The departure of the geoid from the spherical surface, owing to its oblateness, is so small that in the figures which we shall use for illustration it would be less than the thickness of a line.
The oblateness of the geoid is well understood from accepted hypotheses to be the result of the once more rapid rotation of the planet when its materials were more plastic, and hence more responsive to deformation.
Granting that the geoid or mean surface of the ocean is a uniform spheroid, the distribution of land and water approximately indicates a division of the surface of the globe into two areas, one of elevation and one of depression.
Compared with the "spheroid of reference" the surface of the geoid is in general depressed over the oceans and raised over the great land masses.
This condensation is, in practice, the same as to the geoid itself.
In 1849 Stokes showed that the normal elevations N of the geoid towards the ellipsoid are calculable from the deviations [Delta]g of the acceleration of gravity, i.
The reduction of triangulation base lines from the geoid to the reference ellipsoid.
In 1849, Stokes derived a theorem whereby the distance N of the geoid from the ellipsoid of reference can be obtained from an integration of gravity anomalies over the surface of the earth.
In physical geodesy, gravimetric data are used for the determination of the geoid and components of deflections of the vertical.
The geoid is above the ellipsoid of reference under mountains and it is below the ellipsoid on the oceans, where the geoid coincides with mean sea level.
The geoid is defined as the average level of the sea, which is thought of as extended through the continents.
The plumb line is perpendicular to the geoid, and the components of angle between the perpendiculars to geoid and reference ellipsoid are deflections of the vertical.
They have been in operation from the earliest known eons to the present time, and their tendency is ever to deform the geoid and produce irregularity of the terrestrial surface.
They have been in active operation since the dawn of geologic time, and the processes individually and combined ever tend to restore the geoid by obliterating the relief produced by deformation.
