Positioning And Coordinates System In Directional Wells
Ellipsoid

is the name of the volume obtained when an ellipse is rotated about one of its axes. Specifically, an oblate spheroid is an ellipse rotated about the shorter (semi-minor) axis. The Earth is not an exact ellipsoid, and deviations from this shape are continually evaluated. the curvature of the Earth's surface is not uniform due to irregularities in the gravity field  

Geodetic Datum

is a definition of a model for the surface of the earth.
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