datumsystem

A datumsystem, in geodesy and cartography, is a reference framework used to assign coordinates to points on or near the Earth's surface for maps, surveys, and navigation. It defines the position of the origin, the orientation and shape of a reference surface (the ellipsoid) that approximates the Earth, and how coordinates relate to real locations. A datumsystem is typically part of a coordinate reference system, which may also include a map projection to convert curved surface coordinates into a flat map.

A datum comprises a geodetic datum (the mathematical model of the Earth), a reference ellipsoid (an idealized smooth surface), and a defined relationship between the ellipsoid and the Earth's actual gravity field, sometimes via a geoid or vertical datum for elevations. The combination determines how latitude, longitude, and height are measured and interpreted.

Datumsystems are categorized as global or local. Global geodetic datums, such as WGS84, serve worldwide positioning and navigation. Local or regional datums, such as NAD83 in North America or ETRS89 in Europe, align more closely with a specific country's geoid and control points. Vertical datums, like mean sea level or NAVD88, provide reference levels for elevations.

Because the Earth is dynamic, datums can drift over time, especially global systems tied to plate tectonics. When combining data from different datums, a datum transformation is required to convert coordinates between systems, preserving positional accuracy in GIS, surveying, and navigation applications.