Ramification of Datum and Ellipsoidal Parameters on Post Processed Differential Global Positioning System (DGPS) Data – A Case Study

Abstract

The World Geodetic System 1984 (WGS 84) is the global reference frame upon which Global Positioning System (GPS) observations are referenced to. Observations in the frame are in latitudes, longitudes and ellipsoidal heights. For use in a particular locality, it is necessary to convert these into the local coordinate system. In Ghana, the datum and ellipsoidal parameters for Ghana War Office and Clarke 1880 are used for the transformation of the GPS coordinates from the WGS 84 datum system to the Ghana National Grid System. This paper therefore examines post processed DGPS data using these two datum and ellipsoidal definitions separately. Dual frequency DGPS data were collected from 19 survey pillars (ie SGW D224/14/1 to SGW D224/14/19) and post processed using Clarke 1880 datum and ellipsoidal parameters. The raw data was also post processed using Ghana War Office datum and ellipsoidal parameters. The results for the two indicated an average difference of 7.23 m (23.72 ft) for 19 stations. This certainly does not fall within the tolerance set by Survey and Mapping Division (SMD) of the Lands Commission for cadastral plans production, which is +/- 0.9114 m (+/-3 ft). A composite plan using the post processed data from the two datum and ellipsoidal definitions was produced. The plan shows an overlapping area by the use of the two coordinates to plot, an indication of a potential conflict between ownership on paper which may not necessary exist on the ground.

Keywords: Differential, Global Positioning System, Datum and Ellipsoidal Parameters