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 modelling of the Earth approximates the geoid to an oblated 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 there is a large number of ellipsoids 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, by giving the position and the orientation of the ellipsoid relative to the center of the Earth.

The following sections describe the use of ellipsoids and datum in JViews Maps.