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

TLDR

DEM error nomenclature is discussed, emphasizing that surface quality metrics must align with the intended DEM application. The study evaluates the quality of digital elevation models derived from photogrammetric surveys of the clear‑water, shallow, gravel‑bed North Ashburton River. An automated refraction‑correction algorithm corrects submerged point errors, and the influence of image‑collection parameters on DEM quality is examined and validated against independent ground‑truth data. Digital photogrammetry combined with image analysis can generate high‑resolution DEMs of gravel riverbeds, but submerged topographic accuracy is strongly limited by water depth and the inherent ambiguity in defining gravel surface elevation.

Abstract

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.