A spheroid (also called ellipsoid of revolution) is a three-dimensional geometric shape generated by rotating an ellipse around one of its axes, producing a surface that mathematically approximates the shape of Earth and other planets for geodetic and navigation purposes. In GNSS and mapping applications, reference spheroids provide the mathematical surfaces upon which coordinate systems are defined and position calculations are performed.

Earth’s actual shape is an oblate spheroid, flattened at the poles and bulging at the equator due to centrifugal force from planetary rotation. The equatorial radius exceeds the polar radius by approximately 21 kilometers. A reference spheroid is defined by specific parameters, most commonly the semi-major axis (equatorial radius) and either the semi-minor axis (polar radius) or the flattening factor (which quantifies the degree of oblateness).

The WGS84 spheroid, used by GPS and most global GNSS applications, defines a semi-major axis of 6,378,137 meters and flattening of 1/298.257223563. The GRS80 spheroid, adopted by many national datums including NAD83, uses nearly identical parameters. Historically, many regional spheroids were developed to best fit Earth’s surface in specific areas, Clarke 1866 for North America, Bessel 1841 for parts of Europe, Airy for Great Britain, before global satellite measurements enabled accurate worldwide spheroid definitions.

For GNSS users, the reference spheroid matters because geographic coordinates (latitude, longitude) and ellipsoidal heights are defined relative to it. Different spheroids yield different coordinates for the same physical point. When working across systems using different spheroids or datums, proper mathematical transformations must be applied. Modern practice has largely converged on WGS84/GRS80-compatible spheroids for global applications, but legacy data in historical systems and local mapping products may use regional spheroids that require careful handling during data integration.