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The astronomical tide generating forces cause an elastic deformation of the solid Earth. Compared to the model of the rigid Earth, the amplitudes of the partial tides of the solid Earth are amplified and a phase shift takes place. In addition, the ocean tides affect the gravity measurements by the direct attraction of the moving water masses and indirectly by the resulting deformation of the crust due to the water load (ocean load tides). In general, close to the ocean the tidal loading effect is much smaller than the body tides but still affects gravity to very large distances from the coast (Jentzsch 1997).

21 m above floor level). 200 m above floor level or above the ground mark. For the reduction of the absolute gravity value to the floor mark, the observed gravity difference (hereafter called gradient) is needed. Following the IfE standard procedure, the vertical gravity gradient is determined with two LaCoste and Romberg gravimeters with integrated feedback systems (Schnüll et al. 1984) or with a Scintrex Autograv CG3M (since 2002) using a tripod of about l m height. By observing the difference 10 times with each relative meter, the gravity Fig.

000 m, the vertical gravity gradient γ is obtained. Here, a linear gravity change with height is assumed (constant gradient). For geodynamic research, often a more accurate knowledge about the vertical gravity change is required by considering a second-degree polynomial for the height dependency. In those cases, gravity differences g are measured between variable height levels h above the ground mark (cf. Fig. 5). A leastsquares adjustment of observation equations provides an overconstrained solution for the coefficients γ 1 and γ 2 describing the linear and quadratic part of the polynomial: g(hi , hj ) = γ1 (hj − hi ) + γ2 (h2j − h2i ).