Geodesy is the branch of science concerned with the determination of the size and shape of the
Earth. Geodesy involves the processing of survey measurements on the curved surface of the
Earth, as well as the analysis of gravity measurements. Knowing the exact location of a pixel on
the Earth’s surface (its spatial location) is an essential component of remote sensing. It requires a
detailed knowledge of the size and the shape of the Earth.

The Earth is not a simple sphere. Topographic features such as mountain ranges and deep oceans
disturb the surface of the Earth. The ideal reference model for the Earth’s shape is one that can
represent these irregularities and identify the position of features through a co-ordinate system. It
should also be easy to use.