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Mapping the Surface of Sheet Flow Water in the Everglades

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. BACKGROUND The conceptual basis for Airborne Laser Swath Mapping (ALSM), which is also referred to as LIDAR (light detection and ranging), LADAR (laser detection and ranging), and airborne laser altimetry, has existed for at least two decades. However, until a few years ago only federal agencies such as the Department of Defense and the National Aeronautics and Space Administration could afford to build and operate the large, heavy, power demanding early generation instrumentation. The development of compact energy efficient diode pumped Nd:YAG lasers, Inertial Measurement Units (IMU) composed of fiber optic gyroscopes and solid state accelerometers, and high performance personal computers during the mid 1990's suddenly made it possible to build ALSM units that could be operated from light dual, or even single, engine aircraft. The attendant order-of-magnitude drop in operating costs for the first time allowed academic researchers access to this powerful technology, setting off a remarkable growth in studies of Earth’s topography, bridging such traditional specialties as geodesy, geophysics, hydrology, civil engineering and biology (Gutelius et. al., 1998; Carter et. al., 1998: Shrestha, et. al., 1998)