Figure of the Earth
In geodesy, the figure of the Earth is the size and shape used to model planet Earth. The kind of figure depends on application, including the precision needed for the model. A spherical Earth is a well-known historical approximation that is satisfactory for geography, astronomy and many other purposes. Several models with greater accuracy (including ellipsoid) have been developed so that coordinate systems can serve the precise needs of navigation, surveying, cadastre, land use, and various other concerns.
For the historical development of the concept, see Spherical Earth and Flat Earth.Equatorial radius (called semimajor axis), and polar radius (called semiminor axis);
and ;
eccentricity
and .
