- Airborne Lasers Map Beach Erosion
Ruth A. Mendonsa
Residents of coastal communities are no strangers to the ravages of hurricanes, and beach erosion is one of the problems these storms leave in their wake. The National Oceanic and Atmospheric Administration's Coastal Services Center, its Aircraft Operations Center, NASA and the US Center for Coastal Geology and Regional Marine Studies are working with state coastal zone managers to tackle this problem. They are using airborne lasers to map the East and West coasts.
Lidar is helping to provide more accurate mapping of US shorelines -- and speeding up the process.
To provide accurate, cost-effective information on coastal topography, erosion and shoreline position, the Coastal Services Center launched the Airborne Lidar Assessment of Coastal Erosion project. The NASA group developed an airborne laser altimeter, called the Airborne Topographic Mapper, which is being used to map beach elevation and monitor coastal erosion from hurricanes and other storms.
Making conventional land surveys in this application is difficult because of tidal variations and shifting sands. Photogrammetry using aerial photography presents a problem because there is limited contrast between beaches and dunes, and very often vegetation obscures the view.
Airborne lasers, on the other hand, often can penetrate vegetation canopies. They also reflect well off of the sand and rock, allowing accurate determination of absolute elevation and topography.
In lidar surveying, the laser emits high-frequency beams toward the Earth's surface through an opening in the plane's belly. The laser records the time difference between the emission of the beam and the reception of the reflected laser signal. The aircraft travels over the beach at about 135 mph, surveying from the water line to a predetermined inland point.
NASA's Airborne Topographic Mapper measures ground elevation with an accuracy of less than 15 cm by combining its measurements with those from global positioning system receivers. The laser measures the distance between the aircraft and the ground with sampling rates from 2000 to 5000 pps. Postflight processing of the global positioning data recorded from the aircraft and land-based receivers fixes the aircraft location at the time of each spot sampling. These data are combined with the Airborne Topographic Mapper's data to produce digital elevation models containing X-Y-Z positions for every data point recorded.
Annual surveys of this type can help scientists understand the long-term erosion trends and estimate the effects of beach renourishment and erosion-control devices. The National Oceanic and Atmospheric Adminis-tration will develop base data from which comparisons of shoreline change and sand volume can be made.
Coastal managers on the West Coast are particularly interested in the results because they expect that El Niño-related storms will cause tremendous changes in their shoreline this winter.
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