Thus, it is important to select a central meridian that runs through the center of the area of interest on a map. Likewise for a map of the world, the central meridian determines which continents are placed in the center of the projection. Note, for example, that North and South America are at the center of the world maps shown on the U.S.G.S. Map Projections poster, while many wall maps of the world place Europe and Africa in the center. The three projection perspectives are Gnomonic, Orthographic , and Stereographic . The appearance of the graticule differs for each perspective.
Poster gives good graphic examples of each.
Gnomonic The light source for the Gnomonic perspective is from the center of the earth through to the spherical surface where it is projected onto a plane. In the Orthographic perspective, the spherical surface is transformed to a projection plane from infinity, that is, as if a light source were shown from an infinite distance through the globe and onto a planar surface. An example of an orthographic projection is the Oblique Aspect Orthographic Projection. In the Stereographic projection the perspective is a point at the opposite end of the globe. An example of a stereographic projection is the North Polar Stereographic Projection .