The surface of the Earth is complex, even if the topography is removed so that only the geoid (the gravity surface approximating to mean sea level) is taken as the shape of the Earth. In fact, because of Earth’s internal composition, which leads to local gravity anomalies, the geoid is very irregular. Therefore, the most widely used model of the Earth approximates the geoid to an oblate ellipsoid (the spheroid), or even simpler, a sphere.

Of course since the use of an ellipsoid is an approximation, there is no universal ellipsoid that fits the geoid everywhere. For a given location, the best fitting ellipsoid can have different dimensions compared to other locations. That is why a large number of ellipsoids are used in maps.

Defining an ellipsoid is not sufficient. It is also necessary to define the spatial relationship (position and orientation) between the ellipsoid and the geoid. This is achieved through the definition of a geodetic datum, or horizontal datum, giving the position and the orientation of the ellipsoid relative to the center of the Earth.