European Petroleum Survey Group, October 2001

The Problem: Since the advent of satellite positioning users have been faced with the need to transform coordinates delivered by the satellite based systems to those based on the national geodetic system of the country or area concerned. Transformations between Co-ordinate Reference Systems based on different geodetic datums (sometimes colloquially referred to as “datum shifts”) used within oil company operations are normally expressed as three geocentric translations (colloquially known as shifts – dX, dY, dZ) and, less frequently, as four, five, seven or ten parameter transformations additionally involving rotations and a scale change. Often, the three parameter translations that are used to change co-ordinates and heights derived by the ubiquitous GPS system to the local national geodetic and mapping system are available from the local national survey department or NGA list. (NGA is the National Geospatial Intelligence Agency of the USA, formely NIMA and DMA). But in the absence of prior knowledge through the local source or NGA list, or from a previous reliable determination, it may be necessary to determine values for the datum transformation required.