Definition
An oblate spheroid defined by a semi-major axis and a flattening value, approximating mean sea level well enough for coordinate computation. Different ellipsoids fit different regions best, which is why datums proliferated historically: Airy 1830 suits the British Isles, Clarke 1866 suited North America, and WGS84 is a best global fit. Latitude and longitude are meaningless without stating which ellipsoid and datum they refer to.
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