# Physical geodesy

The **physical geodesy** includes those branches of higher geodesy , where physical aspects against geometrical come to the fore.

Your main tasks are to determine the shape of the earth and to measure and model the earth's gravity field . The methodology can be two- or three-dimensional ( scalar or vector field of gravitational acceleration).

In the field of research, physical geodesy contributes to the exploration of the earth's interior (e.g. density of rock layers ), but also to the gravitational field and the interference from satellite orbits . Some topics overlap with those of geophysics .

Knowledge of the gravitational field is important for the practice of surveying , because almost all geodetic measurements are linked to the gravitational field. For example, the measurement instruments are in front of the measurement along the true plumb line leveled , which is principally influenced by all compositions in the environment and therefore runs irregularly on closer inspection.

The differences between reality and a simple mathematical model are shown e.g. B. in the deviation of the true plumb line from the normal ellipsoid , the **plumb deviation** (ξ, η, *English: vertical deflection* ). In gravimetry, on the other hand, the disturbances are given in the form of gravity anomalies (Δg, *gravity anomalies* ).

Vertical deviation and gravity anomaly can reach a magnitude of 10 to 50 " or a few in the mountains . These effects of the gravitational field and geoid undulations have an effect of up to 20 cm per kilometer on a surveying network in rough terrain . The *centimeter geoid has* been aimed for since around 1995 because otherwise the increasing GPS measurements would not be able to be combined with terrestrial height measurements .
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The calculations of the measurement points must abstract from these irregularities - that is, "reduce" the disturbances - by executing them on mathematically defined surfaces and bodies such as planes, spheres or ellipsoid of revolution. The reduction can only be omitted if the survey area is smaller than a few hundred meters and is not in the high mountains .

Physical geodesy has to be particularly careful in larger technical projects ( power plants , tunnel construction or the alignment of a high-speed line ).