Editing Geoid (section)

==Description==
The geoid surface is irregular, unlike the reference ellipsoid (which is a mathematical idealized representation of the physical Earth as an [[ellipsoid]]), but is considerably smoother than Earth's physical surface. Although the "ground" of the Earth has excursions on the order of +8,800 m ([[Mount Everest]]) and −11,000 m ([[Marianas Trench]]), the geoid's deviation from an ellipsoid ranges from +85&nbsp;m (Iceland) to −106&nbsp;m (southern India), less than 200 m total.<ref name="TexasGeoid">{{cite web | url=http://www2.csr.utexas.edu/grace/gravity/gravity_definition.html | title=Earth's Gravity Definition | publisher=Center for Space Research ([[University of Texas at Austin]]) / Texas Space Grant Consortium | work=GRACE – Gravity Recovery and Climate Experiment | date=11 February 2004 | access-date=22 January 2018}}</ref>

If the ocean were of constant density and undisturbed by tides, currents or weather, its surface would resemble the geoid. The permanent deviation between the geoid and [[mean sea level]] is called [[ocean surface topography]]. If the continental land masses were crisscrossed by a series of tunnels or canals, the sea level in those canals would also very nearly coincide with the geoid. [[Geodesist]]s are able to derive the heights of continental points above the geoid by [[spirit leveling]].

Being an [[equipotential surface]], the geoid is, by definition, a surface upon which the force of gravity is perpendicular everywhere, apart from temporary tidal fluctuations. This means that when traveling by ship, one does not notice the [[#Undulation|undulation of the geoid]]; neglecting tides, the local vertical (plumb line) is always perpendicular to the geoid and the local horizon [[tangential component|tangential]] to it. Likewise, spirit levels will always be parallel to the geoid.

===Simplified example===
[[File:Earth_Gravitational_Model_1996.png|thumb|upright=1.5|Map of the undulation of the geoid in meters (based on the [[EGM96]] gravity model and the [[WGS84]] reference ellipsoid).<ref>{{cite web|url=http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html|title=WGS 84, N=M=180 Earth Gravitational Model|work=NGA: Office of Geomatics|publisher=National Geospatial-Intelligence Agency|access-date=17 December 2016|archive-date=8 August 2020|archive-url=https://web.archive.org/web/20200808025531/https://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html|url-status=dead}}</ref>]]
[[File:Geoida.svg|thumb|{{olist
    |Ocean
    |Ellipsoid
    |Local plumb line
    |Continent
    |Geoid
}}]]
Earth's gravitational field is not uniform. An [[oblate spheroid]] is typically used as the idealized Earth, but even if the Earth were spherical and did not rotate, the strength of gravity would not be the same everywhere because density varies throughout the planet. This is due to magma distributions, the density and weight of different [[geological]] compositions in the [[Earth's crust]], mountain ranges, deep sea trenches, crust compaction due to glaciers, and so on.

If that sphere were then covered in water, the water would not be the same height everywhere. Instead, the water level would be higher or lower with respect to Earth's center, depending on the integral of the strength of gravity from the center of the Earth to that location. The geoid level coincides with where the water would be. Generally the geoid rises where the Earth's material is locally more dense, exerts greater gravitational force, and pulls more water from the surrounding area.