A hybridized geoid model for Ashanti Region, Ghana
Ebenezer Tetteh Doku, Cosmas Yaw Asante, John Ayer, Richard Clophy Amoah
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.
