Remote measurement of river morphology via fusion of LiDAR topography and spectrally based bathymetry

Authors


Carl Legleiter, Department of Geography, University of Wyoming, 1000 E. University Ave., Dept. 3371, Laramie, WY 82071 USA. E-mail: Carl.Legleiter@uwyo.edu

ABSTRACT

This study developed and evaluated a hybrid approach to remote measurement of river morphology that combines LiDAR topography with spectrally based bathymetry. Comparison of filtered LiDAR point clouds with surveyed cross-sections indicated that subtle features on low-relief floodplains were accurately resolved by LiDAR but that submerged areas could not be detected due to strong absorption of near-infrared laser pulses by water. The reduced number of returns made the active channel evident in a LiDAR point density map. A second dataset suggested that pulse intensity also could be used to discriminate land from water via a threshold-based masking procedure. Fusion of LiDAR and optical data required accurate co-registration of images to the LiDAR, and we developed an object-oriented procedure for achieving this alignment. Information on flow depths was derived by correlating pixel values with field measurements of depth. Highly turbid conditions dictated a positive relation between green band radiance and flow depth and contributed to under-prediction of pool depths. Water surface elevations extracted from the LiDAR along the water's edge were used to produce a continuous water surface that preserved along-channel variations in slope. Subtracting local flow depths from this surface yielded estimates of the bed elevation that were then combined with LiDAR topography for exposed areas to create a composite representation of the riverine terrain. The accuracy of this terrain model was assessed via comparison with detailed field surveys. A map of elevation residuals showed that the greatest errors were associated with underestimation of pool depths and failure to capture cross-stream differences in water surface elevation. Nevertheless, fusion of LiDAR and passive optical image data provided an efficient means of characterizing river morphology that would not have been possible if either dataset had been used in isolation. Copyright © 2011 John Wiley & Sons, Ltd.

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