Vassilios N. Grigoriadis

Remote sensing is nowadays an essential component of the Earth Observation System. New satellite missions provide a value added in environmental monitoring, as the remote part of an integrated with...

In the frame of the ModernGravNet project, a new experimental gravimetric quasi-geoid model for the broader Greek region has been developed. The computations were based on the remove-compute-restore approach, 1D spherical fast Fourier transform method with the Wong-Gore modification, XGM2019 as the reference geopotential model, and residual terrain modeling (RTM) scheme. The model’s evaluation was conducted using GNSS/leveling data across the Greek mainland, taking into account the geoid separation term. Furthermore, a velocity field model, derived from 16 years of continuous observations at 227 GNSS permanent stations in Greece and the adjacent areas, was incorporated into the evaluation process to assess temporal vertical changes. This model reflects the dynamic geometrical properties associated with the crust’s geophysical processes. As the gravimetric geoid was computed using gravity data from various time periods and the leveling data refer to an adjustment carried out four decades ago, the velocity field has been utilized to examine temporal changes in the GNSS/leveling data relative to the gravimetric geoid heights, aiming for more precise comparisons. The results obtained show an improvement in the comparisons up to 3 cm in terms of standard deviation. The study highlights the importance of adopting a dynamic geoid model and emphasizes the need for its thorough evaluation.

The advent of SWOT mission data in combination with the recent altimetry satellites (CryoSat-2, SARAL-AltiKa, Sentinel-3A, Sentinel-3B) provide additional value in regional Dynamic Ocean Topography (DOT) estimation both in the resolution and the accuracy of the final product. The near-real-time records support the phase of the operational altime-try. Two areas in the Western Mediterranean were chosen in order to study the signal and error covariance of each altimetric product from 2017 to 2024. The repeated altimetric tracks were used for the spectral estimation of the signal and error Power Spectral Density (PSD) and covariance. Various covariance models (exponential, Gaussian, ARMA) were tested to fit the data-driven estimation. A similar computation scheme was tested for ma-rine gravity data using simulated repeated fields. The final analytic gravity covariance from the periodogram PSD approach was tested against the fitted empirical one. Finally, gravity and altimetry PSD information is used in the DOT estimation through a tailored Multiple-Input-Multiple-Output-System (MIMOS).

In the present study, we first examine the contribution of different, satellite-only or combined, global geopotential models in geoid computation employing the remove–compute–restore approach. For this reason, two test areas of about 100 km² each, one in northern and one in central Greece, were selected, and gravity measurements were conducted. These new gravity measurements were used along with the selected geopotential models to produce the reduced gravity field for the study areas. The classical and spectral residual terrain modeling effect is also removed to derive the residual gravity field. The latter is used for geoid computation using the 1D fast Fourier transform. The validation of the geoid models is carried out with gravity/GNSS/leveling measurements, which were conducted in two traverses located in the study areas. Special attention is given to the tidal approach, the geoid separation term as well as the coordinate reference system. Next, the northern study area is extended by incorporating gravity measurements obtained during the last five decades, and geoid models are recomputed. Lastly, using the geoid models computed, reference geopotential values are computed for both areas. From the results achieved for both study areas, the combined model XGM2019 provides the best overall statistical results with differences of 0.065 m and 0.036 m in terms of root mean square error. The incorporation of not recent data into the solutions leads to a degradation in accuracy by about 1.2 cm in terms of standard deviation. Lastly, the computed reference geopotential values present discrepancies between the two study areas, revealing network inconsistencies as well as the dependency on the geopotential model used for the geoid computations.

Recent Digital Surface, Elevation, and Bathymetric Models (DSM/DEM/DBM) aim to provide high-resolution and accurate height and depth information needed for a variety of surveying, geodetic, geophysical, and other applications. In this study, first we aim to validate in two test areas some of the most used models, i.e., ASTER GDEM; AW3D30 DSM; Copernicus DEM; EU-DEM; GEBCO 2020; NASADEM HGT; SRTM15+ and SRTM Global, using GNSS; spirit leveling; and gravity measurements. The validation is performed along two traverses of 14.5 and 12.0 km each in Northern and Central Greece, respectively. Since these models are based on geoid heights obtained from global geopotential models, we also investigate their influence on the validation results. Next, we carry out comparisons between GEBCO 2020, SRTM15+, and the Greek Seas DTM, with depths derived from in situ coastal measurements in six different areas in Northern Greece. From the analysis, we conclude that the heights obtained from the Copernicus DEM provide the best overall results in terms of mean value and standard deviation while also showing consistent results in the two test areas. Similarly, the Greek Seas DTM shows better consistency with the measured depths in the coastal test areas.

In the frame of the “Modernization of the Hellenic Gravity Network - ModernGravNet” project, relative and absolute gravity measurements were carried out at selected 1st and 2nd order benchmarks of the Hellenic gravity network. These measurements are used first for the evaluation of the network. Then, as the official network gravity values are referenced to the Potsdam gravity system, transformation parameters are determined for converting official values to the new gravity system as it is defined by the absolute gravity measurements. A northeast to southwest trend is revealed from corrections computed from the parametric modeling. Moreover, global geopotential models are assessed at the network benchmarks as a first step towards the development of a new geoid model for Greece and successively the establishment of a national geoid-based vertical datum.

Gravity field and steady-state Ocean Circulation Explorer (GOCE) data are strongly affected by noise and long-wavelength errors outside the satellite measurement bandwidth (MBW). One of the main goals in utilizing GOCE data for gravity field modeling is the application of filtering techniques that can remove gross errors and reduce low-frequency errors and high-frequency noise while preserving the original signal. This paper aims to present and analyze three filtering strategies used to de-noise the GOCE Level 2 data from long-wavelength correlated errors and noise. These strategies are Finite Impulse Response (FIR), Infinite Impulse Response (IIR), and Wavelet Multi-resolution Analysis (WL), which have been applied to GOCE residual second order derivatives of the gravity potential. Several experiments were performed for each filtering scheme in order to identify the ideal filtering parameters. The outcomes indicate that all the suggested filtering strategies proved to be effective in removing low-frequency errors while preserving the signals in the GOCE MBW, with FIR filtering providing the overall best results.

In the frame of the “Modernization of the Hellenic Gravity Network” project, we aim at computing a high resolution and accuracy geoid for Greece. For this reason, we selected initially two test areas in northern and southern Greece covering an area of about 100 km2 each, where gravity and GNSS/leveling measurements were carried out. Based on these recent, well documented and reliable measurements, we investigate the use of different techniques for the determination of the geoid, including Least-Squares Collocation, FFT and Input-Output Systems, following the Remove-Compute-Restore approach. For the remove/restore part, we examine different Residual Terrain Modeling schemes along with the use of older and recent Global Geopotential Models. Moreover, we compute the geoid-quasigeoid separation term using different approaches. We then validate the results obtained against the new GNSS/leveling measurements across the test areas and draw conclusions towards the determination of a regional geoid for Greece.

Within the frame of the “Modernization of the Hellenic Gravity Network” project, the homogenization of the Hellenic Vertical Datum is investigated. Two study areas in northern and southern Greece were selected, where the zero-level geopotential value Wo is estimated for each area. Additionally, a combined value is also estimated using a weighted least squares adjustment of Helmert orthometric heights and surface gravity values, that were recently measured, as well as recent global geopotential models. The biases in the vertical datum between northern and southern Greece are investigated through the comparison with a global conventional value. The validation of the results can lead to valuable conclusions on the possibility of a contemporary definition of the Hellenic Vertical Datum.