Geographic Information Systems: An Introduction, 3rd Edition

6.1 Datum

To determine an exact position on the Earth's surface, one has to know the shape and size of the Earth. As we know, the Earth is not a perfect sphere, but more like an ellipsoid with flattening (ellipticity) at the poles (Figure 6.1). The shape of the Earth is therefore, for practical purposes, often expressed by the shape of the ellipsoid. Ellipsoids of various sizes have been used, depending on the degree of accuracy with which it was possible to measure the shape of the Earth. The Greek Eratosthenes (276 - 194 B.C.) is known as the first to determine the Earth's shape with satisfactory accuracy. In the 1600s and early 1700s scientists eagerly discussed the shape of the Earth, debating whether it was flattened at the poles or at the equator. These discussions ended after the French Academy of Science sent out two expeditions, one to South America and one to Northern Sweden. The results showed that the Earth was flattened at the poles. Since that time many different ellipsoid sizes have been computed and used (see Figure 6.2).

Figure 6.1

The Earth is often described using a rotation ellipsoid. The lengths of the semimajor axis (a) and semi-minor axis (b) have gradually changed through history as more exact measurements of the Earth's shape have become available.

In 1924 a common international ellipsoid was accepted by the International Association of Geodesy, but it has ...