Working with Dynamic Datums

There is a widespread misconception within the spatial science community that the three geocentric datums widely used in Australia, GDA94, ITRF and WGS84 are identical “for all practical purposes”. This is a fair assumption for navigation and many mapping purposes, however users of wide area differential GPS and precise positioning services with a precision of less than a metre are beginning to notice discrepancies between the datums. This is due to the tectonic movement of the Australian plate. While all three are geocentric datums, WGS84 and ITRF are dynamic in nature with site coordinates constantly changing to reflect tectonic movement on a global scale. Conversely, GDA94 is a static datum with coordinates fixed at the beginning of 1994 to the ITRF realisation at that time. During the intervening 13 years, the Australian Plate has moved by up to a metre and as a consequence GDA94 coordinates are now offset from WGS84 and ITRF. This paper discusses the implications of use and misuse of positioning technology with regard to the integrity and longevity of survey coordination and spatial data. Practical methods of dealing with the divergence of GDA94 and ITRF/WGS84 are presented.