Glossary for Geospatial Science

datum:  A mathematical description of a smooth surface that closely fits the mean sea-level surface for an area of interest, which provides an imaginary, perfectly flat horizontal surface for use as a map base with x,y coordinate measurements.  A datum is derived from a chosen ellipsoid, and provides the surface to which a cartographer refers ground control measurements.  Maps of large extent must use consistent parameters for ellipsoid and datum to insure consistency between the map projection and ground control.  (Map Projections Used by the U. S. Geological Survey, Geological Survey Bulletin 1532, Second Edition, U. S. Government Printing Office, Washington, D.C., 1982, pp. 13-15).  (See also: ellipsoid.)