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The estimation of the zero-height geopotential level of a local vertical datum (LVD) is a key task towards the connection of isolated physical height frames and their unification into a common vertical reference system. Such an estimate resolves, in principle, the 'ambiguity' of a traditional crust-fixed LVD by linking it with a particular equipotential surface of Earth's gravity field under the presence of an external geopotential model. The aim of this paper is to study the estimation scheme that can be followed for solving the aforementioned problem based on the joint inversion of co-located GPS and leveling heights in conjunction with a fixed Earth gravity field model. Several case studies with real data are also presented that provide, for the first time, precise estimates of the LVD offsets for a number of Hellenic islands across the Aegean and Ionian Sea.

The aim of this study is to present the results of several 'external' quality tests for the most recent (at the time of writing this paper) global geopotential models (GGMs) using precise GPS and leveled orthometric heights over the area of Greece. The tested GGMs include the GRACE-based combined model GGM03C, the latest EIGEN-type combined models EIGEN-GL04C and EIGEN-GL05C, the ultra-high resolution model EGM08 that was released last year by the US National GeospatialIntelligence Agency, and also the older NASA/NIMA/OSU's EGM96 model. The evaluation tests are based on comparisons of absolute and relative geoid undulations that are computed from the selected GGMs and the external GPS/levelling data. The test network covers the entire part of the Hellenic mainland and it consists of more than 1500 benchmarks which belong to the Hellenic national triangulation network, with direct levelling ties to the Hellenic vertical reference frame. The spatial positions of these benchmarks have been recently re-determined at cm-level accuracy (with respect to ITRF00) through a nation-wide GPS campaign that was organized in the frame of the HEPOS project. Our results show that the EGM08 model offers a remarkable improvement for the agreement among geoidal, ellipsoidal and orthometric heights in the mainland part of Greece, compared to the performance of other combined GGMs over the same area. Finally, our study gives a preliminary (yet realistic) accuracy assessment for GGM/GPS-aided orthometric height determination, over different baseline lengths, throughout the Hellenic mainland.

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This paper presents an overview of the evaluation results for the new Earth Gravitational Model (EGM08) that was recently released by the US National Geospatial-Intelligence Agency, using GPS and leveled orthometric heights in the area of Greece. Various comparisons of geoid undulations obtained from the EGM08 model, other combined geopotential models and GPS/leveling data have been performed in both absolute (at individual points) and relative (for baselines of varying length) sense. The test network covers the entire part of the Greek mainland and it consists of more than 1500 benchmarks that belong to the Hellenic national triangulation network, with direct leveling ties to the Hellenic vertical reference frame. The spatial positions of these benchmarks have been recently determined at cm-level accuracy (with respect to ITRF2000) through an extensive national GPS campaign that was organized in the frame of the HEPOS project. Our results suggest that EGM08 offers a major improvement (more than 50%) in the agreement level among geoidal, ellipsoidal and orthometric heights over the mainland part of Greece, compared to the performance of previous global geopotential models for the same area.