The use of conventional survey methods to monitor large, gravel river
beds has traditionally led to a reliance on repeat measurements of
cross-sections which, unless very closely spaced, may give unreliable
information about three-dimensional channel morphology and
morphological change. Provided certain technological limitations can be
overcome, remote survey techniques, such as digital photogrammetry and
airborne laser scanning, remove the spatial and temporal constraints
typically associated with ground-based surveys, allowing high spatial
resolution, distributed, elevation mapping of gravel river beds. This
paper develops the use of digital photogrammetry for the survey of a
3.3 km reach of the braided Waimakariri River, New Zealand, which, when
combined with image analysis of water colour to infer water depth,
provides a Digital Elevation Model (DEM) of the entire river bed.
Central to the successful application of this method is DEM
post-processing. Errors take two forms: (i) individual point errors
associated with incorrect stereo-matching during automated data
collection; and (ii) spatially-variable systematic errors that are
associated with uncertainties in sensor position and orientation as
determined during the bundle adjustment. An automated post-processing
procedure is developed to deal with individual point errors and this
improves DEM surface quality in terms of accuracy, precision and
internal reliability. Systematic errors in the final DEM surface were
reduced by applying a simple correction based on surveyed photo-control
point elevations.