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:

• Complete datum conversion is based on seven parameter transformations that include three translation parameters, three rotation parameters and a scale parameter.
• Simple three parameter conversion between latitude, longitude, and height in different datums can be accomplished by conversion through Earth-Centered, Earth Fixed XYZ Cartesian coordinates in one reference datum and three origin offsets that approximate differences in rotation, translation and scale.
• The standard Molodensky formulas can be used to convert latitude, longitude, and ellipsoid height in one datum to another datum if the Delta XYZ constants for that conversion are available and ECEF XYZ coordinates are not required.

Horizontal Datum

Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude or another coordinate system.

Vertical Datum

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.