Projections - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 82

Basic geometric shapes (cone, cylinder, and plane) serve as developable surfaces,
shown here with a reference globe. The resulting projections of latitude and longitude lines is
shown in the rightmost column.

tude and longitude on a flat surface. All projections using pseudodevelop-
able surfaces can only be described mathematically. They cannot be created
in any mechanical manner.
A key characteristic of all projections, whether developable surface or
pseudodevelopable, is called “aspect” (Figure 4.6). Aspect refers to the orien-
tation of the developable surface to the earth. Various conventions have
come and gone in cartography over time. For future users of GIS, I think it is
most pragmatic to distinguish among equatorial, transverse, oblique, and
polar aspects. The differences refer either to the orientation of the projec-
tion to a region of the earth (equatorial or polar) or to the developable sur-
face of that type of projection—for example, transverse Mercator projections
are rotated 90 from the Mercator projection's usual equatorial orientation.

The basic differences are best visualized in a figure showing the different
aspect for each developable surface. The consequences for distortion and
accuracy are discussed later in this chapter.
Some possible aspects for conical, cylindrical, and planar projection
include equatorial and polar. Equatorial orientation has the projection's cen-
ter positioned somewhere along the equator. Polar aspect occurs only with

°