Earth Ellipsoid
lastmod 2018-08-17created 2014-07-092 edits
An earth ellipsoid is a mathematical figure approximating the shape of the Earth, used as a reference frame for computations in geodesy, astronomy, and the geosciences.
It is an ellipsoid of revolution whose short axis (the axis connecting the North and South Poles) is approximately aligned with the Earth's axis of rotation. The earth ellipsoid is defined by the equatorial axis a and the polar axis b. Additional parameters include the mass function J2 (consistent with the gravity formula) and the rotation period (usually 86164 seconds).
Mean earth ellipsoid and reference ellipsoids
A data set describing the global average of the Earth's surface curvature is called the 'mean earth ellipsoid'. It refers to the theoretical coherence between longitude and the meridional curvature of the geoid. The latter is close to mean sea level, and therefore the earth ellipsoid has the same volume as the geoid.
Types of ellipsoids
Representative ellipsoids include the Bessel ellipsoid and WGS84.
| Name | Equatorial axis (m) | Polar axis (m) | Inverse flattening |
| Bessel 1841 | 6,377,397.155 | 6,356,078.963 | 299.1528128 |
| WGS 1984 | 6,378,137.00 | 6,356,752.3142 | 298.257223563 |