The standard ellipsoids are all used to approximate the overall shape of the Earth as a simple mathematical figure, based on the approximate average level of the world's oceans. read more
While the sphere is a close approximation of the true figure of the Earth and satisfactory for many purposes, geodesists have developed several models that more closely approximate the shape of the Earth so that coordinate systems can serve the precise needs of navigation, surveying, cadastre, land use, and various other concerns. read more