What is a Geoid Though?
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 an equipotential surface.
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.
Definition (1) is preferred.