Assessment of recent GOCE-based global geopotential models and EGM2008 in Niger Republic

In this study, we assessed recent GOCE-based Global Geopotential Models (GGMs) and EGM2008 in Niger.  The combined GGMs EIGEN_6C4, GECO and EGM2008 were evaluated up to their maximum degree and order (d/o) 2,190 to select the one for gravity database densification. The following pure satellite GGMs were assessed for the modelling of the long and medium wavelengths in geoid computation: GGM05G, ITU_GGC16, EIGEN_6S4v2 and the fifth releases from direct (DIR5), space-wise (SPW5) and time-wise (TIM5) approaches. The GGMs are compared to terrestrial gravity data and geoid heights from GNSS/Levelling points before and after applying spectral enhancement method (SEM) by residual terrain model (RTM) for combined models and by RTM and the coefficients of selected combined GGM for pure satellite models. The agreements of combined GGMs with terrestrial gravity data and GNSS/Levelling points, in terms of root mean square (RMS) are about 4.88 to 5.02 mGal and 0.14 to 0.16 m, respectively. EIGEN_6C4 was selected as it showed the best performance in terms of geoid height differences and the probability of 3-sigma rule for gravity anomaly differences. At d/o 200, DIR5 showed a good agreement with terrestrial gravity data (5.04 mGal) and GNSS/Levelling points (0.15 m) after applying SEM, it was then retained. All GOCE-based models exhibited a good performance in long and medium wavelengths confirming the good recovery of the gravity field by the spatial gravity mission in these spectral bands

Impact of datum transformation on local variations of geometric geoid in Niger

In this study, we have conducted an investigation on the impact of the coordinates’ transformation on local variations of geometric geoid. The study area is limited by 1°43′12″ to 4°00′37″ East and 13°01′57″ to 14°31′20″ North in the southwest of the Niger Republic. We used 39 network GPS/levelling points es­tablished by the Japan International Cooperation Agency (JICA) and the National Geographic Institute of Niger (IGNN), including the DOPPLER point ANG302/no.65. Using other coordinates of point no. 65 pro­vided by IGNN, we transformed the points into WGS84 and computed a new geometric geoid model. The comparison of the new model with EGM2008 geoid up to d/o 2160 gives the STD of 15 cm and the RMS of 16cm. Local variations of the geometric geoids, were compared to that of EGM2008 geoid. The comparison through basic statistics, trend lines and 3D overlaps, showed a similar trend between the geometric geoid from the transformed coordinates and that of EGM2008. On the contrary, the JICA-IGNN geometric geoid generated an opposite and exaggerated trend. The Jarque-Bera test confirms that the three samples follow a normal distribution at the significance level α = 5%. The equality of variances between EGM2008 and JICA-IGNN geoids has been rejected by the Fisher’s F-Test/two-tailed at α = 10%. However the test confirms the variances equality between EGM2008 and the transformed geometric geoid at α = 5% and α = 10%. The two-tailed Student’s T-Test at α = 5% also confirms the equality of means between EGM2008 geoid and transformed geometric geoid samples

Remote Sensing by Satellite Gravimetry

Over the last two decades, satellite gravimetry has become a new remote sensing technique that provides a detailed global picture of the physical structure of the Earth. With the CHAMP, GRACE, GOCE and GRACE Follow-On missions, mass distribution and mass transport in the Earth system can be systematically observed and monitored from space. A wide range of Earth science disciplines benefit from these data, enabling improvements in applied models, providing new insights into Earth system processes (e.g., monitoring the global water cycle, ice sheet and glacier melting or sea-level rise) or establishing new operational services. Long time series of mass transport data are needed to disentangle anthropogenic and natural sources of climate change impacts on the Earth system. In order to secure sustained observations on a long-term basis, space agencies and the Earth science community are currently planning future satellite gravimetry mission concepts to enable higher accuracy and better spatial and temporal resolution. This Special Issue provides examples of recent improvements in gravity observation techniques and data processing and analysis, applications in the fields of hydrology, glaciology and solid Earth based on satellite gravimetry data, as well as concepts of future satellite constellations for monitoring mass transport in the Earth system

Determination of the reference height surfaces in the regions with sparse gravity data

The research aims to create a methodology that would lead to a mathematical model of referent surfaces and apply that created model in areas where it is not possible to provide a large number of observations, Global geopotential models, digital models of topographic masses, and available observations were used to model the Earth's potential field parameters to be distributed appropriately and be sufficient in the relevant area. Then, the reference surfaces (geoid and quasi-geoid) are determined from observations and newly modeled parameters of the Earth's gravity potential. The research methods that were used are the method of analysis and synthesis of existing results, mathematical modeling (interpolation, extrapolation, prediction, ...), the process of experiments, the technique of comparing the obtained results, the theory of Earth's gravitational potential, through statistical methods and collocations. The experimental part of the research was performed in two steps using already existing and well-distributed data called Area 01 as the first step, while in the second step, Area 02 was used for sparse data region or the rest area of Egypt territory. The results conclude that it is possible to model the Earth's gravity potential parameters in Area 02, with small numbers and irregular spatial distribution of observation results, in sufficient numbers and with sufficient accuracy. The obtained accuracy for Area 02 geoid was ± 0,479 cm as internal accuracy in terms of Standard deviation compared with 162 GPS/H points. Based on Area 01 results, geoid solutions for Area 01 have been fitted using 80 GPS/leveling stations. In comparison, 100 GPS/leveling stations were used as an external check; the obtained internal accuracy by the LSC technique in terms of the standard deviation was ±0.338 m of the differences, which is considered rather satisfactory. In contrast, external check accuracy was ±0.262 m, considered adequately related to the poor distribution of the available GPS/leveling data in this area. Evidently, from the two obtained Area 01 and Area 02 geoid accuracies, Area 01 geoid has better accuracy, nearly 30%, than Area 02 geoid as an internal check