6.6: GIS Data

6.6.1 Datums and Geometric Earth Models

When producing maps, the surface of the Earth has to be mapped onto a plane. This is called a projection. A projection from a curved surface like the Earth to a plane cannot be done without distortion; therefore, various projection types with different advantages and disadvantages have been developed and are still in use throughout the world. Every map in the world has been made with one or another of such projections, e.g., UTM, Gauss Kruger, or Lambert Conformal. The same area mapped by two different projections may look slightly or completely different.

A spheroid is a surface obtained by rotating an ellipse around one of its axes. In geography, the shape of the Earth is considered to be a spheroid. However, the Earth is not a perfect spheroid; therefore, several spheroid models have been adopted to approximate the surface in different parts of the Earth. These spheroids are slightly different in shape and size. They are uniquely defined by their equatorial and polar radii, and they are identified by names like Clarke 1866, Hayford, and others.

Every map is associated with a projection and a spheroid. This is called a geodetic datum (from Latin, singular for data-given things). Hundreds of different datums have been used to frame position descriptions since the first estimates of the Earth's size were made by Aristotle. Datums have evolved from those describing a spherical Earth to ellipsoidal models derived from years of...