# A New Model of Quasigeoid for the Baltic Sea Area
## Data
### Gravity Data
In 1994 and 2004, point data from the land and sea area of Denmark, point data from the Eastern part of Germany, point sea measurements and mean anomalies from gravimetric maps from the Kaliningrad area, and point data from Lithuania and Latvia were obtained during our training at the KMS in Denmark. Gravity data are in the GRS80 system and are referred to as IGSN71 [9]. Gravity data from Sweden were obtained from the Bureau Gravitemtique International and include g value as well as free-air anomalies and some other useful information. Data are expressed in the GRS80 system and IGSN71. Atmospheric corrections have been made to this data. Gravity anomalies are in the zero-tide tidal system [29]. Gravimetric data were obtained from GETECH, a West-East European gravity project (WEEGP) commenced in April 1991 and follows the completion of all available land and marine gravity data for the area 35\({}^{\circ}\)\(<\)\(\phi\)\(<\) 84\({}^{\circ}\) and 25\({}^{\circ}\)\(<\)\(\lambda\)\(<\) 60\({}^{\circ}\). The database was used to generate an 8 \(\times\) 8 km grid of interpolated free-air and Bouguer values for the region. All gravity data have been reprocessed using the IGSN71 gravity datum and GRS80 gravity formula. A Bouguer density of 2.67 g/cm\({}^{2}\) was used. Terrain and Earth curvature corrections have been applied to the Bouguer gravity data. Terrain corrections were calculated to a distance of 166.7 km for each gravity station [31]. In 2004, ZGP CBK PAN obtained gravimetric data from GETECH for the area of 14\({}^{\circ}\)\(<\)\(\phi\)\(<\) 30\({}^{\circ}\), 44\({}^{\circ}\)\(<\)\(\lambda\)\(<\) 60\({}^{\circ}\). Inour calculations, GETECH data were used only for parts of Ukraine and Belarus. Probably, gravity anomalies are in the mean-tidal system. Gravity data were from the EGM2008 model [32], and additionally for the data gaps in the Gulf of Riga, the free-air gravity anomalies in a grid \(2^{\prime}\times 2^{\prime}\) were computed from the geopotential model EGM2008. All gravimetric data referred to the normal gravity defined by the GRS80 system and referenced the IGSN71 gravity network. Moreover, where necessary, atmospheric and terrain corrections were added. Gravity data were from Poland, the southern Baltic region, and GETECH data were converted from a mean-tidal system to a zero-tide system according to the formula [33]  
\[g_{z}=g_{m}+30.4-91.2 sin^{2}\varphi\ \upmu\text{Gal} \tag{11}\]  
Gravity data from Finland have been converted from a tide system to a zero-tide system according to the formula [33]. \[g_{z}=g_{n}+(\delta-1)\left(-30.4+91.2 sin^{2}\varphi\right)\ \upmu\text{Gal} \tag{12}\]  
Combined Data Set  
To calculate the geoid/quasigeoid in the Baltic region from the above-mentioned data, a joint gravimetric data set was created, which is shown in Figure 2 (statistics in Table 1). Figure 2: Distribution of gravity data (markings in a figure: GREEN CROSS: mean Faye’s from the Institute of Geodesy and Cartography in Warsaw; LIGHT BLUE CROSS: mean free-air anomalies from the Space Research Centre PAS; VIOLET CROSS: from the Finish Geodetic Institute; ROSE CROSS: shipborne gravity and land data from Kort and Matrikelstyrelsen in Denmark; BROWN CROSS: gravity data from the Bureau Gravitérique International; DARK BLUE CROSS: gravimetric data from GETECH; PURLE CROSS: airborne gravity data were from Kort and Matrikelstyrelsen in Denmark).