# A New Model of Quasigeoid for the Baltic Sea Area
## Data  
### Gravity Data  
Our calculations use gravimetric data from various sources, and these data are described below. Mean Faye's anomalies in 1\({}^{\prime}\)\(\times\) 1\({}^{\prime}\) blocks for Poland's territory are from the Institute of Geodesy and Cartography in Warsaw. These data have been calculated from gravimetric point data carried out in the years 1957-1979, from the detailed survey and gravimetric profiles referred to as the Potsdam gravity system. Horizontal coordinates were expressed in the national \"Borowa Gora\" geodetic reference system. All gravimetric measurements have been linked to the national leveling network (Kronstadt60) with an accuracy of +5 cm. Gravimetric measurement accuracy is estimated as \(\pm\) 0.11 mGal [23]. Then, all data were subsequently corrected to the IGSN71 network, transformed to the ETRS89 reference system [24], and corrected for the terrain corrections and atmospheric correction. Then, mean Faye's anomalies were calculated in 1\({}^{\prime}\)\(\times\) 1\({}^{\prime}\) blocks using elevation data from the gravimetric database and the DTED2 model by collocation method [25]. Gravity anomalies are in the mean-tide system [26]. Mean free-air anomalies in 1\({}^{\prime}\)\(\times\) 1\({}^{\prime}\) blocks for the Baltic south area are from the Space Research Centre PAS. These anomalies were calculated from a gravimetric survey taken between 1978 and 1980 by the teams of the Moscow Research Institute. Measurements were made along the profiles using gravimeters of Soviet construction. The point position error in the horizontal plane was estimated \(\pm\)80 m. The depth of the sea was determined by sonar with an accuracy \(\pm\) of 1.4 m. After taking into account the necessary corrections, the error of determining free-air anomalies was estimated as \(\pm\)0.57 mGal. The geographical coordinates are determined in the \"Borowa Gora\" horizontal reference frame and the heights in the Kronstadt60 vertical system. Finally, anomalies were calculated based on the formula for the GRS80 normal field and referred to as IGSN71. These data do not contain atmospheric corrections. Mean free-air anomalies in a 1\({}^{\prime}\)\(\times\) 1\({}^{\prime}\) grid were calculated with the collocation method [27]. Computed gravity anomalies are in the mean-tidal system. Mean 5\({}^{\prime}\)\(\times\) 5\({}^{\prime}\) free-air gravity anomalies from Finland were provided by the Finish Geodetic Institute. Anomalies in 5\({}^{\prime}\) by 5\({}^{\prime}\) blocks were generated from the Finnish gravity register. The anomalies are free-air anomalies computed using the gravity formula of the GRS80 system. No terrain corrections were applied. The coordinates referred to grid cell center points, while the grid cell edges are on \"even\" latitudes/longitudes (\"Forsberg convention\"). The heights used are in the Finnish height system NN60. Horizontal coordinates are in the Finnish Map Grid Coordinate System (KKJ), which differ by several hundred meters from both ED50 and ED87 coordinates. The accuracy of anomalies computed in such a way is estimated to fluctuate within \(\pm\)2 mGal [28]. Gravity anomalies are in the tide-free system [29]. Airborne gravity data were from Kort and Matrikelstyrelsen in Denmark. A set of point aerial gravimetric profiles, from the project on the Baltic Sea, was carried out in 1999. The measurements were performed with the use of a high-class gravimeter stabilized on a vibration-damping platform. Data from airborne gravimetric measurements (free-air anomalies) are linked to gravimetric points in the IGSN71 system at airports. The position is determined by GNSS observation related to numerous reference stations linked to the ITRF94. Consistent r.m.s. error estimates for r surveys range from \(\pm\)1.5 to \(\pm\)2 mGal [30]. Gravity anomalies are in the zero-tide tidal system [29]. Shipborne gravity and land data were from Kort and Matrikelstyrelsen in Denmark.