As for the transverse Mercator, the conic, and thegnomonic projections, a glance at the appearance ofmeridians and parallels on any one of these indicatesnot only that direction is different in different parts ofthe map, but that the direction of North (for example)in one part of the map may be precisely opposite tothat of north in another. Let’s call the two types ofconformality we have mentioned directional con-formality and distance conformality. Someauthorities hold that directional conformality is all thatis required for a conformal projection. A Mercatorprojection has this type of conformality, and this factmakes that type of projection highly advantageous fornavigational charts. A navigator is primarilyinterested in determining geographical location of hisship; and the principal disadvantage of Mercatorprojection—the north-south compared to east-westdistance distortion (which increases with latitude)–isnegligible in navigational practice. This statementapplies only to navigation in customary latitudes,however, since Mercator projection of the polarregions (above about 80-degrees latitude) isimpossible.For surveying and other purposes in which dis-tance measurements must be consistent in every direc-tion, Mercator projection presents disadvantages. Tounderstand these, you have only to reflect on the factthat no distance scale could be consistently applied toall parts of a Mercator projection, which means thatno square grid system could be superimposed on aMercator projection; however, the transverseMercator projection, as it is used in conjunction withthe UTM military grid, provides relatively small-areamaps that are virtually conformal, both direction-wiseand distance-wise.POLYCONIC PROJECTIONIn polyconic projection a near approach todirection conformality is obtained in relatively small-area maps by projecting the area in question onto morethan one cone. A central meridian on the map isstraight; all the others are slightly curved and not quiteparallel. Similarly, the parallels are slightly curvedand not quite parallel; therefore, they are not preciselyperpendicular to the meridians. An example of apolyconic map projection is shown in figure 9-24.Polyconic projection is extensively used for thequadrangle maps (familiarly called quad sheets) ofareas of the United States published by the GeologicalSurvey. For most of the built-up areas of the States,these maps are available on a scale of 1:24,000,Figure 9-24.—Polyconic projection of North America.showing areas extending for 7°30’ of latitude andlongitude. An index map is available, which givesyou the quadrangle divisions and the name of the mapthat covers a particular area.That polyconic projection is not conformaldistance-wise is indicated by the fact that one of thesequad sheets, though it shows an area that is square onthe ground, is oblong rather than square. The verticalor latitudinal length of the map is always greater thanthe horizontal or longitudinal length. The reason isthat latitude is measured along a meridian, which isalways a great circle, while longitude is measuredalong a parallel; and every parallel other than theequator is less than a great circle.An understanding of the concept of the great circleis essential to a thorough understanding of map and9-21
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