Datum represents a mathematical model that provides a smooth approximation of the Earths surface. 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.
Datum is also called a geodetic coordinate system - a coordinate system that is referenced directly to a datum. Some coordinate systems only specify the ellipsoid; these are non-geodetic. Coordinate systems that specify the complete datum are geodetic.
The Global Positioning System (GPS) uses an earth centered datum called the World Geodetic System 1984 or WGS 84. Datum conversions to and from WGS 84 are accomplished by following methods:
Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude or another coordinate system.
Vertical datums measure elevations or depths. In engineering and drafting, a datum is a reference point, surface, or axis on an object against which measurements are made.