High-resolution gravimetric geoid modeling in the era of satellite and airborne gravimetry

Abstract

With the advances in positioning and inertial navigation systems, the accuracy obtained from the airborne gravimetry technique has reached very important levels that aid high-resolution gravity field modeling. The data obtained from the airborne gravimetry is of great importance in complementing the deficiencies of terrestrial data in mountainous areas and land-sea transitions in coastal areas where modeling the geoid are most troublesome. In this thesis, high-resolution regional gravimetric geoid modeling was investigated in light of the recent advancements in the field of gravimetry, more specifically satellite and airborne gravimetry. With recently developed GOCE-based global geopotential models and advanced stochastic techniques to model the regional gravity field as a solution of the geodetic boundary value problem, it is possible to achieve a high-resolution geoid model which can alter the traditional vertical reference system realization. In this regard, four studies are carried out in two test regions: Colorado, the USA, and Turkey. The first study focused on the contribution of airborne gravity measurements to gravimetric geoid modeling in a high topography, Colorado, USA, via the least squares modification of Stokes (LSMSA) and Hotine (LSMHA) integrals with additive corrections techniques. The study included filtering the high-frequency airborne gravimeter data with minimum loss of signal and downward continuing it to the Earth's surface by Least Squares Collocation method with a planar logarithmic covariance model. The reduced data was optimally combined with the satellite data from the global geopotential model and terrestrial gravity data to calculate a high-accuracy gravimetric geoid model. In this combination, the error variance of each data set was taken into account to stochastically determine the variance of input gravity anomaly/disturbance data set for Stokes and Hotine integrals. To clarify the importance of airborne gravity data to the study, three gravity data sets were created: terrestrial-only, airborne-only, and combined. The computed gravimetric geoid models were tested with highly accurate GPS/leveling benchmarks collected for the validation of models along a profile passing through the rough topography of the Colorado mountains. The results indicated the contribution of airborne gravity data over the mountainous regions, clearly. In conclusion, we obtained two gravimetric geoid models calculated using combined data set via LSMSA and LSMHA methodologies whose absolute accuracies are 2.69 cm and 2.87 cm, respectively. In the rest of the thesis, we focused on improving the accuracy of the gravimetric geoid model in Turkey. The first study that concerns the geoid model of Turkey dealt with the downscaling of low-resolution gravity anomaly data set, which originally has ~9 km resolution, to a spatial resolution of ~2 km. This task was achieved via the proper modeling of the topographic attraction on gravity using planar and spherical approaches for Bouguer gravity anomalies. While the planar approach was implemented for the computation of complete Bouguer gravity anomalies using classical terrain correction based on the mass-prism technique, the spherical approach was applied using a global model for the topographic attraction that is SRTM2Gravity. Based on these two approaches, the low-resolution complete Bouguer anomalies were enriched to higher-resolution data set, and consequently, surface gravity anomalies were calculated from planar and spherical complete Bouguer anomalies. Three gravimetric geoid models were calculated via the LSMSA technique, a low-resolution reference geoid with a planar approach, and two high-resolution geoids via planar and spherical approaches. Based on the accuracy assessment at 100 homogeneously distributed GPS/leveling benchmarks, the accuracy of the best-performing geoid was found as 8.6 cm using spherical approximation. The performance of gravimetric geoid models using the down-scaled surface gravity anomalies was significantly better compared to the low-resolution solution, the spherical approach being slightly better than the planar one. Hence, the success of the down-scaling was proven in terms of the accuracy achieved by the high-resolution gravimetric geoid models.