PrintLike ellipsoids, geoids, and datums, there are many projections to choose from, as well as many options for customizing the projection you choose. Before you decide, it will help to understand the characteristics of different projections. Projections are generally defined by their class, case, and aspect. All three of these characteristics refer to the way in which the developable surface relates to the reference globe.
A projection’s class refers to which developable surface was used to create the projection. Was the developable surface a cone (conic class), plane (planar class/azimuthal), or cylinder (cylindric class)?
The projection class you use will depend, among other factors, on the location of the region you intend to map. Planar projections, for example, are often used for polar regions.
As shown by the figure below (Figure 3.4.2), a map will contain no distortion at the location where the reference globe touches the developable surface, and distortion increases with distance from this location.
Even among projections of the same class, there is more than one way to create a projection with the selected developable surface. A projection’s case refers to how this surface was positioned on the reference globe. If the developable surface touches the globe at only one point or line, this is called a tangent projection. If it touches at two, this is called a secant projection.
Aspect refers to where the developable surface is placed on the globe. If it is placed over one of the Poles (North or South), this is called a polar aspect projection. If the center is along the equator, this creates an equatorial projection. If the developable surface is placed anywhere else, we call this an oblique projection.
No matter what its class, case, and aspect, the projection process always creates distortion. Different projections, however, have different types of distortion. In the next section, we discuss these differences.