CDW photogrammetry of low relief fluvial features: Accuracy and implications for reach-scale sediment budgeting

Terrestrial photogrammetry enables rapid survey to be undertaken in the flu vial environment, a crucial factor when assessing highly dynamic features. In addition, it permits the generation of terrain surfaces at a level of de tail which, given equal time constraints in the field, far exceeds the deta il permitted by conventional tacheometric survey. This study details the le vels of accuracy that can be achieved using the RolleiMetric Close-range Di gital Workstation (CDW) photogrammetric survey technique on a variety of lo w relief fluvial geomorphic features (lateral, point and mid-channel bars) ranging from 10 m(2) to 100 m(2). The CDW software processes image data fro m an array of photographs and establishes a set of three-dimensional co-ord inates for the photographed object. The accuracies achieved across all bar surfaces were well within the resolution defined by the maximum size of the gravel on the bar surface (mean errors were between 0.026 and 0.057 m). Th e use of CDW photogrammetry does not require experienced personnel; however , care must be taken during the survey to minimize edge effects and to ensu re that target density is sufficient to produce a terrain model that is acc urate to within the limits of the surface sediment size. Guidance on target density as a function of terrain roughness is given to aid the user. A met hodology is suggested that will allow CDW to be incorporated into sediment budgeting techniques, improving the level of achievable accuracy. (C) 1998 John Wiley & Sons, Ltd.