Mapping the Surface of Sheet Flow Water in the Everglades

Abstract

University of Florida (UF) researchers used airborne laser swath mapping (ALSM) to map the height of vegetation, relative to the surface of sheet flow water, in the Everglades. In less than three hours of flying, ALSM provided sub-decimeter precision measurements of the height and slope of the water surface over an area greater than 50 square kilometers. The RMS scatter of the measurements about the best fitting slope along a 14 kilometer transect was less than 4 cm. Two features of the UF ALSM system were of particular advantage in this project: coverage of the nadir provided by the oscillating mirror scanner, and intensity values of the return signals. Openings in the grasses allowed a small percentage of laser shots, at the nadir of the aircraft, to be reflected from the calm surface of the slowly flowing water. These nearly specular reflections produced intensity values five to ten times as strong as the diffuse reflections from the grasses. The ALSM observations had to be carefully calibrated for the unusually strong return signals, but once this was done the signal-to-noise ratio was excellent and the geometry could not have been better for determining the height of the water surface. This project demonstrated the unique capabilities of ALSM to quickly and accurately map remote wildlife habitats, in this case that of the Cape Sable Seaside Sparrow, an endangered species that builds nests within 15 cm of the surface of the water. Managers hope to use such measurements to plan and control the release of water in order to avoid inundating nests, hopefully helping the Cape Sable Seaside Sparrow to avoid extinction