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Vertical datum
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<p>In <a href="/facts/Geodesy/EJH0Te0m">geodesy</a>, <a href="/facts/Surveying/qchGcnRC">surveying</a>, <a href="/facts/Hydrography/FP9yHxSv">hydrography</a> and <a href="/facts/Navigation/sGGAJ0AQ">navigation</a>, vertical datum or altimetric datum is a reference <a href="/facts/Coordinate_surface/Mtd0q4SP">coordinate surface</a> used for <a href="/facts/Vertical_position/HpaNYcWC">vertical positions</a>, such as the <a href="/facts/Elevation/h2d0u9nd">elevations</a> of Earth-bound features (<a href="/facts/Terrain/r2lduLeI">terrain</a>, <a href="/facts/Bathymetry/vnYgY6qK">bathymetry</a>, <a href="/facts/Water_level/tUYQsFkI">water level</a>, and built structures) and <a href="/facts/Altitude/B5eNVurK">altitudes</a> of <a href="/facts/Geocentric_orbit/geC4BF1x">satellite orbits</a> and in <a href="/facts/Altitude_in_aviation/B5eNVurK">aviation</a>. In <a href="/facts/Planetary_science/ocX6rp0d">planetary science</a>, vertical datums are also known as zero-elevation surface or zero-level reference. </p><p>Commonly adopted criteria for a vertical datum include the following approaches: </p> <ul><li><a href="/facts/Tides/zNstvWPv">Tides</a>, based on <a href="/facts/Sea_level/mDhuI7Xe">sea level</a> when specific conditions occur, such as <a href="/facts/NOAA/IvcuqNJg">NOAA</a>'s <a href="/facts/National_Geodetic_Survey/8XhGwogo">National Geodetic Survey</a>-produced <a href="/facts/Tidal_datum/919PWLJr">tidal datums</a>;</li> <li><a href="/facts/Gravimetric/kPrbm8wx">Gravimetric</a>, based on a <a href="/facts/Geoid/6aR04HQl">geoid</a>; or geometric, based on the same <a href="/facts/Earth_ellipsoid/l88dGWLD">Earth ellipsoids</a> that are used in computing a horizontal datum, such as NOAA's planned gravimetric and <a href="/facts/Global_Navigation_Satellite_Systems/lVgPlF5v">Global Navigation Satellite Systems</a> (GNSS)-based <a href="/facts/Datum_of_2022/6DIYmdXo">Datum of 2022</a> set to be released that year by the National Geodetic Survey.</li></ul> <p>In the United States, prominent vertical datums for professional uses include the <a href="/facts/National_Geodetic_Vertical_Datum_of_1929/tzU5kdgc">National Geodetic Vertical Datum of 1929</a> and the <a href="/facts/North_American_Vertical_Datum_of_1988/XuS8PeTj">North American Vertical Datum of 1988</a>. </p>