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ETRS89 has become the common standard for harmonized horizontal coordinates in the European countries. In contrast, the national height networks are still determined by leveling with respect to different tide gauges and standards (height types, tide systems). The differences in the definitions cause height discrepancies up to a few decimeters along the borders. National geoid models are defined accordingly.Nowadays, GNSS-based height determination is widely used, which also increases the demand for geoid models as a height reference across borders. For large parts of the continent, the United European Leveling Network provides harmonized heights based on the standards of the EVRS. However, EUREF has yet to adopt a corresponding height reference surface officially.Thus, the EUREF Working Group “Unified European Height Reference” was formed by resolution in 2021 with the objective to enhance the usability of European heights for practical applications such as civil engineering, digital elevation models, etc. The main goal is to establish a European Height Reference Surface (EHRS) which is tailored to a consistent dataset of GNSS-leveling control points (EHRS_CP) referring to the latest ETRS89 and EVRS realizations.Furthermore, comprehensive information about the national integrated spatial reference systems, including heights and geoid models, shall be made available through the Information and Service System for European Coordinate Reference Systems (CRS-EU).The paper gives an overview of the rationale and current activities of the working group. Preliminary comparisons with selected gravimetric quasigeoid models provide an outlook on the increased accuracy of the new GNSS-leveling dataset., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

This technical report describes the levelling and GNSS (Global Navigation Satellite System) results for stations on the MPQ (Max-Planck-Institut für Quantenoptik) campus in Garching, aiming at the derivation of relativistic redshift corrections for novel clock comparison experiments. The underlying observations were carried out mainly in the year 2016, but supplementary information and data were also considered until the end of 2018. The (relative) accuracy of the levelled heights within the internal network on the MPQ campus is estimated to be better than 1 – 2 mm, which is based on the raw double-run levelling discrepancies and loop misclosures involving also stations on rooftops of buildings. The accuracy of the GNSS (ellipsoidal) heights is estimated to be better than 1 cm. The consistency between the levelled and GNSS heights was evaluated internally by approximating the quasigeoid by a horizontal plane as well as externally by comparing with a gravimetric quasigeoid model, yielding maximum residuals of only 2 – 3 mm.

International audience; The frequency stability and uncertainty of the latest generation of optical atomic clocks is now approaching the one part in 1018 level. Comparisons between earthbound clocks at rest must account for the relativistic redshift of the clock frequencies, which is proportional to the corresponding gravity (gravitational plus centrifugal) potential difference. For contributions to international timescales, the relativistic redshift correction must be computed with respect to a conventional zero potential value in order to be consistent with the definition of Terrestrial Time. To benefit fully from the uncertainty of the optical clocks, the gravity potential must be determined with an accuracy of about 0.1m2s−2, equivalent to about 0.01 m in height. This contribution focuses on the static part of the gravity field, assuming that temporal variations are accounted for separately by appropriate reductions. Two geodetic approaches are investigated for the derivation of gravity potential values: geometric levelling and the Global Navigation Satellite Systems (GNSS)/geoid approach. Geometric levelling gives potential differences with millimetre uncertainty over shorter distances (several kilometres), but is susceptible to systematic errors at the decimetre level over large distances. The GNSS/geoid approach gives absolute gravity potential values, but with an uncertainty corresponding to about 2 cm in height. For large distances, the GNSS/geoid approach should therefore be better than geometric levelling. This is demonstrated by the results from practical investigations related to three clock sites in Germany and one in France. The estimated uncertainty for the relativistic redshift correction at each site is about 2×10−18.

Geodesy, with its three core areas positioning and reference systems, Earth rotation determination, and gravity field modeling, is striving for a relative accuracy of at least 10−9 for all relevant quantities, and to a great extent this goal has already been reached (10−9 corresponds to about 6 mm relative to the Earth’s radius and \( 10^{ - 8} \,{\text{ms}}^{ - 2} = 1\,\upmu {\text{Gal}} \) in terms of gravity). Regarding gravity field modeling, the highest accuracy demands are from geodesy, especially Global Navigation Satellite System (GNNS) positioning, oceanography, and geophysics. In this context, the geoid and quasigeoid are of major interest; e.g., these quantities are required for the transformation between the purely geometric GNSS (ellipsoidal) heights and gravity field related heights as well as for the modeling of the (mean) dynamic ocean topography (DOT), requiring accuracies at the level of about 1 cm or even below. In this way, the importance of geoid and quasigeoid modelling has increased considerably—also for economic reasons—and as early as 1982 Torge (1982) postulated a “renaissance of the geoid.”