Gravity Change in Finland 1962-2010 from the Comparison of Legacy Relative Measurements with New Absolute Measurements Using the A10-020 Gravimeter

The gravity change associated with the Fennoscandian Postglacial Rebound (PGR) has been studied for 50 years now, with both relative and absolute gravity measurements. High-precision relative gravity measurements on the specially designed Fennoscandian Land Uplift Gravity Lines began in 1966. First absolute-gravity measurements with laboratory-type instruments were made in 1976. Here we report on a new regionally dense dataset: the comparison of legacy relative measurements in the Finnish First Order Gravity Net (FOGN) with absolute-gravity measurements with the A10-020 gravimeter. The FOGN was first measured in 1962 using a Worden Master gravimeter, and re-surveyed in 1988 using two LaCoste&Romberg model G gravimeters. It was re-measured in 2009-2010 using the A10-020 free-fall gravimeter of the Institute of Geodesy and Cartography. The FOGN covers the whole country and consists of 50 outdoor stations in public buildings, typically on church steps. About 30 stations from 1962 were still intact in 2009/10, and at some additional stations there is a history of local relative ties to replacement sites now occupied with the A10-020. The vertical PGR rates at the sites are up to 1 cm/yr, and thus the total gravity change in the 47 years can amount to 80 microgals. Since the legacy measurements are relative, only the differences of gravity change are estimable, and consequently the expected maximum signal is less, about 60 microgals. We compare the observed gravity change in the FOGN with estimates of vertical motion from continuous GNSS, from repeated precise leveling and from tide gauges, and with gravity change predicted from PGR models. At seven locations the gravity change estimated from the FOGN can also be compared with time series of absolute-gravity measurements with laboratory-type instruments.