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.)