Flinders bar until the required data are acquired.
The iron sections of Flinders bar should be continuous and placed at the top of the tube with the longest section at the top. Wooden spacers are used at the bottom of the tube.
Having adjusted the length of Flinders bar, place the spheres on the bracket arms at an approximate position. If the compass has been adjusted previously, place the spheres at the position indicated by the previous deviation table. In the event the compass has never been adjusted, place the spheres at the midpoint on the bracket arms.
The next adjustment is the positioning of the heeling magnet using a properly balanced dip needle. Section 637 discusses this procedure.
These three dockside adjustments (Flinders bar, quadrantal spheres, and heeling magnet) will properly establish the conditions of mutual induction and shielding of the compass. This minimizes the steps required at sea to complete the adjustment.
611. Expected Errors
Figure 607 lists six different coefficients or types of deviation errors with their causes and corresponding correctors. A discussion of these coefficients follows:
The A error is caused by the miscalculation of azimuths or by physical misalignments rather than magnetic effects of unsymmetrical arrangements of horizontal soft iron. Thus,

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MAGNETIC COMPASS ADJUSTMENT

checking the physical alignments at dockside and making careful calculations will minimize the A error. Where an azimuth or bearing circle is used on a standard compass to determine deviations, any observed A error will be solely magnetic A error because such readings are taken on the face of the compass card rather than at the lubber’s line of the compass. On a steering compass where deviations are obtained by a comparison of the compass lubber’s line reading with the ship’s magnetic heading, as determined by pelorus or gyro, any observed A error may be a combination of magnetic A and mechanical A (misalignment). These facts explain the procedure in which only mechanical A is corrected on the standard compass, by realignment of the binnacle, and both mechanical A and magnetic A errors are corrected on the steering compass by realignment of the binnacle. On the standard compass, the mechanical A error may be isolated from the magnetic A error by making the following observations simultaneously:
1. Record a curve of deviations by using an azimuth (or bearing) circle. Any A error found will be solely magnetic A.
2. Record a curve of deviations by comparison of the compass lubber’s line reading with the ship’s magnetic heading as determined by pelorus or by gyro. Any A error found will be a combination of mechanical A and magnetic A.
3. The mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total A found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount. It is neither convenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyro may be removed by rotating the binnacle in the proper direction.

The B error results from both the fore-and-aft permanent magnetic field across the compass and a resultant unsymmetrical vertical induced effect forward or aft of the compass. The former is corrected by the use of fore-and-aft B magnets, and the latter is corrected by the use of the Flinders bar forward or aft of the compass. Because the Flinders bar setting is a dockside adjustment, any remaining B error is corrected by the use of fore-and-aft B magnets.
The C error results from the athwartship permanent magnetic field across the compass and a resultant unsymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship C magnets, and the latter by the use of the Flinders bar to port or starboard of the compass. Because the vertical induced effect is very rare, the C error is corrected by athwartship C magnets only.
The D error is due only to induction in the symmetrical arrangements of horizontal soft iron, and requires correction by spheres, generally athwartship of the compass.
E error of appreciable magnitude is rare, since it is caused by induction in the unsymmetrical arrangements of horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres, as described in section 620.
As stated previously, the heeling error is adjusted at dockside with a balanced dip needle (see section 637).
As the above discussion points out, certain errors are rare and others are corrected at dockside. Therefore, for most ships, only the B, C, and D errors require at sea correction. These errors are corrected by the fore-and-aft B magnets, athwartship C magnets, and quadrantal spheres respectively.
612. Study Of Adjustment Procedure
Inspecting the B, C, and D errors pictured in Figure 612a demonstrates a definite isolation of deviation effects on cardinal compass headings.

Figure 612a. B, C, and D deviation effects.

MAGNETIC COMPASS ADJUSTMENT

91

Figure 612b. A and B deviation.

For example, on 090° or 270° compass headings, the only deviation which is effective is that due to B. This isolation, and the fact that the B effect is greatest on these two headings, make these headings convenient for B correction. Correction of the B deviation on a 090° heading will correct the B deviation on the 270° heading by the same amount but in the opposite direction and naturally, it will not change the deviations on the 000° and 180° headings, except where B errors are large. However, the total deviation on all the intercardinal headings will be shifted in the same direction as the adjacent 090° or 270° deviation correction, but only by seven-tenths (0.7) of that amount, since the sine of 45° equals 0.707. The same convenient isolation of effects and corrections of C error will also change the deviations on all the intercardinal headings by the seven-tenths rule.
Note that only after correcting the B and C errors on the cardinal headings, and consequently their proportional values of the total curve on the intercardinal headings, can the D error be observed separately on any of the intercardinal headings. The D error may then be corrected by use of the spheres on any intercardinal heading. Correcting D error will, as a rule, change the deviations on the intercardinal headings only, and not on the cardinal headings. Only when the D error is excessive, the spheres are magnetized, or the permanent magnet correctors are so close as to create excessive induction in the spheres will there be a change in the deviations on cardinal headings as a result of sphere adjustments. Although sphere correction does not generally correct deviations on cardinal headings, it does improve compass stability on these headings.
If it were not for the occasional A or E errors, adjusting observed deviations to zero on two adjacent cardinal headings and then on the intermediate intercardinal heading would be sufficient. However, Figure 612b, showing a combination of A and B errors, illustrates why the adjusting procedure must include correcting deviations on more than the three essential headings.
Assuming no A error existed in the curve illustrated in Figure 612b, and the total deviation of 6° E on the 090° heading were corrected with B magnets, the error on the 270° heading would be 4° E due to B overcorrection. If this 4° E error were taken out on the 270° heading, the error on

