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GPS terminology)

Cartesian coordinate system where the origin is (nominally) the center of the earth. It is actually the centre of a datum which is thought to best fit the size and shape of the earth. For GPS calculations it is defined and named WGS 84. The X direction is the intersection of the prime meridian (Greenwich) with the equator. The vectors rotate with the earth. The Z direction is from the centre of the earth to the north pole.