meridians as does the track of a vessel under a truecourse. On the globe the parallels become shortertoward the poles, and their length is proportionate tothe cosine of latitude. In the Mercator projection theparallels are equally long. This means that any parallelis increased by 1/cos q, or sec q, where q is the latitudein degrees. To have the same scale along the parallelsas along the meridians, you must increase each degreeof latitude by the secant of the latitude. In this math-ematical transformation, the tangent cylinder conceptwas not employed, nor is it ever employed, in theMercator projection. A Mercator projection table isused to plot the meridional distances. For intensivestudy on elements of map projection, you may refer tospecial publications published by the U.S. Coast andGeodetic Survey that deal with this subject.Universal Transverse Mercator Military GridAn extensive application of the transverseMercator projection is in a grid reference system formilitary maps called the universal transverseMercator (UTM) military grid system. In thissystem a reference plane grid, like those used in ourstate grid systems, is imposed on transverse Mercatorprojections of relatively small areas. The basicFigure 9-14.—(A) Grid zone designations of the military grid reference system; (B) 100,000-meter-square designations in the UTMmilitary grid system,9-13
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