Editing Physical geodesy (section)

==Geoid==
{{main|Geoid}}
[[File:Earth_Gravitational_Model_1996.png|thumb|Map of the undulation of the geoid in meters (based on the [[EGM96]])]]
Due to the irregularity of the Earth's true gravity field, the equilibrium figure of sea water, or the [[geoid]], will also be of irregular form. In some places, like west of [[Ireland]], the geoid—mathematical mean sea level—sticks out as much as 100 m above the regular, rotationally symmetric reference ellipsoid of GRS80; in other places, like close to [[Sri Lanka]], it dives under the ellipsoid by nearly the same amount. 
The separation between the geoid and the reference ellipsoid is called the ''[[undulation of the geoid]]'', symbol <math>N</math>.

The geoid, or mathematical mean sea surface, is defined not only on the seas, but also under land; it is the equilibrium water surface that would result, would sea water be allowed to move freely (e.g., through tunnels) under the land. Technically, an ''equipotential surface'' of the true geopotential, chosen to coincide (on average) with mean sea level.

As mean sea level is physically realized by tide gauge bench marks on the coasts of different countries and continents, a number of slightly incompatible "near-geoids" will result, with differences of several decimetres to over one metre between them, due to the [[dynamic sea surface topography]]. These are referred to as ''[[vertical datum]]s'' or ''height [[datum (geodesy)|datum]]s''.

For every point on Earth, the local direction of gravity or [[vertical direction]], materialized with the [[plumb line]], is ''perpendicular'' to the geoid (see [[astrogeodetic leveling]]).