POLYCONIC PROJECTION

As for the transverse Mercator, the conic, and the gnomonic projections, a glance at the appearance of meridians and parallels on any one of these indicates not only that direction is different in different parts of the map, but that the direction of North (for example) in one part of the map may be precisely opposite to that of north in another. Let’s call the two types of conformality we have mentioned directional con- formality and distance conformality. Some authorities hold that directional conformality is all that is required for a conformal projection. A Mercator projection has this type of conformality, and this fact makes that type of projection highly advantageous for navigational charts. A navigator is primarily interested in determining geographical location of his ship; and the principal disadvantage of Mercator projection—the north-south compared to east-west distance distortion (which increases with latitude)–is negligible in navigational practice. This statement applies only to navigation in customary latitudes, however, since Mercator projection of the polar regions (above about 80-degrees latitude) is impossible. For surveying and other purposes in which dis- tance measurements must be consistent in every direc- tion, Mercator projection presents disadvantages. To understand these, you have only to reflect on the fact that no distance scale could be consistently applied to all parts of a Mercator projection, which means that no square grid system could be superimposed on a Mercator projection; however, the transverse Mercator projection, as it is used in conjunction with the UTM military grid, provides relatively small-area maps that are virtually conformal, both direction-wise and distance-wise. POLYCONIC PROJECTION In polyconic projection a near approach to direction conformality is obtained in relatively small- area maps by projecting the area in question onto more than one cone. A central meridian on the map is straight; all the others are slightly curved and not quite parallel. Similarly, the parallels are slightly curved and not quite parallel; therefore, they are not precisely perpendicular to the meridians. An example of a polyconic map projection is shown in figure 9-24. Polyconic projection is extensively used for the quadrangle maps (familiarly called quad sheets) of areas of the United States published by the Geological Survey. 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 and longitude. An index map is available, which gives you the quadrangle divisions and the name of the map that covers a particular area. That polyconic projection is not conformal distance-wise is indicated by the fact that one of these quad sheets, though it shows an area that is square on the ground, is oblong rather than square. The vertical or latitudinal length of the map is always greater than the horizontal or longitudinal length. The reason is that latitude is measured along a meridian, which is always a great circle, while longitude is measured along a parallel; and every parallel other than the equator is less than a great circle. An understanding of the concept of the great circle is essential to a thorough understanding of map and 9-21