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This question already has an answer here:

Are the datum and the ellipsoid the same? What is a 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. in this article....

marked as duplicate by PolyGeo, Antonio Falciano, simo, Vince, Fezter Mar 10 '14 at 11:35

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    Please refer to this Q&A to get a response (or at least part of it) : gis.stackexchange.com/questions/664/… – simo Mar 10 '14 at 8:14
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    It might also help if you can tell us what research you've already done. – BradHards Mar 10 '14 at 11:05
  • actually this is not a duplication. he asked about ellipsoid-vs-datum and the Q you refer to is projection-vs-datum. the answer is: ellipsoid is a 3d mathematical surface. a datum consists of an ellipsoid + more parameters. for example the same ellipsoid can be set at different positions and orientations and then you'l have different datums – dowi Sep 28 '14 at 8:25
  • For some reason I was unable to re-open this question when I tried to just now. In any event, I think it is a duplicate of gis.stackexchange.com/questions/175293/… instead of the Q&A to which it is currently assigned. – PolyGeo Dec 31 '15 at 1:27