Geodetic datum and geoid

The international standard ISO 19111:2007 called Geographic Information: Spatial Referencing by Coordinates defines datum as: "a set of parameters that defines the position of the origin, the scale, and the orientation of a coordinate system."

Global and regional datums

A datum can be a horizontal datum or a vertical datum, either global or local. Jekeli (2006) showed in his previous work that a traditional datum is defined by eight parameters: three to define its origin (for example, the center of mass of the Earth), three to define its orientation, and two to define the ellipsoid.

As you could see, an ellipsoid is not enough to define a global datum. We need an ellipsoid and a Terrestrial Reference System that has the defined origin and orientation with respect to the Earth, as in the following figure:

An example of global datum is the World Geodetic System 1984, known as WGS84.

A regional datum aligns the ellipsoid to a regional surface in such way to have...