National Gravity Networks

National gravity activities of the GEO Gravity Group support Danish authorities in keeping the fundamental geodetic infrastructure of Denmark and Greenland up to date. The GEO Gravity Group is the only institution in Denmark capable of monitoring, maintaining and modernizing these national gravity networks. In this capacity we act both as advisers and operators.

The gravity data are used as input in modelling and improving the national geoid - a national realization of the global geodetic “zero-height-level” surface. Good coverage of the Earth’s surface by gravity data of high quality - both on land and at sea - is required to improve a national geoid model. Furthermore, the quality of the national geoid model depends on broader regional data coverage. Denmark is located just on the edge of an area with substantial post-glacial land uplift which clearly affects gravity networks in the neighboring Scandinavian and Baltic countries. Although monitoring timewise change of gravity data is less severe in Denmark it nevertheless conforms to joint regional data standards by the Nordic Geodetic Commission (NKG) – a regional organization coordinating this work.

Absolute gravity observations in Denmark are largely collocated with Danish GNSS CORS stations so that measured gravity changes can be compared with a corresponding height change. We use relative gravimetry for quality assessment of the network by revisiting selected subsets of these stations with modern instruments. This dense data set is used for computation of the national geoid model, to further densify data coverage, and to provide reference stations in harbors and airports. We also conduct relative gravity measurements on '5D points' – a physical geodetic network of stations used for national levelling in Denmark. Gravity measurements on these 5D points should make it possible, in cases of misalignment between the geometric and gravimetric geoids, to diagnose the reason for the misalignment.

The same gravity data can be used to map geological structures in the subsurface, jointly with seismic data, for example, whether for exploration or academic purposes.