Datums

A geodetic datum is a fixed reference point according to which the size and shape of Earth are measured. They are also used in measuring gravitational pull and rotation. There are at least three kinds of geodetic datums used in geography. Vertical datums form the zero surface for vertical measurements of altitude and elevation. Horizontal datums define the size and shape of Earth and the origin and orientation of a horizontal coordinate system. Complete datums can provide both vertical and horizontal zero origins and coordinate system definitions. Some complete geodetic datums define Earth's shape, horizontal and vertical coordinate system origins, gravity fields, and physical constants such as rotational velocity and Earth's gravitational constant.

Without a vertical datum, the altitude of a point is ambiguous, and ...