Physical earth models depend on the gravity field.
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
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
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
much regular, "normal", surface which approximates the geoid while being more regular in an
mathematical or physical sense.