Geographic Information System (GIS) - Terminology (Continued) - Datum
The Earth is shaped like a flattened sphere. This shape is called an ellipsoid. A datum is a model of the earth that is used in mapping.
The datum consists of a series of numbers that define the shape and size of the ellipsoid and it's orientation in space. A datum is chosen to give the best possible fit to the true shape of the Earth.
There are a large number of datums in use. Many of them are optimised for use in one particular part of the world. An example is the Geodetic 1949 datum that has been used in New Zealand. Another example, familiar to GPS users, is the WGS-84 datum. WGS-84 is an example of a datum that is used globally.
Latitude and longitude are commonly used to refer to a specific location on the surface of the Earth. It is important to keep in mind that latitude and longitude are always specified in terms of a datum. The latitude and longitude of your current position are different for different datums. For example, the Cathedral in central Christchurch is at 172.63658E, 43.53103S in WGS-84 coordinates and 172.36344E, 43.53270S in Geodetic 1949 coordinates. If you are working with latitude/longitude coordinates and you get an error of a couple of hundred metres, you most likely are using the wrong datum.
New Zealand is changing to a new coordinate system for general mapping. New Zealand Transverse Mercator projection (NZTM) is replacing New Zealand Map Grid, for general mapping. NZTM uses a new official NZGD2000 datum. Unless you are a mapping or survey professional, NZGD2000 and WGS-84 can be considered to be the same. There's more about NZTM and NZGD200 in our NZTM article