Geodesy
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:
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.
