How to Read a Nautical Chart, 2nd Edition (Includes ALL of Chart #1): A Complete Guide to Using and Understanding Electronic and Paper Charts (INTERNATIONAL MARINE-RMP) - Softcover

Calder, Nigel

Softcover

ISBN 10: 0071779825 ISBN 13: 9780071779821

Publisher: International Marine/Ragged Mountain Press, 2012

Synopsis

Authoritative, practical, and hands-on information on reading and relying on electronic and paper nautical charts

The classic How to Read a Nautical Chart explains every aspect of electronic and paper nautical charts: how a chart is assembled, how to gauge the accuracy of chart data, how to read charts created by other governments, how to use information such as scale, projection technique and datum that every chart contains; how not to get fooled or run aground by overzooming. Nigel Calder teaches you how to squeeze every ounce of information out of a nautical chart (on your GPS, chartplotter, or nav station) and understand the limits of accuracy for all charts, paper and electronic, raster and vector.

This much-awaited second edition addresses the changes in the world of electronic charting, integrated onboard navigation systems, as well as radar overlays and AIS and their interfacing with charts. A new chapter onthe Wiki Revolution explores the mechanisms allowing you to incorporate user-generated content into navigational products and share this content with others―harnessing a user-base never before possible. Calder also explores how 3D-technology and real-time depth and weather information is creating interactive charting capacities that are fundamentally changing how we navigate. Calder's unique, practical insights will help you to navigate safely for years to come.

About this title

About the Author

Niger Calder is the author of Marine Diesel Engines (1987, 1991) and Boatowner's Mechanical and Electrical Manual (1989, 1995), the success of which has made him the most sought-after marine how-to writer in the U.S. and U.K. He has published more than 800 magazine articles in SAIL, Cruising World, Ocean Navigator, Yachting World, etc., and lectures on cruising to sell-out crowds. He is also the author of The Cruising Guide to The Northwest Caribbean (IM, 1991) and Cuba: A Cruising Guide (Imray, 1996). He will be testing his newest ideas with his family during a six-month cruise to the Caribbean and South America this winter.

Excerpt. © Reprinted by permission. All rights reserved.

HOW TO READ A NAUTICAL CHART

A Complete Guide to Using and Understanding Electronic and Paper Charts

By Nigel Calder

The McGraw-Hill Companies, Inc.

Copyright © 2012 Nigel Calder
All rights reserved.

ISBN: 978-0-07-177982-1

Contents

Acknowledgments and Art Credits
Introduction
PART 1. THE LIMITS OF ACCURACY
Chapter 1. Fundamental Chart-Making Concepts
Chapter 2. Horizontal Chart Accuracy
Chapter 3. Vertical Chart Accuracy
Chapter 4. Regulations and Revolution
PART 2. SYMBOLOGY
Chapter 5. Introduction to INT-1
Chapter 6. Topography
Chapter 7. Hydrography
Chapter 8. Aids and Services
PART 3. APPENDIX
Index to INT-1
Glossary and Acronyms
Common Chart Abbreviations
Bibliography
General Index

Excerpt

CHAPTER 1

Fundamental Chart-Making Concepts

UNTIL RECENTLY, there has been little need for chart users to understand thetechnology of chart-making, particularly its limitations, because the tools usedby navigators to determine the position of their vessels were inherently lessaccurate than those used to conduct and display the surveys on which charts arebased. Realizing the limits of accuracy of their tools, navigators tended to bea cautious crowd, giving hazards a wide berth and typically taking proactivemeasures to build in an extra margin of safety for errors and unforeseen events.

Knowing this, and knowing that navigation in inshore waters was by reference tolandmasses and not astronomical fixes, surveyors were more concerned withdepicting an accurate relationship of soundings and hydrographic featuresrelative to the local landmass (coastline) than they were with absolute accuracyrelative to latitude and longitude. The surveyor's maxim was that it is muchmore important to determine an accurate least depth over a shoal or danger thanto determine its geographical position with certainty. Similarly, thecartographer, when showing an area containing many dangers (such as a rockyoutcrop), paid more attention to bringing the area to the attention of thenavigator, so it could be avoided by a good margin, than to accurately showingevery individual rock in its correct position.

All this changed with the advent of satellite-based navigationsystems—notably the global positioning system (GPS). Now a boat's position(latitude and longitude) can be fixed with near-pinpoint accuracy and, in thecase of electronic navigation, accurately displayed on a chart in real time.This encourages many navigators (myself included) to "cut corners" more closelythan they would have done in the past. With such an attitude, it is essentialfor the navigator to grasp both the accuracy with which a fix can be plotted(whether manually or electronically) and the limit of accuracy of the chartitself—together they determine the extent to which it is possible to cutcorners in safety.

