A datum is a set of parameters defining a coordinate system and a set of control points whose geometric relationships are known either through measurements or calculation (Dewhurst,1990). All datums are based on a spheroid, which approximates the shape of the earth. Each datum includes both an ellipsoid, which specifies the size and shape of the earth, and a base point for latitude and longitude. If two maps use different datums, points on the map may not line up.