Editing Geodetic datum (section)

=== Vertical datum ===
{{main|Vertical datum}}
{{further|Chart datum}}

A '''vertical datum''' is a reference surface for [[vertical position]]s, such as the [[elevation]]s of Earth features including [[terrain]], [[bathymetry]], [[water level]], and human-made structures.

An approximate definition of [[sea level]] is the datum [[World Geodetic System|WGS 84]], an [[reference ellipsoid|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{{nbsp}}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 [[Geography of Mars#Zero elevation|martian datums]] have been used to locate places there.