a definition of the origin, orientation, and scale of the coordinate system; usually a system of coordinate positions on a surface (horizontal datum) or heights above or below a surface (vertical datum).[1]
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[a]ny numerical or geometrical quantity or set of such quantities which may serve as reference or base for other quantities. Where the concept is geometric, the plural form is "datums" in contrast to the normal plural "data."[2]
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a reference point, set of points or surface, that provides known locations from which the locations of other features are determined.[3]
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[a] standard position or level that measurements are taken from.[4]