Characteristics of Projections
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)?
Which class of projection 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 5.4.2), 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.
Figure 5.4.3 illustrates the difference between a tangent and 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, all projections have distortion. Just by nature of transforming from a 3D globe to a 2D projection, distortion is inevitable. Different projections, however, have different types of distortion. In the next section, we discuss these differences.
When all else is equal, secant projections have less distortion than tangent projections. Why?
- Chapter 9: Elements of Map Projections. Slocum, Terry A., Robert B. McMaster, Fritz C. Kessler, and Hugh H. Howard. 2009. Thematic Cartography and Geovisualization. Edited by Keith C. Clarke. 3rd ed. Upper Saddle River, NJ: Pearson Prentice Hall. Harris, Johnny. 2016.
- “All Maps Are Wrong. I Cut Open a Globe to Show Why.” Vox.