text: Vertical datum A vertical datum is a reference surface for vertical positions, such as the elevations of Earth features including terrain, bathymetry, water level, and human-made structures.
  An approximate definition of sea level is the datum WGS 84, an ellipsoid, whereas a more accurate definition is Earth Gravitational Model 2008 (EGM2008), using at least 2,159 spherical harmonics. Other datums are defined for other areas or at other times; ED50 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.
  Geodetic coordinates
  In geodetic coordinates, Earth's surface is approximated by an ellipsoid, and locations near the surface are described in terms of geodetic latitude (), longitude (), and ellipsoidal height ().
  Earth reference ellipsoid
  Defining and derived parameters
  The ellipsoid is completely parameterised by the semi-major axis  and the flattening .
  From  and  it is possible to derive the semi-minor axis , first eccentricity  and second eccentricity  of the ellipsoid
  Parameters for some geodetic systems
  The two main reference ellipsoids used worldwide are the GRS80
  and the WGS 84.
  A more comprehensive list of geodetic systems can be found here.