The many paths to a common ground: A comparison of transformations between GDA94 and ITRF

Digital positioning data are currently collected and stored in a variety of reference frames or datums. Before different data can be compared or combined, they must be brought together onto the same datum. The practice of transforming from one datum to another is certainly not difficult and the necessary parameters are available in many different software packages. To complicate this issue, today’s datum of choice may well be a global, and therefore dynamic, datum such as the International Terrestrial Reference Frame (ITRF). In a dynamic datum it is important to note both the datum realisation (e.g. ITRF2000, ITRF2005, ITRF2008, etc.) and the epoch at which the data are valid. We must also be able to propagate the data from one epoch to another, if desired, by taking tectonic motion into account. Since the Geocentric Datum of Australia (GDA94) was introduced there have been several refinements of ITRF, and also refinements of the transformation parameters between GDA94 and ITRF. As a result, there are many different combinations of transformation and propagation by which the data can be ‘moved’ from GDA94 to a particular ITRF and vice versa. This paper demonstrates that differences up to several centimetres in both horizontal and vertical coordinates can result from following different ‘paths of transformation’ and suggests that some but not all users need to be careful of the methods employed by their software. Additionally, what is often not considered is the effect of the formal errors in the transformation procedure on the estimated error of the output coordinates. We discuss and demonstrate these effects for several sample transformations to give the reader a better understanding of the effect of transformations on the quality of their data.