REMOTE SENSING OF CLEAR-WATER, SHALLOW, GRAVEL-BED RIVERS USING DIGITAL PHOTOGRAMMETRY

The digital elevation model (DEM] quality that can be obtained from a digital photogrammetric survey of a reach of the clear water, shallow, gravel-bed North Ashburton River, New Zealand is assessed. An automated correction procedure is used to deal with point errors associated with submerged topography, based on a correction for refraction at an air-water interface. The effects of collection parameter variation upon DEM quality are also considered. The accuracy and precision of DEMs of submerged topography are evaluated 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 quality of submerged topographic representation is heavily dependent upon the water depth at the time of image acquisition. It is suggested that differences between the digital photogrammetric surface and the "actual" riverbed surface (as determined by terrestrial ground survey] will, in part, reflect the problem of defining what is the true elevation of a gravel-covered surface. A digital photogrammetric survey will generally see the tops of gravel cobbles, while a hand-held survey staff will tend to record the elevation between stones. The nomenclature of errors is also discussed, and it is concluded that the measure of surface quality adopted should be consistent with the application for which the DEM is to be used.