Figure 9-20.-Conic projection.Figure 9-21.—Appearance of meridians and parallels on aconic projection.9-19indicate that the map covers parts of both. Note, too,that the direction of grid north (that is, the directionof the north-south grid lines in the map) varies fromthat of true north by O“39’E and from the magneticnorth by l“15’W.CONIC PROJECTIONTo grasp the concept of conic projection, againimagine the earth as a glass sphere with a light at thecenter. Instead of a paper cylinder, image a paper coneplaced over the Northern Hemisphere tangent to aparallel, as shown in figure 9-20. The North Pole willbe projected as a point at the apex of the cone. Themeridians will radiate outward from the North Pole asstraight lines. The parallels will appear as concentriccircles, growing progressively smaller as latitude in-creases. When the cone is cut along a meridian andflattened out, the meridians and parallels will appearas shown in figure 9-21. In this case, the NorthernHemisphere was projected onto a cone placed tangentto the parallel at 45°N, and the cone was cut along the180th meridian.GNOMONIC PROJECTIONTo grasp the concept of gnomonic projection,again imagine the lighted sphere—this time witha flat-plane paper placed tangent to the North Pole(fig. 9-22). The North Pole will project as a point fromwhich the meridians will radiate outward as straightlines; and the parallels will appear as concentriccircles, growing progressively smaller as latitudeincreases. The difference between this and conicFigure 9-22.-Gnomonic projection.
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business