HR: 16:40h
AN: H34B-03 INVITED     [Abstracts]
TI: Stream Morphologic Measurements from Airborne Laser Swath Mapping: Comparisons with Field Surveys, Traditional DEMs, and Aerial Photographs
AU: * Snyder, N P
EM: noah.snyder@bc.edu
AF: Dept. of Geology and Geophysics, Boston College, 140 Commonwealth Ave., Chestnut Hill, MA 02467 United States
AU: Schultz, L L
EM: schultli@bc.edu
AF: Dept. of Geology and Geophysics, Boston College, 140 Commonwealth Ave., Chestnut Hill, MA 02467 United States
AB: Precise measurement of stream morphology over entire watersheds is one of the great research opportunities provided by airborne laser swath mapping (ALSM). ALSM surveys allow for rapid quantification of factors, such as channel width and gradient, that control stream hydraulic and ecologic properties. We compare measurements from digital elevation models (DEMs) derived from ALSM data collected by the National Center for Airborne Laser Mapping (NCALM) to field surveys, traditional DEMs (rasterized from topographic maps), and aerial photographs. The field site is in the northern Black Mountains in arid Death Valley National Park (California). The area is unvegetated, and therefore is excellent for testing DEM analysis methods because the ALSM data required minimal filtering, and the resulting DEM contains relatively few unphysical sinks. Algorithms contained in geographic information systems (GIS) software used to extract stream networks from DEMs yield best results where streams are steep enough for resolvable pixel-to-pixel elevation change, and channel width is on the order of pixel resolution. This presents a new challenge with ALSM-derived DEMs because the pixel size (1 m) is often an order of magnitude or more smaller than channel width. We find the longitudinal profile of Gower Gulch in the northern Black Mountains (~4 km total length) extracted using the ALSM DEM and a flow accumulation algorithm is 14% longer than a traditional 10-m DEM, and 13% longer than a field survey. These differences in length (and therefore gradient) are due to the computed channel path following small-scale topographic variations within the channel bottom that are not relevant during high flows. However, visual analysis of shaded-relief images created from high-resolution ALSM data is an excellent method for digitizing channel banks and thalweg paths. We used these lines to measure distance, elevation, and width. In Gower Gulch, the algorithm-derived profile is 10% longer than that measured by tracing the thalweg on the ALSM imagery. By obtaining longitudinal profiles and widths from the same dataset, ALSM represents a major step forward from previous remote-sensing morphologic analyses, in which profiles are measured using traditional DEMs or topographic maps, and widths from aerial photographs. These combined analyses would be particularly powerful in regions of low relief and/or wide channels were traditional DEMs have limited utility.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 1855 Remote sensing (1640)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005