Identification of Large Woody Debris in Acoustic Bathymetry Data
by Laurent White, B.S. and Ben R. Hodges, Ph.D.
ABSTRACT
The more frequent use of two-dimensional hydrodynamic river models
also requires more
detailed bathymetry surveys. For smooth bathymetries, there is little
difficulty in developing
accurate translations from survey data to model; however, in rivers
with significant bottom
structure (e.g., large woody debris), simple data averaging and
interpolation methods may
lead to misrepresentation of the bottom bathymetry. It is necessary
to identify in the data set
what is true bathymetry from what is caused by large woody debris.
Two groups of methods
are investigated to serve our objective: statistical techniques
and filtering techniques. While
the former are appealing for their simplicity and direct applicability
in modeling, they fail
at consistently treating spikes (hypothesized to be large woody
debris signature) in the data
set. Among filtering techniques, linear filters are turned down
due to their inherent trade-off
between edge retention and spike rejection. Two nonlinear filters
are examined. Median
and erosion filters are specifically designed to preserve sharp
edges while eradicate spikes.
Finally, median filtering is preferred to erosion filtering, for
the former leaves large-scale
bathymetric features virtually undisturbed.
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