A map projection is a way of representing an area of the globe on a flat surface. It involves the transfer of meridians and parallels from the curved spherical surface of the globe, or part of the globe, to a flat surface. The fundamental problem is to make this transfer within acceptable limits of accuracy.
Certain geometric surfaces such as the cone and the cylinder can be cut lengthwise and unrolled to make a flat sheet. But because the earth has a compound-curvature geometric form, the surface cannot lie flat without serious distortion. Consequently, it is impossible to make a perfect map projection.
The problem of distortion may be considered negligible for very small maps (those six kilometres across or less), but increases as the map area becomes larger. Because of distortion, many maps, in particular those covering large areas, display changes in scale and as a result show different area coverages in different parts of the map projection.
Map projections may be classified according to the form of the geometric surface on which the projection is made.
For more information, see Map Projections.