Because the earth is not a perfect sphere (it is wider at the equator than at the poles), an ellipsoid is often used to model its shape. The reference ellipsoid is defined by its dimensions for the major and minor axes and the amount of flattening at the poles [See Figure 4].

Ellipsoids that model the earth are very near to being spherical, so close that they can be called a spheroid. Since the flattening occurs at the poles due to the centrifugal force of the rotation of the earth, the figure may be further defined as an oblate spheroid.

Specific ellipsoids are better suited for specific situations. For a relatively small area such as a county, the earth's surface can be thought of as a plane (or flat surface). On the other hand, when high accuracy of large areas is needed, it is necessary to use a more accurate and reliable model of the earth such as an ellipsoid or geoid. (Maling, 1989) Reference Ellipsoids are used around the world, depending on the region of interest, because of the varying earth curvature in different locales [See Table 2].

Another description of the earth is a geoid. The Geoid is a representation of the earth's gravity field. "A Geoid is the equipotential surface of the earth's gravity field which best fits, in a least squares sense, global mean sea level," (NGS,2000 Geoid). Essentially this is a representation of the surface of the earth in terms of sea level for every position on earth, in a more complex manner than an ellipsoid.

When combining or integrating data in a GIS, the various themes (inputs) should share the same projection, datum, and ellipsoid to ensure that all of the features will be in accurate alignment. Not all data come so homogeneous.