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datum
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[geodesy]
The reference specifications of a measurement system, usually a system of coordinate positions on a surface (a horizontal datum) or heights above or below a surface (a vertical datum).
[coordinate systems, photogrammetry]
In imagery, the origin and orientation of lines on a surface. Datums are used as part of a coordinate reference system to provide a frame of reference when measuring location on the earth's surface.
[georeferencing]
A collection of highly accurate control points used for georeferencing map data so that it aligns with the geodetic latitude and longitude coordinate system; a standard position of level from which measurements are taken.