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Remotely Sensed Topographic Data for River Channel Research 135
representation and the threshold at which a point becomes erroneous when judged with
respect to the approximate representation. As different surfaces will have different degrees
of topographic variability, these issues may be surface-specific. However, the development
of a priori recommendations (e.g. expected topographic variability over a given spatial
scale) and/or the combination of different data sources (e.g. using map-derived elevations
to construct an approximate terrain model which is used to filter data derived using higher
resolution remote sensing) is worth further exploration.
Finally, a major theme in this chapter has been the detection and management of locally
systematic error. Section 6.3.2 emphasizes the importance of local versus global estimates
of data quality. When a study is interested in distributed process modelling, and process rates
are defined by local gradients in surface topography, getting those gradients right is crucial.
In turn, this requires individual data points to be correct, which emphasizes the importance
of using local topographic variability in the data correction procedure, rather than the black
box smoothing typical of some DEM products. However, it is equally important to design
topographic surveys that consider how a particular data collection method interacts with a
particular surface of interest. If this cannot be done using a priori reasoning, then it should
be done through appropriate pilot survey, so that data collection parameters are optimized
before the expense of a full survey. This will prevent potential users of remotely sensed
topographic data from being disappointed by its apparent failure when the failure arises
simply because the survey has not been designed properly, and the data have not been
processed adequately.
Acknowledgements
The study was partially funded by the New Zealand Foundation for Research, Science and
Technology under contracts CO1818 and CO1X0014. Maurice Duncan, NIWA, assisted
with field data collection and bathymetric data analysis. B. Fraser, D. Pettigrew and W.
Mecchia, Environment Canterbury, assisted with field surveys. Air photography was by Air
Logistics, Auckland, while AAM Geodan, Brisbane, undertook the airborne laser scanning.
G. Chisholm, Trimble Navigation N.Z. Ltd., assisted with GPS equipment set-up. RMW
was supported by a NERC studentship. SNL was supported by the Royal Society and the
University of Leeds.
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