Lietuvos teritorijos kvazigeoido parametrų tikslinimas 1998 metais

Abstract

In this article the results of the accuracy analysis of the Digital Geoid Model of Lithuanian territory are presented and recommendations for the rational development of the Vertical geodetic control are given. The geoid model for Lithuanian territory was calculated by the Geoid Determination Working Group Of the Nordic Countries Geodetic Commission three times: in 1994, 1996 and 1998. The main efforts were directed to collecting the gravity data and determination of the most reliable benchmarks set. Gravity data of the terrain, Baltic Sea and Curonian lagoon, totally about 12000 points, and DTM with a grid of 1 km have been prepared. The average density of the gravity data is 1 point per 5 sq km. The sources of the gravity data were the gravimetric maps at the scale of 1:200000 and catalogues of the gravity points. The gravity data used are of the period of 1954-74. Also, some data from the surrounding areas were involved, totally about 3000 points. The accuracy of ellipsoidal heights ensures the Lithuanian GPS Network, which consists of zero, first and second order stations, totally 1078 points. The Zero Order GPS Network covering the territories of Estonia, Latvia and Lithuania was established in 1992, during the international EUREF BAL’92 GPS campaign. The First Order GPS Network was designed and observed in 1992-93 by the Geodetic Institute, Vilnius Technical University. The Second Order GPS Network was measured and adjusted in 1994-96 by Geodetic Institute as well. The density of Lithuanian National GPS Network is 1 station per 60 sq km. According to the software for the adjustment of the GPS networks the accuracy of the ellipsoidal heights of the stations of the state GPS network is 2-5 mm. But these parameters do not reflect the real co-ordinating accuracy. More realistic accuracy was calculated from the differences of the double co-ordinating. In this case the 20 mm accuracy of the ellipsoidal heights was achieved. The GPS network was a basis for positioning the benchmarks of the first and second order levelling lines. Totally 120 benchmarks were co-ordinated, from which 70 most reliable ones were chosen for the geoid fitting. The GRAVSOFT software package was used for calculations. The geoid model of Lithuania fit at the 3 cm level was obtained.