Accuracy assessment of digital elevation models (DEMs) plays an important role in facing their use in geoscience applications. This study investigates the vertical accuracy of most recently published versions of global DEMs over Croatia: 1 arc-minute global relief model (ETOPO1), Global 30 Arc-Second Elevation (GTOPO30), SRTM30+, Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010), Altimeter Corrected Elevations 2 DEM (ACE2 DEM), Shuttle Radar Topography Mission GDEM (SRTM GDEM), and Advanced Spaceborne Thermal Emission and Reflection GDEM (ASTER GDEM). Geodetic ground control points (benchmarks) allowed the comparison in terms of vertical accuracy. Additionally, the differences between models in the identical points were determined and analysed after adjusting the models to the same resolution. The results have yielded information about the overall accuracy of models; the accuracy depending on height, land cover, and slope; presence of large and systematic errors; and mutual agreement between the models.Overall vertical accuracies according to the root mean square error over Croatia are: ETOPO1 27.6 m, GTOPO30 21.6 m, SRTM30+ 21.3 m, GMTED2010 7.4 m, ACE2 DEM 4.5 m, SRTM GDEM 3.8 m, and ASTER GDEM 7.1 m. The highest accuracy was shown by SRTM GDEM version 3, which is far better than the previously released versions. All models have proved to be worse in the rough and forest areas, whereas in flat and barelands, new-generation global DEMs SRTM GDEM, and ASTER GDEM are highly accurate. IntroductionIn the past decade, global digital elevation models (global DEMs) have become invaluable sources of information in geosciences (e.g. Raaflaub and Collins 2006;Toutin 2008). Global DEMs have been significantly improved in comparison with those first released in the mid1990s, Global 30 Arc-Second Elevation (GTOPO30) and 1 arc-minute global relief model (ETOPO1), with the new-generation model releases, the Shuttle Radar Topography Mission GDEM (SRTM GDEM) in 2007, with a resolution of 3 arc seconds, and the Advanced Spaceborne Thermal Emission and Reflection GDEM (ASTER GDEM) in 2009, with a resolution of 1 arc second. As all models deviate from reality, global DEMs are usually used without an estimation of their accuracy and reliability and without considering errors that can affect results (Wechsler 2003). The goals of this work were to quantify and compare vertical accuracy, assess dependence between vertical accuracy and topographic characteristics (land cover and slope), and assess the differences between all global DEMs that were published from the mid-90s over Croatia. Testing of global DEMs was done for most recent versions of

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Abstract. The AlpArray Gravity Research Group (AAGRG), as part of the European AlpArray program, focuses on the compilation of a homogeneous surface-based gravity data set across the Alpine area. In 2017 10 European countries in the Alpine realm agreed to contribute with gravity data for a new compilation of the Alpine gravity field in an area spanning from 2 to 23∘ E and from 41 to 51∘ N. This compilation relies on existing national gravity databases and, for the Ligurian and the Adriatic seas, on shipborne data of the Service Hydrographique et Océanographique de la Marine and of the Bureau Gravimétrique International. Furthermore, for the Ivrea zone in the Western Alps, recently acquired data were added to the database. This first pan-Alpine gravity data map is homogeneous regarding input data sets, applied methods and all corrections, as well as reference frames. Here, the AAGRG presents the data set of the recalculated gravity fields on a 4 km × 4 km grid for public release and a 2 km × 2 km grid for special request. The final products also include calculated values for mass and bathymetry corrections of the measured gravity at each grid point, as well as height. This allows users to use later customized densities for their own calculations of mass corrections. Correction densities used are 2670 kg m−3 for landmasses, 1030 kg m−3 for water masses above the ellipsoid and −1640 kg m−3 for those below the ellipsoid and 1000 kg m−3 for lake water masses. The correction radius was set to the Hayford zone O2 (167 km). The new Bouguer anomaly is station completed (CBA) and compiled according to the most modern criteria and reference frames (both positioning and gravity), including atmospheric corrections. Special emphasis was put on the gravity effect of the numerous lakes in the study area, which can have an effect of up to 5 mGal for gravity stations located at shorelines with steep slopes, e.g., for the rather deep reservoirs in the Alps. The results of an error statistic based on cross validations and/or “interpolation residuals” are provided for the entire database. As an example, the interpolation residuals of the Austrian data set range between about −8 and +8 mGal and the cross-validation residuals between −14 and +10 mGal; standard deviations are well below 1 mGal. The accuracy of the newly compiled gravity database is close to ±5 mGal for most areas. A first interpretation of the new map shows that the resolution of the gravity anomalies is suited for applications ranging from intra-crustal- to crustal-scale modeling to interdisciplinary studies on the regional and continental scales, as well as applications as joint inversion with other data sets. The data are published with the DOI https://doi.org/10.5880/fidgeo.2020.045 (Zahorec et al., 2021) via GFZ Data Services.