The next chapter discusses factors that affect the limits of chart accuracy.However, I first want to explore the extent to which electronic navigationdevices actually give us the plotting accuracy we believe they do. This is bestdone by understanding the basic concepts of mapmaking and chart-making.

A Little History

As early as the third century b.c., Erastothenes and other Greeks establishedthat the world is a sphere, created the concepts of latitude and longitude, anddeveloped basic mapmaking skills. It was not until the sixteenth century a.d.that there were any advances in mapmaking techniques, which occurred largely asa result of steady improvements in the equipment and methods used for makingprecise astronomical observations and for measuring distances and changes inelevation on the ground. From this time, instruments were available formeasuring angles with great accuracy.

The core surveying methodology that developed is noteworthy because it remainedessentially unchanged until recent decades—for both cartographic andinshore hydrographic surveys—and is the basis of many of the charts westill use. A survey started from a single point whose latitude and longitudewere established by astronomical observations. For accurate surveys, theseobservations required heavy, bulky, and expensive equipment, as well as multipleobservations by highly trained observers over a considerable period of time.From the starting point, a long baseline was precisely measured using carefullycalibrated wooden or metal rods or chains. The surveyors measured all changes invertical elevation in order to be able to discount the effects of them on thehorizontal distances covered. In this way, a precise log of horizontal distanceswas maintained, resulting in baseline measurements that were accurate toinches—sometimes over a distance of many miles. The process was slow andpainstaking, and often took years to complete.

Once a baseline had been established, angular measurements were taken from bothends to a third position. Knowing the length of the baseline and the two angles,spherical trigonometry established the distances to the third point withouthaving to make field measurements. The sides of the triangle thus establishedwere now used as fresh baselines to extend the survey, again without having tomake actual distance measurements in the field. The measured baselines plus theprocess of triangulation provided the horizontal distances on the ground. Withone or more precise astronomical observations at a different point to theoriginal one, it was possible to mathematically establish a latitude andlongitude framework and apply it to the results of the survey—there was noneed to obtain astronomical fixes for all the intermediate points, therebyavoiding the time, expense, and difficulties involved.

By the seventeenth century, it was possible to make sufficiently accurateastronomical observations and distance measurements to discover that in one partof the world a degree of latitude as measured astronomically (i.e., withreference to the stars) does not cover the same distance on the ground as itdoes in another part of the world. This would be impossible if the world were aperfect sphere.

From Sphere to Ellipsoid

How to model this nonspherical world? This was more than an academic question.To make maps, national surveyors now universally used an astronomicallydetermined starting point and a measured baseline, working away from thebeginning point by the process of triangulation (see art page 13).

As the surveyors progressed farther afield, if the mapped latitudes andlongitudes were to be kept in sync with the occasional astronomical observations(i.e., real-life latitudes and longitudes), there had to be a model showing therelationship between the distance on the ground and latitude and longitude, andindicating how this relationship changed as the surveyors moved away from theirastronomically determined starting point. This model had to be such that withavailable trigonometrical and computational methods, the mapmakers could adjusttheir data to accurately calculate changing latitudes and longitudes oversubstantial distances—in other words, the model had to be mathematicallypredictable.

The model that was adopted, and which is used to this day even with satellite-based mapmaking and navigation, is an ellipsoid (also called aspheroid). In essence, an ellipsoid is nothing more than a flattenedsphere, characterized by two measurements: its radius at the equator and thedegree of flattening at the poles. Clearly, the key questions become: What isthis radius, and what is the degree of flattening?

During the nineteenth century, the continents were first accurately mapped basedon this concept of the world as an ellipsoid. For each of the great surveys,preliminary work extending over years used astronomical observations andmeasured baselines to establish the key dimensions of the ellipsoid that was tounderlie the survey. In the United Kingdom, a geodesist (a person whodoes this type of research) named Sir George Airy developed an ellipsoid (knownas Airy 1830) that became the basis for an incredibly detailed survey of theBritish Isles. His ellipsoid is still used today (2012) for the British Isles,since it fits the actual shape of this part of the world very well (better thanmodern satellite-derived ellipsoids, which are described later in this chapter).

Using this ellipsoid, the surveyors commenced at a precisely determinedastronomical point on Salisbury Plain, measured a baseline, and triangulatedtheir way across the British Isles. The accuracy of the survey work and theellipsoid was such that when western Ireland was reached decades later, and theoriginal baseline was checked by computation from the Irish baseline 350 milesaway, the two values differed by only 5 inches!

