Abstract:
Although satellite-based positioning techniques, especially Global Positioning System (GPS) are
used extensively in Ghana together with the conventional methods of positioning fixing, there is
little information about the geoid-ellipsoid separation (N), a parameter required to transform
measured ellipsoidal heights to the locally used geoidal heights for constructional purposes. This
deficiency compels users of GPS in the country to accept the default value (zero) offered by their
respective software developers during post processing of GPS data, which results in relatively
lower accuracy in height measurement. To overcome this challenge and to enable easy
conversion of heights from orthometric to ellipsoidal height, this study sought to determine the
actual value of N for the study area. Ellipsoidal and geoidal heights were determined for selected
points using GPS receivers and trigonometric leveling from which the average separation
between the two surfaces was generated from the dataset. The determination of the separation
was accomplished with the aid of the Earth Gravitational Model 1996 (EGM96). A plot of the
two heights showed strong correlation with the intercept representing the value of N. The most
probable value (MPV) of the height differences between the ellipsoidal and the geoidal height
yielded 8.40 m where as the line of best fit from a regression analysis of the scatter plot gave
9.07 m with 95% confidence interval. A sensitivity test was carried out using the determined
value of N instead of the default value previously used for reprocessing of the GPS data yielded
better accuracy of 0.05 mm.
