surveying iii (topographic and geodetic surveys) - Modern Prepper

The Airy theory, announced by G. B. Airy in 1855, proposed that continents andislands are resting hydrostatically on highly plastic or liquid material, with roots orprojections penetrating the inner material of the earth just as icebergs extenddownward into the water (Figure 1-13). The greater the elevation of mountain massesabove the earth, the deeper the penetration of the roots. It has been called the Rootsof Mountain Theory, and has the support of some geologists.Figure 13. Airy's Theory of Compensationj. The theoretical value of gravity at a point on the ellipsoid depends on the size and shape of theellipsoid and the observed value of gravity at the equator (978 gals). It also varies with the latitude ofthe observation point, assuming that the earth is a surface without mountains and oceans, having novariations in rock densities or in the thickness of the crust. The theoretical value of gravity representsthe force of the earth's attraction due to gravitation, minus the centrifugal force due to the rotation of theearth.k. Since the earth is not an ellipsoid and there are variations in both the crust material and theterrain, the observed gravity of the geoid varies from point to point. Gravity observations must be madeso that the distance between the geoid and theEN0593 1-20