the 090° heading would then be 4° E due to B undercorrection. To eliminate this endlessly iterative process and correct the B error to the best possible flat curve, split this 4° E difference, leaving 2° E deviation on each opposite heading. This would, in effect correct the B error, leaving only the A error of 2° E which must be corrected by other means. It is for this reason that, (1) splitting is done between the errors noted on opposite headings, and (2) good adjustments entail checking on all headings rather than on the fundamental three.
613. Adjustment Procedures At Sea
Before proceeding with the adjustment at sea the following precautions should be observed:
1. Secure all effective magnetic gear in the normal seagoing position.
2. Make sure the degaussing coils are secured, using the reversal sequence, if necessary (See section 643).
The adjustments are made with the ship on an even keel, swinging from heading to heading slowly, and after steadying on each heading for at least 2 minutes to avoid Gaussin error.
Most adjustments can be made by trial and error, or by routine procedure such as the one presented in section 601. However, the procedures presented below provide analytical methods in which the adjuster is always aware of the errors’ magnitude on all headings as a result of his movement of the different correctors.
Analysis Method. A complete deviation curve can be taken for any given condition, and an estimate made of all the approximate coefficients. See section 615. From this estimate, the approximate coefficients are established and the appropriate corrections are made with reasonable accuracy on a minimum number of headings. If the original deviation curve has deviations greater than 20°, rough adjustments should be made on two adjacent cardinal headings before recording curve data for such analysis. The mechanics of

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MAGNETIC COMPASS ADJUSTMENT

Heading by compass
Degrees 000 045 090 135 180 225 270 315

1
Original deviation
curve
Degrees 10.5 E. 20.0 E. 11.5 E. 1.2 W. 5.5 W. 8.0 W. 12.5 W. 6.8 W.

2
Anticipated curve after
first correcting A = 1.0° E
Degrees
9.5 E. 19.0 E. 10.5 E. 2.2 W. 6.5 W. 9.0 W. 13.5 W. 7.8 W.

3
Anticipated curve after
next correcting B = 12.0° E
Degrees
9.5 E. 10.6 E. 1.5 W. 10.6 W. 6.5 W. 0.6 W. 1.5 W. 0.6 E.

4
Anticipated curve after
next correcting C = 8.0° E
Degrees
1.5 E. 5.0 E. 1.5 W. 5.0 W. 1.5 E. 5.0 E. 1.5 W. 5.0 W.

5
Anticipated curve after
next correcting D = 5.0° E
Degrees
1.5 E. 0.0 1.5 W. 0.0 1.5 E. 0.0 1.5 W. 0.0

Figure 613a. Tabulating anticipated deviations.

6
Anticipated curve after
next correcting E = 1.5° E
Degrees
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

applying correctors are presented in Figure 601. A method of tabulating the anticipated deviations after each correction is illustrated in Figure 613a. The deviation curve used for illustration is the one which is analyzed in section 615. Analysis revealed these coefficients:

A = 1.0˚ E

B

= 12.0˚ E

C

= 8.0˚ E

D = 5.0˚ E

E

= 1.5˚ E

One-Swing Method. More often it is desirable to begin adjustment immediately, eliminating the original swing for deviations and the estimate of approximate coefficients. In this case the above problem would be solved by tabulating data and anticipating deviation changes as the corrections are made. Figure 613b illustrates this procedure. Note that a new column of values is started after each change is made. This method of tabulation enables the adjuster to calculate the new residual deviations each time a corrector is changed, so that a

record of deviations is available at all times during the swing. Arrows indicate where each change is made.
Since the B error is generally greatest, it is corrected first. Therefore, on a 090° heading the 11.5° E deviation is corrected to approximately zero by using fore-and-aft B magnets. A lot of time need not be spent trying to reduce this deviation to exactly zero since the B coefficient may not be exactly 11.5° E, and some splitting might be desirable later. After correcting on the 090° heading, the swing would then be continued to 135° where a 9.2° W error would be observed. This deviation is recorded, but no correction is made because the quadrant error is best corrected after the deviations on all four cardinal headings have been corrected. The deviation on the 180° heading would be observed as 5.5° W. Since this deviation is not too large and splitting may be necessary later, it need not be corrected at this time. Continuing the swing to 225° a 0.0° deviation would be observed and recorded. On the 270° heading the observed error would be 1.0° W, which is compared with 0.0° deviation on the opposite 090° heading. This could be split, leaving 0.5° W deviation on both 090° and 270°, but since this is so small it may be left uncorrected. On 315˚ the observed deviation would be 1.2° E. At 000° a deviation of 10.5°

Heading

First observation

Degrees
000 045 090 135 180 225 270 315

Degrees
... ... 11.5 E.→ ... ... ... ... ...

Observed deviations
after correcting B = 11.5° E
Degrees
10.5 E.→ ...
0.0 9.2 W. 5.5 W. 0.0 1.0 W. 1.2 E.

Anticipated deviations
after correcting C = 8.0° E
Degrees
2.5 E. 6.4 E.→ 0.0 3.6 W. 2.5 E. 5.6 E. 1.0 W. 4.4 W.

Anticipated deviations
after correcting D =5.0° E
Degrees
2.5 E. 1.4 E.→ 0.0 1.4 E. 2.5 E. 0.6 E. 1.0 W. 0.6 E.

Anticipated deviations
after correcting A = 1.0° E
Degrees
1.5 E. 0.4 E. 1.0 W.→ 0.4 E. 1.5 E. 0.4 W. 2.0 W. 0.4 W.

Anticipated deviations
after correcting E = 1.5° E
Degrees
0.0 0.4 E. 0.5 E. 0.4 E. 0.0 0.4 W. 0.5 W. 0.4 W.

Figure 613b. Tabulating anticipated deviations by the one-swing.