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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 © 2012Nigel Calder
All rights reserved.
Fundamental Chart-Making Concepts
UNTIL RECENTLY, there has been little need for chart users to understand the technology of chart-making, particularly its limitations, because the tools used by navigators to determine the position of their vessels were inherently less accurate than those used to conduct and display the surveys on which charts are based. Realizing the limits of accuracy of their tools, navigators tended to be a cautious crowd, giving hazards a wide berth and typically taking proactive measures 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 to landmasses and not astronomical fixes, surveyors were more concerned with depicting an accurate relationship of soundings and hydrographic features relative to the local landmass (coastline) than they were with absolute accuracy relative to latitude and longitude. The surveyor's maxim was that it is much more important to determine an accurate least depth over a shoal or danger than to determine its geographical position with certainty. Similarly, the cartographer, when showing an area containing many dangers (such as a rocky outcrop), paid more attention to bringing the area to the attention of the navigator, so it could be avoided by a good margin, than to accurately showing every individual rock in its correct position.
All this changed with the advent of satellite-based navigation systems—notably the global positioning system (GPS). Now a boat's position (latitude and longitude) can be fixed with near-pinpoint accuracy and, in the case of electronic navigation, accurately displayed on a chart in real time. This encourages many navigators (myself included) to "cut corners" more closely than they would have done in the past. With such an attitude, it is essential for 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 chart itself—together they determine the extent to which it is possible to cut corners 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 navigation devices actually give us the plotting accuracy we believe they do. This is best done 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 established that the world is a sphere, created the concepts of latitude and longitude, and developed basic mapmaking skills. It was not until the sixteenth century a.d. that there were any advances in mapmaking techniques, which occurred largely as a result of steady improvements in the equipment and methods used for making precise astronomical observations and for measuring distances and changes in elevation on the ground. From this time, instruments were available for measuring angles with great accuracy.
The core surveying methodology that developed is noteworthy because it remained essentially unchanged until recent decades—for both cartographic and inshore hydrographic surveys—and is the basis of many of the charts we still use. A survey started from a single point whose latitude and longitude were established by astronomical observations. For accurate surveys, these observations required heavy, bulky, and expensive equipment, as well as multiple observations by highly trained observers over a considerable period of time. From the starting point, a long baseline was precisely measured using carefully calibrated wooden or metal rods or chains. The surveyors measured all changes in vertical elevation in order to be able to discount the effects of them on the horizontal distances covered. In this way, a precise log of horizontal distances was maintained, resulting in baseline measurements that were accurate to inches—sometimes over a distance of many miles. The process was slow and painstaking, and often took years to complete.
Once a baseline had been established, angular measurements were taken from both ends to a third position. Knowing the length of the baseline and the two angles, spherical trigonometry established the distances to the third point without having to make field measurements. The sides of the triangle thus established were now used as fresh baselines to extend the survey, again without having to make actual distance measurements in the field. The measured baselines plus the process of triangulation provided the horizontal distances on the ground. With one or more precise astronomical observations at a different point to the original one, it was possible to mathematically establish a latitude and longitude framework and apply it to the results of the survey—there was no need to obtain astronomical fixes for all the intermediate points, thereby avoiding the time, expense, and difficulties involved.
By the seventeenth century, it was possible to make sufficiently accurate astronomical observations and distance measurements to discover that in one part of the world a degree of latitude as measured astronomically (i.e., with reference to the stars) does not cover the same distance on the ground as it does in another part of the world. This would be impossible if the world were a perfect 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 astronomically determined starting point and a measured baseline, working away from the beginning point by the process of triangulation (see art page 13).
As the surveyors progressed farther afield, if the mapped latitudes and longitudes 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 the relationship between the distance on the ground and latitude and longitude, and indicating how this relationship changed as the surveyors moved away from their astronomically determined starting point. This model had to be such that with available trigonometrical and computational methods, the mapmakers could adjust their data to accurately calculate changing latitudes and longitudes over substantial distances—in other words, the model had to be mathematically predictable.
The model that was adopted, and which is used to this day even with satellite- based mapmaking and navigation, is an ellipsoid (also called a spheroid). In essence, an ellipsoid is nothing more than a flattened sphere, characterized by two measurements: its radius at the equator and the degree of flattening at the poles. Clearly, the key questions become: What is this radius, and what is the degree of flattening?
During the nineteenth century, the continents were first accurately mapped based on this concept of the world as an ellipsoid. For each of the great surveys, preliminary work extending over years used astronomical observations and measured baselines to establish the key dimensions of the ellipsoid that was to underlie the survey. In the United Kingdom, a geodesist (a person who does this type of research) named Sir George Airy developed an ellipsoid (known as Airy 1830) that became the basis for an incredibly detailed survey of the British 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 than modern satellite-derived ellipsoids, which are described later in this chapter).
Using this ellipsoid, the surveyors commenced at a precisely determined astronomical point on Salisbury Pl
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..
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