CONIC PROJECTION

Figure 9-20.-Conic projection. Figure 9-21.—Appearance of meridians and parallels on a conic projection. 9-19 indicate that the map covers parts of both. Note, too, that the direction of grid north (that is, the direction of the north-south grid lines in the map) varies from that of true north by O“39’E and from the magnetic north by l“15’W. CONIC PROJECTION To grasp the concept of conic projection, again imagine the earth as a glass sphere with a light at the center. Instead of a paper cylinder, image a paper cone placed over the Northern Hemisphere tangent to a parallel, as shown in figure 9-20. The North Pole will be projected as a point at the apex of the cone. The meridians will radiate outward from the North Pole as straight lines. The parallels will appear as concentric circles, growing progressively smaller as latitude in- creases. When the cone is cut along a meridian and flattened out, the meridians and parallels will appear as shown in figure 9-21. In this case, the Northern Hemisphere was projected onto a cone placed tangent to the parallel at 45°N, and the cone was cut along the 180th meridian. GNOMONIC PROJECTION To grasp the concept of gnomonic projection, again imagine the lighted sphere—this time with a flat-plane paper placed tangent to the North Pole (fig. 9-22). The North Pole will project as a point from which the meridians will radiate outward as straight lines; and the parallels will appear as concentric circles, growing progressively smaller as latitude increases. The difference between this and conic Figure 9-22.-Gnomonic projection.