What is a geodetic datum?
Computing the coordinates of points on the Earth’s surface requires a reference framework, known as a geodetic datum, as a basis for computation. A geodetic datum consists of a set of three dimensional (3D) coordinate axes (x,y,z), whose origin is within a few hundred meters of the center of mass of the Earth, and a reference ellipsoid that is an ellipsoid of revolution. Only two datums have been used in North America since 1927 to provide the basis for computing horizontal positions, the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). NAD 27 was the official datum of the United States from 1927 until it was replaced in 1986 (not 1983) by NAD 83. A national geodetic organization such as NGS or USC&GS has two primary jobs, to define a national datum and to realize that datum. Defining a datum consists of (1) specifying the origin and orientation of the datum’s coordinate axes relative to the Earth and (2) specifying the the size and shape of the refe
