A
Geodetic datum or
geodetic system is a
coordinate system, and a set of reference points, used to locate places on the
Earth (or 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 (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.