Editing Geodetic datum (section)

=== 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 [[North American Datum#North American Datum of 1927|NAD 27]] and [[NAD 83]], usually create this binding with a series of physically monumented [[geodetic control network | geodetic control points]] of known location. Global datums, such as [[WGS 84]] 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 NAD{{nbsp}}83 can differ from NAD{{nbsp}}27 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 [[NAD 83]] datum used in North America and the [[ETRS89]] datum used in Europe, is a common standard datum.{{citation needed|date=November 2016}}