Geographic Information Systems: An Introduction, 3rd Edition

6.3 Map Projection

Georeferenced data may be drawn on maps only when referenced to a plane surface, not to the curved surface of the Earth. Various projections are used to represent the curved surface of the Earth on the plane surface of a map. Map projections can sometimes be more closely related to map drawing than to position finding. They are classified in three groups according to the underlying geometrical conversions involved (Figure 6.7):

• Cylindrical
• Conical
• Azimuthal or plane surface

As shown in Figure 6.7, the three different projection surfaces can be unfolded to one plane and thus function as a plane map. It should be remembered, however, that curved surfaces are always distorted when imaged into a plane, and the deformations multiply with the increasing size of the area represented. In the planes' touch line or the touch point with the Earth is however the scale factor equal to 1. This can be used when choosing a projection in relation to the shape of the area to be mapped.

A map projection can be defined as a mathematical flat surface representation of the geodetic ellipsoid. There are different projection formulas that preserve either shape, area likeness or distance in a particular direction. Length preservation, where the scale is the same in all directions, is not possible. Cylindrical, conical and azimuthal projections can be made equidistant, equal in area, or conformal. Because fixed mathematical formulas are used, it is possible to convert to and from different ...