text: Horizontal datum A horizontal datum is a model used to precisely measure positions on Earth; it is thus a crucial component of any spatial reference system or map projection. A horizontal datum binds a specified reference ellipsoid, a mathematical model of the shape of the earth, to the physical earth. Thus, the geographic coordinate system on that ellipsoid can be used to measure the latitude and longitude of real-world locations. Regional horizontal datums, such as NAD27 and NAD83, usually create this binding with a series of physically monumented  geodetic control points of known location. Global datums, such as WGS84 and ITRF, are typically bound to the center of mass of the Earth (making them useful for tracking satellite orbits and thus for use in satellite navigation systems.
  A specific point can have substantially different coordinates, depending on the datum used to make the measurement. For example, coordinates in NAD83 can differ from NAD27 by up to several hundred feet. There are hundreds of local horizontal datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas. The WGS 84 datum, which is almost identical to the NAD83 datum used in North America and the ETRS89 datum used in Europe, is a common standard datum.