A horizontal datum is a mathematical surface (a particular ellipsoid) used for geodetic computations and to define locations. World-wide there are several hundred of these horizontal datums defined.
In the past datum systems apply only to a specific country or localized area of the Earth’s surface. Recent years have seen that globalisation is leading to the definition of global datum systems (or geocentric datum systems), like the International Terrestrial Reference System (ITRS), and several implementations, such as ITRF2000 or WGS84.
geodetic datum, geographic coordinate system
Explain the relevance of reference surfaces, coordinate systems, and coordi-nate transformations in mapping (level 1 and 2).