Abstract
In this study, we computed and presented grid maps of high-resolution terrain corrections and residual terrain model (RTM) as short-wavelengths of the gravity field and the geoid in Niger. We constructed RTM elevations from mean elevation surfaces corresponding to ~100 km and ~9 km of spatial scales and 3 arc-seconds SRTM data. The computations are performed at gravity stations and 1.5 arc-minute regular grid, out to 10 and 200 km for inner and outer zones respectively with the standard density of 2670 kg/m-3. The study area is characterized by low values of terrain effects. The indirect effects are lower than 10 cm for ~9 km and reach 1.8 m for ~100 km. In Niger, 98.44% of indirect effects are lower than 1 cm and 98.2% of direct effect are lower than 5 mgal for ~9 km. For ~100 km, 85.87% of indirect effects are lower than 10 cm and 89.77% of direct effects are lower than 5 mgal for ~100 km, and 98.77% of terrain corrections are lower than 1 mgal. We found out that height discrepancies between gravity stations and SRTM influences the precision of terrain effects. The results are value for applications in geodesy and geophysics that require accurate interpretations.
Highlights
The gravity reductions serve as tool for geoid determination in geodesy, interpolation and/or extrapolation of the gravity and the investigation of the Earth’s crust in geophysics and geology (Heiskanen & Moritz, 1967)
To better construct Residual Terrain Model (RTM) elevations taking into account local terrain characteristics, we considered the reference surfaces determined by the filter low pass
For d/o 200, 85.87% of indirect effects are lower than 10 cm, and 12.75% range from 10 to 30 cm, 89.77% of direct effects are lower than 5 mgal and 6.66% range from 5 to 10 mgal. 98.77% of terrain corrections are lower than 1 mgal, 1.105% range from 1 to 5 mgal in Niger
Summary
The gravity reductions serve as tool for geoid determination in geodesy, interpolation and/or extrapolation of the gravity and the investigation of the Earth’s crust in geophysics and geology (Heiskanen & Moritz, 1967). The Residual Terrain Model (RTM) was introduced by Forsberg (1984) for RCR approaches, in this technique, topographic irregularities relative to a smooth mean elevation surface, with resolution comparable to that of the used GGM, are computationally removed. The Shuttle Radar Topography Mission (SRTM) GDEMs (Jarvis, Reuter, Nelson, & Guevara, 2008; NASAJPL, 2013) showed the best vertical accuracy in Niger and surrounding areas with precise ground control points Their comparison with the gravity database elevations showed important height discrepancies (Ibrahim Yahaya & El Azzab, 2018). Both elevation sources are subject to errors of vertical and/or horizontal positioning, it is reported in several studies that SRTM GDEMs did not require horizontal shift (Varga & Bašić, 2015). RTM effects and terrain corrections are computed at gravity stations and regular grid nodes and recommendation are given for further implementations of the results
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