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Physical Earth Models Geodetic Reference System
Physical earth models depend on the gravity field.
One of the 3 definitions of "figure of the earth" [Moritz 1990, p. 1]:
B) The (part of the earth bounded by the) geoid, which is a level surface coinciding (somewhat loosely speaking) with the free surface of the oceans together with its continuation under the continents. It is the geoid above which "heights above sea level" are measured. A level surface is everywhere horizontal, that is, perpendicular to the direction of the plumb line. Level surfaces are surfaces of constant gravity potential W. , W = const. and the geoid is one of them, W = W0, denoting the constant geoid potential by W0. Again we are disregarding temporal (tidal) variations. Whereas the physical earth's surface, in its picturesque variety and beauty, is very irregular, the geoid is smoother and subject to a mathematical equation, W = W0; however, even the gravity potential W is far from being a simple mathematical function. Therefor, the geoid is referred to a much regular, "normal", surface which approximates the geoid while being more regular in an mathematical or physical sense.