Magyarország 100 évvel ezelőtt létesített első gravimetriai főalappontjának újramérése a Műegyetemen

The first gravity reference station of Hungary was established by Károly Oltay, professor and head of former Department of Geodesy, Technical University of Budapest. The station was located in the pendulum hall and its gravity value (g = 980 852 mGal) was derived in 1915 from relative pendulum measurements with respect to the reference station at Geodetic Institute in Potsdam. The network of European gravity reference stations was adjusted by C. Morelli in the 1940's and he got g = 980 853 mGal for the station established by Oltay. This value only slightly differs from the one obtained by Finnish professor R.A. Hirvonen (g = 980 853.3 mGal), who used isostatic reduction in his calculations. The pendulum hall was reconditioned in 2016, during which the reference station was identified and restored. Gravity was measured at the reference station between 26–27 May, 2016 with the FG5X (No. 251) absolute gravity meter by staff members of the Czech Geodetic Observatory (Pecny, Ondrejov). Vertical gravity gradient was also required to be measured at the station to reduce gravity measurements to the benchmark and to compare with the old g value. The measured vertical gravity gradient (–0.3091 mGal/m) served on the one hand to calculate g for the benchmark (allowing our reference station to be fit into the present operative gravity base network of Hungary [MGH-2013]). On the other hand it facilitated calculating g for the benchmark established by Oltay for comparisons. When the systematic difference of –14 mGal between the reference system of Potsdam and the absolute system (or the value of –13.94 mGal based on local studies) is accounted for, the value obtained by us in 2016 (g = 980 839 431.55 μGal) agrees well with the value obtained by Oltay.