The 2nd Geomagnetic Information Renewal Cycle started in 2017, pursuant to a request from the State Geodetic Administration and Ministry of Defence to ensure actual declination and its annual variation across the territory of Republic of Croatia. A test survey was performed at POKUpsko as part of the project in 2017. The PRM1 Primary Repeat Station had been destroyed, and the survey performed at a secondary location established in 2011, which subsequently became the primary location, known as PRM2. In this paper, the results of 2017 measurements reductions are presented, along with reductions in PRM1 and PRM2 measurements in 2011, and differences between the PRM1 and PRM2 locations, which are necessary to maintain the continuity of measurements at Pokupsko.

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The primary objective of the 1-cm geoid experiment in Colorado (USA) is to compare the numerous geoid computation methods used by different groups around the world. This is intended to lay the foundations for tuning computation methods to achieve the sought after 1-cm accuracy, and also evaluate how this accuracy may be robustly assessed. In this experiment, (quasi)geoid models were computed using the same input data provided by the US National Geodetic Survey (NGS), but using different methodologies. The rugged mountainous study area (730 km × 550 km) in Colorado was chosen so as to accentuate any differences between the methodologies, and to take advantage of newly collected GPS/leveling data of the Geoid Slope Validation Survey 2017 (GSVS17) which is now available to be used as an accurate and independent test dataset. Fourteen groups from thirteen countries submitted a gravimetric geoid and a quasigeoid model in a 1′×1′ grid for the study area, as well as geoid heights, height anomalies, and geopotential values at the 223 GSVS17 marks. This paper concentrates on the quasigeoid model comparison and evaluation, while the geopotential value investigations are presented as a separate paper (Sánchez et al. 2021). Three comparisons are performed: the area comparison to show the model precision, the comparison with the GSVS17 data to estimate the relative accuracy of the models, and the differential quasigeoid (slope) comparison with GSVS17 to assess the relative accuracy of the height anomalies at different baseline lengths. The results show that the precision of the 1′×1′ models over the complete area is about 2 cm, while the accuracy estimates along the GSVS17 profile range from 1.2 cm to 3.4 cm. Considering that the GSVS17 does not pass the roughest terrain, we estimate that the quasigeoid can be computed with an accuracy of ~2 cm in Colorado. The slope comparisons show that RMS values of the differences vary from 2 to 8 cm in all baseline lengths. Although the 2-cm precision and 2-cm relative accuracy have been estimated in such a rugged region, the experiment has not reached the 1-cm accuracy goal. At this point, the different accuracy estimates are not a proof of the superiority of one methodology over another because the model precision and accuracy of the GSVS17-derived height anomalies are at a similar level. It appears that the differences are not primarily caused by differences in theory, but that they originate mostly from numerical computations and/or data processing techniques. Consequently, recommendations to improve the model precision towards the 1-cm accuracy are also given in this paper.

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