<P> In contrast, extracting φ, λ and h from the rectangular coordinates usually requires iteration . A straightforward method is given in an OSGB publication and also in web notes . More sophisticated methods are outlined in geodetic system . </P> <P> Currently the most common reference ellipsoid used, and that used in the context of the Global Positioning System, is the one defined by WGS 84 . </P> <P> Traditional reference ellipsoids or geodetic datums are defined regionally and therefore non-geocentric, e.g., ED50 . Modern geodetic datums are established with the aid of GPS and will therefore be geocentric, e.g., WGS 84 . </P> <P> Reference ellipsoids are also useful for geodetic mapping of other planetary bodies including planets, their satellites, asteroids and comet nuclei . Some well observed bodies such as the Moon and Mars now have quite precise reference ellipsoids . </P>

An ellipsoid model of the earth's shape includes the semi-major axis in the