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.

Digital elevation models (DEMs) are a widely used form of topographic information, with some of the most popular being the Shuttle Radar Topography Mission (SRTM) DEM and the Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model (ASTER GDEM). These two sources of topographical information are the main constituents of the European Union Digital Elevation Model (EU-DEM), which is a relatively new dataset of the EU’s Copernicus Land Monitoring Service. In this context, the purpose of this study was to validate EU-DEM for its vertical accuracy and to compare it with SRTM DEM and ASTER GDEM data. This was achieved in a Geographic Information System (GIS) environment, using extensive—in the order of tens of thousands of points—geodetic Global Navigation Satellite System (GNSS) measurements and appropriate pre-processing steps. The absolute elevation errors results had a Root Mean Square Error (RMSE) of 2.7 m at a 90% confidence level and characterize the performance of EU-DEM from local to regional scale, generally confirming that it is an enhanced source of elevation information when compared with its predecessors.

Following the M 7.0 earthquake that struck the Greek island of Samos and Turkey's western coast, causing extensive damage and casualties, we combined existing knowledge geodatabases concerning historical seismicity and rupture zones with seismological and geodetic measurements as well as with modelling and in situ observations, to provide an assessment of rapid response to the seismic event. In this paper, we demonstrate that in the frame of the gradual provision of information from the individual scientific disciplines, taking into account their respective potential and limitations, a multidisciplinary approach is able to address more efficiently rapid response issues in order to allow effective preliminary interpretation of the earthquake activity, even within the first 24 h of the event. It focuses on the assessment of the timely provision of information by each discipline, evaluating the access to primary data sources as well as the maturity of the techniques in terms of accuracy and rapid data processing. Within a period of less than a week, several constraints were partially compensated for, allowing the delivery of more robust results and interpretation. The study highlights the readiness level of the various domains that has been significantly improved over the past years, including rapid seismological solutions, systematic availability of free and open Earth Observation data and on-demand online processing through dedicated platforms. Their combination with routinely applied inversion modelling and timely in situ observation is leading to improved operational response levels. Supplementary Information The online version contains supplementary material available at 10.1007/s11600-021-00578-6.

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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.

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Many of the old geodetic reference frames which realized in the previous decades using classical observations carry biases. These biases are mainly caused due to the problematic observations and/or the tectonic motion. That is the case of the official Greek geodetic reference frame which consists of classical and satellite observations. Herein, we present a rigorous approach of the reconstruction of the Greek official reference frame based on the modern geodetic reference frames and their ability to express the spatial position and the dynamic change of the stations. We applied the rigorous approach to ninety stations located in Greece and we compare it with the officially accepted procedure. We found a consistency at 59.4cm between the rigorous and the officially accepted approaches, respectively. The associated mean bias estimation was estimated at 51.4 cm, indicating the resistance of a rather large amount of systematic effects. In addition, the observed discrepancies between the two approaches show great inhomogeneity all over the country. Keywords: Geodetic Reference System (GRS), GRS Datum, transformation between two GRS, ETRS89Resumo: Muitos dos antigos sistemas geodésicos de referência que foram realizados nas últimas décadas por meio de observações clássicas apresentam erros. Estes erros são principalmente oriundos de problemas nas observações e/ou movimentos tectônicos. Este é o caso do sistema geodésico de referência Grego, que consiste de observações clássicas e por satélites. Neste trabalho apresenta-se uma abordagem rigorosa para a reconstrução deste sistema geodésico de referência baseada nos sistemas de referência modernos e na sua habilidade de expressar a posição espacial e as mudanças dinâmicas das estações de referência. Foi aplicada a abordagem rigorosa a noventa estações localizadas na Grécia e foi realizada uma comparação com o procedimento oficialmente aceito. Encontrou-se uma consistência de 59,4 cm entre os procedimentos rigoroso e oficial. A estimativa de erro associada foi de 51,4 cm, indicando grande efeito sistemático. Além disso, as discrepâncias observadas entre as duas abordagens demonstram falta de homogeneidade ao longo do país.Palavras-chave: Datum Clássico; Transformação; Campo de velocidade; ETRS89; Sistema de referência.

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