A hybridized geoid model for Ashanti Region, Ghana

Abstract

GNSS positioning techniques deliver accurate three dimensional coordinates in Easting, Northing and geometric heights. However, the use of the geometric heights for topographic mapping is limited due to the unavailability of accurate National geoid models for their conversion into used Normal-Orthometric height systems in Ghana. Though global gravimetric geoids such as EGM2008 are readily available, these are not suitable for direct conversion of geometric heights from GNSS into the useful normalorthometric height system used because local geoids are offset in an unpredictable way from such global geoids. In this investigation, a new hybrid geoid model for the Ashanti Region is developed and tested. The developed model incorporates the contribution of global geopotential models (GGMs) provided by EGM2008 together with GPS/Levelling using a remove and restore-like approach to develop a surface corrector model that separates the deterministic trend from the stochastic errors which then is separately modelled using the Gauss-Markov method. The main idea is thence to model the anomalies after comparing the gravimetric and quasi-geoid models from several collocated points. A Four-parameter Helmert transformation model have been applied to reduce data biases and provide a fitting quasi geoid heights obtained from the GGMs to those from GPS/levelling data. Whereas the gravimetric geoid alone has revealed that the vertical datum offset from the National height datum over Ashanti Region was at the level of 8cm up to 1.0m with a mean offset of 44cm. The Hybrid geoid model (ASHGeoid21) computed through the Remove-Compute-Restore (RCR) procedure resulted in geoid heights that are offset from Local geoid values at nineteen additional checkpoints for up to ± 50cm. with a standard deviation of ±10cm. It is recommended that, the hybrid residual height anomalies of a maximum of 50cm, could be further improved with the inclusion of more GPS/levelling points.