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23 <br />equipment, as the flow was sufficiently low to allow wading throughout the study site. <br />Transect data in the Green were collected by GPS and echo sounding, as in the May <br />survey. <br />Airborne laser mapping <br />Both study areas were surveyed using airborne laser mapping (LIDAR; Measures <br />1991) to describe topography for floodplains, tops of banks, tops of islands, and other <br />near-channel features. LIDAR data were collected during October 23-24, 1999 by a <br />contractor using a proprietary airborne laser mapping system from a small, fixed-wing <br />aircraft. Distancing was accomplished by scanning a laser (4 kHz pulse rate) across the <br />flight path of the aircraft. Aircraft position was measured using an on-board dual <br />frequency GPS receiver. During the airborne survey a second dual frequency receiver <br />collected measurements over benchmark NRPP1. Laser distances and differentially <br />corrected GPS data were post-processed in conjunction with data from an inertial <br />measurement unit and a geoid model (GEOID 96) to calculate ground positions and <br />elevations. During final processing a program was used to identify and remove laser <br />returns from tops of vegetation. All LIDAR data processing was completed by the <br />contractor using proprietary algorithms and programs. Hand editing of the final data file <br />was not performed by the contractor. Imagery was also collected during the airborne <br />survey using a 2,000 x 2,000 pixel digital camera. The digital images were co-registered <br />and georeferenced to the laser data. Accuracy of LIDAR elevation data collected for this <br />study varied from 9 cm to over 100 cm root mean square error across different types of