Evaluation results of the Earth Gravitational Model EGM08 over the Baltic countries A. Ellmann Department of Civil Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn, Estonia J. Kaminskis Geodesy Department, Latvian Geospatial Information Agency, O.Vaciesa iela 43, Riga LV-1004, Latvia E. Parseliunas Geodetic Institute, Vilnius Gediminas Technical University, Sauletekio al. 11, LT-10223 Vilnius, Lithuania H. Jürgenson Estonian University of Life Sciences, Kreutzwaldi 5, Tartu, Estonia T. Oja Department of Geodesy, Estonian Land Board, Mustamäe tee 51, Tallinn, Estonia Abstract. Earth’s geopotential model (EGM) in conjunction with regional terrestrial gravity data are often used in regional geoid determination. Thus, significant enhancements are expected due to release of the new high resolution Earth Gravitational Model EGM08. Accordingly, this study evaluates the performance of the EGM08 model over the Baltic Sea region with emphasis to Estonia, Latvia and Lithuania. Several different sets of the “ground truth” data are used in the comparisons. First, the EGM08-derived height anomalies are compared with an existing regional geoid model. The detected discrepancies range within ± 0.3 m with a mean of - 0.02 m, whereas the standard deviation (STD) of the discrepancies amounts to 0.08 m. The largest discrepancies occur in the areas where only a few data points were available either for the regional geoid modeling or at the EGM08 compilation, or both. Second, the free-air gravity anomalies at the terrestrial data-points are compared with the EGM08-derived anomalies. The STD of the anomaly discrepancies is 2.6 mGal. Finally, the EGM08 model is validated with respect to GPS- levelling data. The STD of detected discrepancies is 0.06 m, with a mean of 0.49 m. Thus, the EGM08 based quantities agree reasonably well with the tested datasets. Evidently, most of the available gravity data in the Baltic Sea region appear to be utilised at the EGM08 construction. Keywords: geopotential model, geoid, GPS- levelling. _____________________________________________________ 1 Introduction A new combined Earth gravitational model EGM08 (Pavlis et al, 2008) was released to the public in 2008. EGM08 takes advantages of recent satellite, terrestrial gravity, elevation and altimetry data. This activity is conducted by the National Geospatial- Intelligence Agency (NGA) of the USA. The resolution of the EGM08 is 5´ (corresponding to 9 km, i.e. to the spectral degree of ca 2160), also the global accuracy of the EGM08 is expected to be superior over earlier EGM-s. Regional improvements of global geoid models can be obtained by modifying Stokes’s integral formula (Stokes, 1849). When solving the Stokes problem, strictly speaking, gravity anomalies over the entire Earth are required. In practice, however, the data availability is limited to some spatial domain (Ω ψ0 ) around the computation point. Modified Stokes’s formula (first proposed by Molodenskii et al., 1960) combines local terrestrial gravity anomalies and the EGM-derived long- wavelength component of the geoid. For instance, a generalized Stokes scheme (cf. Vaníček and Sjöberg, 1991) can be used ( 29 ( 29 ( 29 ( 29 ( 29 0 2 0 2 0 , , 4 2 , 2 1 L L n n L n n R N S gR g R d R g R n ψ ψ πγ γ = Ω =   ′ Ω = ∆ Ω- ∆ Ω Ω+     + ∆ Ω - ∑ ∫∫ ∑ (1) where R is the mean radius of the Earth; ψ is the geocentric angle, the modified Stokes function S L (ψ) 110