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.