A map projection
is a mathematical model for conversion of locations from a three-dimensional
earth surface to a two-dimensional map representation. This conversion
necessarily distorts some aspect of the earth's surface, such as
area, shape, distance, or direction. Projection types are based
on the geometric form used in the transfer from the spherical earth
to a flat surface.

Azimuthal (or Planar)

Conic

Cylindrical

Pseudocylindrical or Compromise Projections

Fiigure 2: Planar,
Conic and Clylindrical projections.

Different types of projections are used for
specific areas of the Earth and minimize the distortions for that
part of the globe. Some of these projections include Mercator, Robinson,
Transverse Mercator, Eckert, and Lambert Conformal Conic.

Map projections are very important when more than one data
source is used. For example, if a base map is in the Mercator
projection and a data set of cities is in the Robinson projection,
the cities will not be displayed in the correct location relative
to the base map.

Figure 3: Animated
Illustration of projection distortion (SIC 2002)