Map Coordinate Referencing and the use of GPS Datasets in Ghana

Abstract

The concepts of coordinate systems required to identify points in space and represent them on maps uses mathematical methods of assigning numbers, called coordinates to each point in space. There are different coordinate systems the commonest being the system of latitudes, longitudes and ellipsoidal heights. The situation is complicated because the latitudes and longitudes of the same point differ slightly depending on the geodetic coordinate system of a country, the result being that different systems of latitude and longitude in use for the same point can disagree in coordinates by more than 200 metres. The GPS for instance uses a WGS84 system and gives latitude and longitude values which can not be integrated directly into the mapping system of a country without suitable mathematical conversions. Similarly, it is not possible to just measure distances from a graticule map without appropriate projection conversions. In order to use data obtained from GPS measurements correctly and effectively in Ghana, we need to use appropriate transformation parameters that relate our Ghana National Survey Mapping coordinate system to that used for the Global Positioning System (GPS). Datum transformation parameters define functional relationship between two reference
frames. This paper looked at geodetic coordinate systems and transformations between the WGS84 and the coordinate systems used in Ghana (the Ghana war office system and also the Clarke1880 system) using the Bursa-Wolf model. Particular attention was given to the derivation of ellipsoidal heights through the use of the Abridged Molodensky formulas.