This force deforms the sphere into an oblate ellipsoid. For example, Io is better modeled by triaxial ellipsoid or prolated spheroid with small eccentricities. However, the Earth's actual shape is closer to an oblate ellipsoid. The geometrical separation between the geoid and the reference ellipsoid is called the geoidal undulation. Most global mean radii are based on the reference ellipsoid, which approximates the geoid. These are true spherical coordinates with the origin at the center of the ellipsoid. For the WGS 84 ellipsoid, the mean curvature equals . The linear map maps this sphere onto an ellipsoid in . Polarizability is a 3-dimensional tensor that can be represented as an ellipsoid. The composition then sends the unit-sphere onto an ellipsoid isometric to . It is also the equatorial radius used for the IERS 2003 ellipsoid. Auxiliary latitudes are often employed in projecting the ellipsoid. Let be the semi-minor axis () of the auxiliary ellipsoid. Using the parameters from WGS84 ellipsoid of revolution, and , . The "authalic" (equal area) radius of the Clarke 1866 ellipsoid is .