Definition
A geodetic datum defines a reference ellipsoid and its orientation and origin relative to the Earth, fixing what a stated latitude and longitude actually means. Local datums such as OSGB36 or ED50 were fitted to a region and can differ from global datums by hundreds of metres. Because a satellite receiver reports on a global datum, positions must be converted before plotting on a chart drawn to a different datum, or the plotted position will be systematically wrong.
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