A set of parameters specifying the reference surface or the reference coordinate system used for geodetic control in the calculation of coordinates of points on the earth. Commonly datums are defined as horizontal and vertical datums separately.
For a local geodetic datum the reference surface is defined by five parameters: the latitude and longitude of an initial point, the azimuth of a line from this point, and the parameters of the reference spheroid.
Absolute datums specify the initial point of the reference ellipsoid to be (ideally) located at the earth's centre of mass. For modern reference systems using datum information given by satellites additional parameters are defined, e.g. gravity models.
See also horizontal control datum.