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A modern nautical chart, compiled from an accurate survey, is necessarily based 

 geographically on some definite form of projection. Numerous types of projections 

 have been devised, each with a particular purpose in view, but the two which have 

 properties peculiarly useful in navigation are the Mercator and the gnomonic. 



1121. The Mercator Projection 



The Mercator projection was invented by Gerhard Kramer (Mercator) in 1569, 

 and practically all modern small-scale nautical charts and many large-scale ones are 

 constructed on it. For nautical charts, then, except in high latitudes, the Mercator 

 projection has attained an importance far above all others. It is a type of projec- 

 tion, derived by mathematical analysis, in which the meridians are straight equally 

 spaced parallel lines, and the parallels are straight parallel lines at right angles to them 

 but whose distances apart for a sphere increase at a rate varying with the secant of 

 the latitude. 



The projection has a number of advantages, among wh,ich are simplicity of con- 

 struction, convenience in plotting positions from the border divisions, and the fact 

 that on it alone a course can be laid off from any meridian or compass rose within its 

 borders. Its principal advantage, however, and the one responsible for its world-wide 

 use for nautical charts, is that any straight line drawn on it in any direction is a rhumb 

 line, or loxodromic curve. The track of a ship on a constant course is a straight line 

 on the projection and it will, theoretically at least, pass all features along that line 

 exactly as they are charted. This is a great advantage in extensive coastal navigation, 

 in that the straight line representing a constant course made good will indicate at once 

 the distance abeam dangers will be passed. 



Disadvantages of the Mercator projection are that it exaggerates areas appreci- 

 ably — seriously where an extensive north and south area is included — and that the 

 scale is constantly changing with latitude, so that a graphic scale cannot be used on the 

 smaller-scale charts, making it necessary to measure distances along the border divisions 

 for the latitudes in which the distance lies. 



Most of the nautical charts published by the Coast and Geodetic Survey and all 

 newly constructed charts are on the Mercator projection. The most recent and best 

 tables, and those used by the Coast and Geodetic Survey, for the construction of 

 Mercator projections are those issued by the International Hydrographic Bureau at 

 Monaco, 1928. 



Much information about the Mercator projection and its construction is contained 

 in The American Practical Navigator (Bowditch), pages 38 to 40, and in Coast and 

 Geodetic Survey Special Publication No. 68, Elements of Map Projection. The latter 

 publication also contains tables to three decimal places, which accuracy is sufficient for 

 all practical purposes. 



1122. The Gnomonic Projection 



The gnomonic projection is a perspective projection upon a tangent plane, the 

 projecting lines radiating from the center of the sphere. Obviously only a limited 

 portion of the globe can be represented on this projection. 



Its one special property of value to seamen is that any line of sight, or part of a great 

 circle, is represented by a straight line on the projection. Thus all plotted bearings, 

 either visual or radio, are straight lines, and by it a mariner can readily determine the 



