A set of quantities from which other quantities are calculated.
For a list of all members of this type, see IDatum Members.
For the OGC abstract model, it can be defined as a set of real points on the earth that have coordinates. EG. A datum can be thought of as a set of parameters defining completely the origin and orientation of a coordinate system with respect to the earth.
A textual description and/or a set of parameters describing the relationship of a coordinate system to some predefined physical locations (such as center of mass) and physical directions (such as axis of spin). The definition of the datum may also include the temporal behavior (such as the rate of change of the orientation of the coordinate axes).