Another British geodesist, Alexander Clarke, went to the United States and wasinstrumental in developing the ellipsoid that has underlain the mapping of NorthAmerica. Known as the Clarke 1866 ellipsoid, it was the basis of mapmaking andchart-making on the North American continent until the advent ofsatellite-derived ellipsoids. Later, Clarke developed an ellipsoid for mappingFrance and Africa (Clarke 1880).

Using the Clarke 1866 ellipsoid, and commencing at a single astronomicallyderived point and a measured baseline at the Meades Ranch in Osborne County,Kansas, the American surveyors from the U.S. Coast and Geodetic Survey (now theNational Geodetic Survey) fanned out, establishing triangulation points andmapping the entire continent as they went. This combination of an underlyingellipsoid, a specific astronomically determined starting point, and a measuredbaseline, together with some clever mathematics, is known as a geodeticdatum; in this case, it is now known as the North American Datum of 1927(NAD 27). Such is the accuracy of the NAD 27 surveys and the correlation ofthe Clarke 1866 ellipsoid with the real world that at the margins of the survey(the northeast and northwest United States—those areas in the lower forty-eight states farthest from the starting point), the discrepancies between mappedand astronomically derived latitudes and longitudes are no more than 40 to 50meters (130–165 ft.).

From Ellipsoid to Geoid

By the end of the nineteenth century, there were numerous ellipsoids in use, allof them differing slightly from one another. This raised another interestingquestion: Surely, they couldn't all be correct, or could they?

The answer lies in a more sophisticated understanding of our planet. Theindividual ellipsoids closely model the shape of the world in the areas in whichthe surveys were conducted, producing a close correlation between mapped andastronomically derived positions, even at the margins of the survey.Nevertheless, although these ellipsoids are based on very accurate measurementsover large areas of land, these are still only small areas of the world. Whenextrapolated to the globe as a whole, the ellipsoids produce increasinglyserious discrepancies between ellipsoid-derived latitudes and longitudes andastronomically derived positions. Geodesists realized that not only is the worldnot a sphere, but it is also not an ellipsoid. In fact, it does not have ageometrically uniform shape at all, but rather has numerous irregular humps andhollows.

Another concept was needed to deal with this shape. It is the geoid, which isdefined as the real shape of the surface of the world if we discount allelevations above sea level. In other words, if we were to bulldoze the mountainsand valleys to sea level, we would have the geoid. In effect, this is the two-dimensional world as surveyed by mapmakers because the vertical element in theearth's topography is discounted when measuring baselines and otherdistances—which are all painstakingly reduced to the horizontal, using sealevel as the base elevation. Whereas an ellipsoid is a mathematically definedregular surface, the geoid is a very irregular (mathematically unpredictable)shape. Regardless of the ellipsoid used to model the world, at different timesthe surface of the geoid will be above or below that of the ellipsoid, aphenomenon known as geoid undulation, or geoid-spheroidseparation.

If we take two positions on an ellipsoid and define them in terms of latitudeand longitude, the distance between them can be mathematically determined.However, no such relationship holds with the geoid. If the geoid undulates abovethe ellipsoid, the horizontal distance between the two points is greater thanthe corresponding distance on the ellipsoid; if the geoid undulates below theellipsoid, the horizontal distance is less.

Astronomically derived positions are real-life points on the surface of theearth that have been determined relative to observable celestial phenomena. Assuch, they are referenced to the mathematically unpredictable geoid, as opposedto mapmakers' positions that are mostly derived from a mathematical model (anellipsoid) of the world. Because of the mathematically unpredictable nature ofthe geoid, there is no mathematical relationship between astronomicallydetermined positions and positions determined by reference to an ellipsoid.The only way to correlate the two is either through individual measurements orby modeling the geoid and ellipsoid and measuring the offsets.

What this means is that there can be no ellipsoid that produces a precisecorrelation between ellipsoid-derived latitudes and longitudes and those derivedastronomically. This is why we currently have more than twenty differentellipsoids in use around the world, each of which forms the basis for adifferent map datum, and none of which are compatible. In their own areas, theseellipsoids and datums create a "best fit" between latitudes and longitudesderived from the ellipsoid and those derived astronomically (those referenced tothe geoid). However, when expanded to worldwide coverage, latitudes andlongitudes based on these ellipsoids exhibit increasingly large discrepanciesfrom those derived astronomically.

A New Age

Geodesists have long tried to resolve these problems. In the eighteenth century,British and French surveyors coordinated the lighting of flares on both sides ofthe English Channel to establish triangulation data that would enable thenational surveys to be brought into sync. More recently in North America,sightings were made off aircraft to tie surveys of Greenland, Cuba, and otheroutlying areas into the NAD 27 grid. With the advent of radio, electronicmethods of accurately measuring relatively long distances on land or across seasallowed further improvements to be made by strengthening the triangulationnetworks with trilateration. However, until the satellite age, it wasnot possible to bridge the distances between continents in a way that wouldeliminate the inevitable discontinuities in mapmaking from one continent toanother.

