As we know from reading the previous page, in Geodesy an ellipsoid is a mathematical model of the earth as a slightly flattened shape. There are several different ellipsoidal models used in defining the shape of the Earth, and these are used for defining geodetic coordinates, three-dimensional coordinate systems defined in terms of latitude and longitude.

In order to navigate with an ellipsoid model, we have to know where the ellipsoid is tied to defined points on the Earth's surface. A datum defines how an ellipsoidal model of the Earth's shape relates to its actual surface. Places where the Earth and the ellipsoid are "attached" to each other are called Realisation Points (or "Realization Points"), and tend to be marked with a small construction such as a concrete post in the ground. Traditionally, these were used by cartographers to triangulate as an accurate point of reference when making maps.

Unfortunately, many different datums were developed over time, each applying to a different country or part of the world. This means that it is not possible to translate a latitude and longitude position to an exact location on the Earth's surface unless you are aware of which datum is being used. Examples of different datums include:

i) Ordnance Survey 1936
- British, based on Airy's 1830 ellipsoid
ii) Hjorsey 1955
- Icelandic, based on the 1924 International ellipsoid
iii) Bogota Observatory
- Colombian, based on the 1924 International ellipsoid
iv) North American Datum of 1983 (NAD83)
- Based on the GRS80 ellipsoid
v) World Geodetic System of 1984 (WGS84)
- Global, developed by US Department of Defense, based on GRS80 and its own ellipsoid.

The differences in these datums and ellipsoids mean that you may sometimes be given a set of coordinates for a location which do not match other ones you may have. This commonly occurs between NAD83, in North America, and WGS84, as much US geographical data is based on NAD83, and most GPS devices use WGS84.

A2B always uses the World Geodetic System of 1984 (WGS84) for defining coordinates. If uploading from a GPS or entering a position manually, not using WGS84 can lead to errors in the hundreds of metres.

You can find out more about navigation and geodesic datums on our links page.