datuums

Datuums are reference frameworks used in geodesy, surveying, and GIS to define the location and elevation of points on the Earth's surface. A datum comprises a coordinate system and a reference surface or ellipsoid against which measurements are made. In practice, datum is often treated as singular; the plural datums is common in English, while data is sometimes used as a plural in general usage. The form datuums appears in some technical or Latinized writings as a plural.

Horizontal datums define positions on the surface of the Earth by adopting a reference ellipsoid and an anchor point or network of control points. Global examples include WGS 84, used by GPS, while regional systems include NAD83 in North America and ETRS89 in Europe. Local datums, such as NAD27, are tied to specific regional networks and can differ substantially from global frames.

Vertical datums define elevations relative to a reference surface, typically mean sea level or a geopotential surface. They distinguish between ellipsoidal height (from the reference ellipsoid) and orthometric height (above the vertical datum surface). Common vertical datums include NAVD88 in North America and European vertical datums tied to mean sea level.

Transformations between datums enable integration of geospatial data from different frames. These conversions rely on parameters and, for time-dependent frames, more complex models to account for tectonic motion and updates to the reference surface. The use of a consistent datum is essential for accurate navigation, mapping, construction, and data analysis.

Historically, datums emerged from surveying networks that anchored maps to fixed surfaces. Today, national and international agencies maintain geodetic networks, with modern global frames realized through systems like the International Terrestrial Reference System (ITRS) and its realizations, supporting time-dependent coordinates to reflect Earth dynamics.