Remote sensing of clear-water, shallow, gravel-bed rivers using digital photogrammetry

The digital elevation model (DEM) quality that can be obtained from a digit al photogrammetric survey of a reach of the clear water, shallow, gravel-be d North Ashburton River, New Zealand is assessed. An automated correction p rocedure is used to deal with point errors associated with submerged topogr aphy, based on a correction for refraction at an air-water interface, The e ffects of collection parameter variation upon DEM quality are also consider ed. The accuracy and precision of DEMs of submerged topography are evaluate d using an independent data set. Results show that digital photogrammetry, if used in conjunction with image analysis techniques, can successfully be used to extract high-resolution DEMs of gravel riverbeds, but that the qual ity of submerged topographic representation is heavily dependent upon the w ater depth at the time of image acquisition. It is suggested that differenc es between the digital photogrammetric surface and the "actual" riverbed su rface (as determined by terrestrial ground survey) will, in part, reflect t he problem of defining what is the true elevation of a gravel-covered surfa ce. A digital photogrammetric survey will generally see the tops of gravel cobbles, while a hand-held survey staff will tend to record the elevation b etween stones. The nomenclature of errors is also discussed, and it is conc luded that the measure of surface quality adopted should be consistent with the application for which the DEM is to be used.