A Practical Software Package for Hybrid Geoid Determination

Abstract

Ellipsoidal heights obtained from the GNSS technique cannot be directly used for the needs of engineering projects due to their dependence on the geodetic datum. In contrast, orthometric heights are physical quantities as they are related to the Earth's gravity field, making them more suitable for practical applications. The transformation between orthometric and ellipsoidal heights is achieved using the geoid height, which is obtained from a geoid model with sufficient accuracy for the region. Therefore, determining a highly accurate geoid model to be used in the transformation from ellipsoidal to orthometric heights is of great importance. Currently, in geodetic applications, the hybrid method which evaluates gravimetric and geometric geoid determination methods together has increasingly been adopted. The hybrid method is an approach that determines a high-accuracy geoid model in the national datum by correcting systematic errors of the gravimetric geoid (such as datum shifts and tilts). In this study, geoid heights at GNSS-levelling points were first obtained from the gravimetric geoid model through the inverse distance weighted interpolation method. Then, the systematic errors between the gravimetric and geometric geoids were modeled using corrective surfaces based on 4, 5, and 7 parameter models. Finally, amongthe obtained corrective surface models, the one with the lowest root mean square error was used for the hybridization process. In addition, the accuracy of the hybrid geoid model was analyzed at selected test points which were not used in hybridization. Numerical results indicate that hybrid geoid determination significantly contributes to the accuracy. Within the scope of the study, the interpolation process and hybrid geoid model generation process were coded in MATLAB software and prepared as a practical graphical user interface.