Vertical datum

In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built structures) and altitudes of satellite orbits and in aviation. In planetary science, vertical datums are also known as zero-elevation surface or zero-level reference.

Commonly adopted criteria for a vertical datum include the following approaches:

• Tides, based on sea level when specific conditions occur, such as NOAA's National Geodetic Survey-produced tidal datums;
• Gravimetric, based on a geoid; or geometric, based on the same Earth ellipsoids that are used in computing a horizontal datum, such as NOAA's planned gravimetric and Global Navigation Satellite Systems (GNSS)-based Datum of 2022 set to be released that year by the National Geodetic Survey.

In the United States, prominent vertical datums for professional uses include the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988.