Editing Sea level (section)

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[[File:Geoida.svg|thumb|{{ordered list |Ocean |[[Reference ellipsoid]] |Local [[Plumb-bob|plumb line]] |[[Continent]] |[[Geoid]]}}]]

It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the [[geoid]]. In the absence of external forces, the local mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's [[gravitation]]al field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits [[gravity anomalies]] such as those measured by NASA's [[GRACE satellites]]. In reality, the geoid surface is not directly observed, even as a long-term average, due to ocean currents, air pressure variations, temperature and salinity variations, etc. The location-dependent but time-persistent separation between local mean sea level and the geoid is referred to as (mean) [[ocean surface topography]]. It varies globally in a typical range of ±{{convert|1|m|ft|0|abbr=on}}.<ref>{{cite web |url=https://science.nasa.gov/earth/climate-change/sea-level-101-what-determines-the-level-of-the-sea/#h-waves-in-the-bathtub |title=Sea Level 101: What Determines the Level of the Sea? |publisher=NASA |date=3 June 2020 |access-date=17 April 2024}}</ref>