Today, all this has changed. Satellites and space-age technology (e.g., electro-optical distance-measuring devices such as lasers) have finally unified theglobe, from a surveyor's perspective. In the past five decades, an incrediblemass of geodetic data has become available from all parts of the world. On thisbasis, a succession of World Geodetic Systems (WGS) was developed (e.g., WGS 66,WGS 72), culminating in WGS 84. (The "66," "72," etc., refer to the year inwhich the system was developed.)

Each has been closer to the truth, allowing further measurements to be made witheven more accuracy. The shift between WGS 72 and WGS 84 was just plottable at ascale of 1:50,000; it is likely that the magnitude of any further change fromWGS 84 will diminish below the threshold of importance, in which case WGS 84will be with us for a long time to come (the center of the WGS 84 ellipsoid isestimated to be less than 2 cm from its reference point, which is the earth'scenter of mass). (Note that in the United States, NAD 83—see page23—is used for some map- and chart-making. For all intents andpurposes, it is the same as WGS 84.)

WGS 84 is another ellipsoid; however, this one was developed as a best fit withthe geoid (real-life sea-level world) as a whole, as opposed to having a bestfit with just one specific region of the geoid. The irony in this is that, giventhe irregularities in the geoid, the divergence between WGS 84 and the geoid isactually greater in many areas than the divergence between older ellipsoids andthe geoid. For example, in North America, the difference between the Clarke 1866ellipsoid and the geoid is generally less than 10 meters (33 ft.), whereas withWGS 84, it is at least 15 meters (49 ft.) and often 30 to 35 meters(100–115 ft.). What this means is that the difference between map-derivedand astronomically derived latitudes and longitudes is greater on a WGS84–based map than it is on a NAD 27 map. But, on a worldwide scale, WGS 84makes a better fit than Clarke 1866 (NAD 27).

However, almost no one uses astronomically derived position-fixing anymorebecause, with the advent first of Transit (NavSat) and then GPS and GLONASS (theRussian equivalent of GPS), after 2,500 years we have finally broken theumbilical cord that tied our mapmaking to the stars. In the new age, we have ourown artificial stars (satellites) and satellite-based survey techniques thatrelate surveyed positions to the WGS 84 ellipsoid. Whereas astronomicallydetermined latitudes and longitudes are absolute—in the sense that everyreal-life point on the globe has a fixed astronomical latitude andlongitude—ellipsoid-derived latitudes and longitudes are only absoluterelative to a particular ellipsoid, which makes them relative in relation to thegeoid. A change in ellipsoidal assumptions alters the ellipsoid-derived latitudeand longitude of real-life points on the globe. (Of course, the astronomicallydetermined latitude and longitude remain the same.)

At first sight, this seems to make it impossible to have precise position fixes.But with a little more thought, it is seen that this relativity of ellipsoid-derived latitudes and longitudes is irrelevant as long as the equipment used toderive a latitude and a longitude bases the calculations on the same ellipsoidas the map or chart (paper or electronic) on which the position is plotted. Ifthe maps and charts are made to a particular set of assumptions and theposition-fixing equipment operates on the same assumptions, the results will beprecise fixes—in some cases, incredibly precise fixes: down tocentimeter-level accuracy on a continental scale!

The rub comes if someone is navigating with satellite-based electronicnavigation equipment that isn't operating on the same set of assumptions asthose used to make a given map or chart. In this case, the bottom line is that amatch is being attempted between two different ellipsoids. In the case of WGS 84and Clarke 1866 (NAD 27), the resulting position error may be as high as 100meters (328 ft.) in the United States; in the case of WGS 84 and the UnitedKingdom's Ordnance Survey (OS), it is also approximately 100 meters (328 ft.);for charts based on the 1950 European Datum (used in Europe), it may be up to300 meters (985 ft.); and, in the case of WGS 84 and the Tokyo datum, used inmuch of eastern Asia, it may be as much as 900 meters (2,955 ft.).

(Continues...)

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Excerpted from HOW TO READ A NAUTICAL CHART by Nigel Calder. Copyright © 2012 by Nigel Calder. Excerpted by permission of The McGraw-Hill Companies, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: International Marine/Ragged Mountain Press
• Publication date: 2012
• ISBN 10: 0071779825
• ISBN 13: 9780071779821
• Binding: Paperback
• Language: English
• Edition number: 2
• Number of pages: 272