Choosing the best path: Global to national coordinate transformations

If you've ever had to put together a jigsaw puzzle of spatial data obtained from different jobs, by different methods, from different eras or from different organisations, then you know the importance of making sure all the jigsaw pieces come from the same box. That is, you're working with all apples or all oranges. Stories abound of the errors that occur from mixing (i.e. ignoring) the datums in which data were observed, processed, archived or supplied to the next user. A lesser known issue of growing importance for users trying to squeeze all they can from new positioning techniques is how the transformation between datums was actually done. This applies to many users, whether they are using GNSS, LiDAR or imagery data to name just a few. Whilst national transformation parameters, endorsed software or the way you did it last time may appear the easiest and most obvious solution, there are many paths for data to travel between datums. Which one should you follow? This paper demonstrates that differences of up to several centimetres in both horizontal and vertical coordinates can result from following different transformation paths. We suggest that some (but not all) users need to be careful of the methods employed. Additionally, the effect of the formal uncertainty in the transformation procedure on the estimated uncertainty of the output coordinates is often ignored, at the user's own risk. If included, formal uncertainty could help solve any discrepancies right away. Using some Australian scenarios, we discuss these issues to give spatial professionals a better understanding of the effect transformations have on the quality of their data.