Gravimetric geoid of a part of south India and its comparison with global geopotential models and GPS-levelling data

Abstract

Precise information of geoid undulations is essential for obtaining the orthometric heights from Global Positioning System (GPS) measurements over any region; apart from providing the information of subsurface density distribution. This paper presents computation of geoid undulations over a part of southern Indian region from terrestrial gravity and elevation data using remove-restore technique that involves spherical Fast Fourier Transform (FFT) to compute 'Stokes' coefficients. Computed geoid undulations are compared with geoid obtained from global geopotential models such as EGM2008 and EIGEN-GRACE02S and measured GPS-levelling records at 67 locations. Statistical analysis of comparison suggests that the computed gravimetric geoid model has a good match with the geoid determined from GPS-levelling with rms of 0.1 m whereas EGM2008 has 0.46 m. The differences of GPS-levelling with EGM2008 at majority of stations fall in the range of ±0.5 m, which indicates that EGM2008 may be used for orthometric height determination with an accuracy of <0.5 m in the south Indian region and offers a reasonably good transformation platform from ellipsoid to local datum. However, local determination of geoid is necessary for better accuracy of orthometric height from GPS. The gravimetric geoid calculated from the available gravity data shows considerable improvement to the global model and can be used to achieve orthometric height with an accuracy of 0.1 m.

Algorithms in Matlab to Compute a Local Geoid Model for Geomatics Purposes

“…-Remove: removal of the contribution of the global model (determined by a spherical harmonic model) and of an additional contribution called "Residual Terrain Correction" (RTC) representing the effect of the masses between the topography and a reference surface; -Compute: calculation of co-geoid undulation residuals; -Restore: addition, in terms of geoid undulation, of both that of the global model and that of the RTC effect, also called "indirect topographic effect". These steps can be summarised as reported below (Srinivas et al, 2012) from the residues of gravity anomalies.…”

An experimental Indian gravimetric geoid model using Curtin University’s approach

“…Researchers and government organisations have made some efforts to develop local gravimetric geoid models for regions in India (Singh 2007;Carrion et al 2009;Srinivas et al 2012;Mishra andGhosh 2016, Singh andSrivastava 2018), but only using the GRAVSOFT package with residual terrain modelling (Forsberg 1985). Despite these efforts, a state-of-the-art national gravimetric geoid model for the whole India remains elusive (Goyal et al 2017).…”

Accuracy of unmodified Stokes’ integration in the R-C-R procedure for geoid computation

“…In an operational context, one widely used method is the R-C-R procedure along with the Residual Terrain Model (RTM) reduction [2,17,26,31,29]. This method was introduced by Forsberg [10] in order to combine rigorously the information of three frequency ranges of the gravity field -the short, intermediate and long wavelength signals -through three steps, including a Stokes' integration, to obtain a geoid model.…”