Stitching Fine Resolution DEMs

High resolution DEMs are being collected at various places across Australia according to need and funding. In some cases the gaps between existing data sets are being filled in and there is a growing need to join these DEMs together. A coherent mosaic provides much more value than an overlapping set of unconnected DEMs. This paper describes the techniques developed to stitch several DEM and bathymetric data sets together to create a continuous coherent DEM of the Murray River corridor from the South Australian/Victorian border to the Murray mouth, including the Lower Lakes and the Coorong. A variety of challenges were encountered in this process: some features were represented well in one DEM and not in the overlapping DEM; some parts of the DEM and bathymetry did not represent real data and needed to be removed; one DEM had non-constant height errors requiring modelling of the difference. Resolving those issues required a substantial amount of manual effort. In one case, bathymetry data collected for the same area at two different times showed significant differences in channel form highlighting the dynamic nature of channel and lake bed bathymetry. The stitching of disparate DEMs could be automated to a substantial degree if two conditions are met. Firstly, the DEMs should overlap rather than abut; this allows analysis of the differences between the DEMs in the overlap area, detection of differences and an assessment of the quality of the join. Secondly, information on the quality of the surface is needed so that informed choices can be made about which data set provides the better quality information in different places; at a minimum it is necessary to know where a DEM is supported by measurements and where it is interpolated. We recommend that data purchasers consider these requirements when specifying data products so that new DEMs can be connected with existing DEMs, creating maximum value from new data acquisitions.