ABSTRACT
Geodesy is the science of the measurement and mapping of the surface of the Earth. The problem of geodesy is to determine the figure and the external gravity of the Earth. The figure of the Earth refers to the physical and mathematical surface of the Earth. The irregular solid surfaces and ocean surfaces of the Earth cannot be modeled in a simple manner. Thus, it was necessary to define a fictitious surface which approximates the total shape of the earth. The surfaces of revolution of this section are the geodesists' answer to the problem. The geoid is the intermediate projection surface between the irregular Earth and the mathematically manageable surface of revolution. It is necessary to present the increased simplification in equations for coordinates, angles, and distances if a spherical model is used. This chapter discusses the accepted radius for the spherical model of the Earth and gives the characteristics of a triaxial ellipsoid model.
