The geoid is that
equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were to be extended through the continents (such as with very narrow canals). According to
C.F. Gauss, who first described it, it is the "mathematical figure of the Earth," a smooth but highly irregular surface that corresponds not to the actual surface of the Earth's crust, but to a surface which can only be known through extensive
gravitational measurements and calculations. Despite being an important concept for almost two hundred years in the history of
geodesy and
geophysics, it has only been defined to high precision in recent decades, for instance by works of
P. Vaníček and others. It is often described as the true physical
figure of the Earth, in contrast to the idealized geometrical figure of a
reference ellipsoid.
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Name for the pear shape of planet Earth, which is not a sphere , nor is it a perfect 'door knob' shaped spheroid due to centrifugal distortion by rotation about the polar axes because there is a sea at the arctic and a land mass at south pole.
Figure of the Earth visualized as a mean sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction of gravity (the plumbline).
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