 The geodetic datum or geodetic system is a coordinate system, and a set of reference points, used to locate places on the Earth for similar objects. An approximate definition of sea level is the datum WGs 84, an ellipsoid, whereas a more accurate definition is Earth gravitational model 2008 GM 2008 using at least 2159 spherical harmonics. Other datums are defined for other arias or at other times, head 50 was defined in 1950 over Europe and differs from WGs 84 by a few hundred meters depending on where in Europe you look. Mars has no oceans and so no sea level, but at least two Martian datums have been used to locate places there. Datums are used in geodesy, navigation, and surveying by cartographers and satellite navigation systems to translate positions indicated on maps paper or digital to their real position on Earth. Each starts with an ellipsoid stretched sphere and then defines latitude, longitude and altitude coordinates. One or more locations on the Earth's surface are chosen as anchor base points. The difference in coordinates between datums is commonly referred to as datum shift. The datum shift between two particular datums can vary from one place to another within one country or region, and can be anything from zero to hundreds of meters or several kilometers for some remote islands. The north pole, south pole and equator will be in different positions on different datums, so true north will be slightly different. Different datums use different interpolations for the precise shape and size of the Earth reference ellipsoids. Because the Earth is an imperfect ellipsoid, localized datums can give a more accurate representation of the area of coverage than WGs 84. OSGB 36, for example, is a better approximation to the geoid covering the British files than the global WGs 84 ellipsoid. However, as the benefits of a global system outweigh the greater accuracy, the global WGs 84 datum was becoming increasingly adopted. Horizontal datums are used for describing the point on the Earth's surface, in latitude and longitude or another coordinate system. Vertical datums measure elevations or depths.