Geodetic systems or map datums are used in geodesy, navigation, surveying and the Global Positioning System to indicate the datum or reference point to which a measured position is relative.

The systems are needed because the earth is not a perfect sphere.

Examples of map datums are:

• WGS 84, 72, 64 and 60 of the World Geodetic System
• OSGB36 of the Ordnance Survey of Great Britain
• ED50 : the European Datum

The main reason that there are a number of datums is that national map making organisations did not have a common surveying reference point and only produced maps for their locality.