1)The equipotential surface of the Earth's gravity field
which best fits, in the
least-squares sense, mean sea level. In practice, the average position of mean sea level and a corresponding average
over the time-varying
geopotential must be accepted.
2)The equipotential surface of the Earth's gravity field
which would coincide
with the ocean surface if the latter were undisturbed and affected only by the Earth's gravity field. This is the
original definition as given by
Listing (1873). The definition is deficient because it assumes that the oceanic surface specified is
3)The equipotential surface coinciding with mean sea level
in the oceans. Since mean sea
level is not an equipotential surface, the definition is inconsistent.
4)The equipotential surface, through a given point, chosen
near mean sea level, that would
exist if only the rotation of the Earth and the Earth's gravitational field affected the potential. This
definition was given by
Jensen (1950). It is a function of the position of the chosen point.