ABSTRACT
The earth has a natural surface that is complicated. In surveying and cartography, it is replaced by an earth ellipsoid or sphere that approximates the natural surface of the earth in overall shape, especially the general curvature. In the computation of map projections, the earth ellipsoid or sphere is called the earth datum; it is also called the datum used for the related map projections. Now we will introduce some of the basic elements and fundamental formulae.
