Editing Geoid (section)

===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.