Ellipsoid (Reference Ellipsoid)

An ellipsoid, in geodesy typically a reference ellipsoid, is a mathematically defined surface obtained by rotating an ellipse about its minor axis. It approximates the overall shape of the Earth, which is slightly flattened at the poles due to rotation. A reference ellipsoid is defined by parameters such as the semi-major axis and flattening.

In geodesy, the reference ellipsoid serves as the fundamental geometric model for defining coordinate systems, geographic coordinates (latitude, longitude, ellipsoidal height), and terrestrial reference frames. It provides the basis for GNSS positioning, map projections, satellite orbit computation, and the transformation between geometric and physical height systems (via the geoid).