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The use of a "first reflection" airborne lidar (light detection and ranging) instrument was initiated in 1995 for beach mapping at Assateague Island. In its simplest form, a lidar sends a short pulse of light energy from a laser to a target. The time it takes for each pulse to complete a round trip to and from the target is converted into a direct measure of the elevation of the target. A first-reflection lidar measures the distance to the leading edge of the first laser reflection, typically the first non-air surface the outgoing light pulse encounters, such as the surface of a sandy beach or the tops of the trees in a forest.
A new research project at the U.S. Geological Survey (USGS) is now assessing the capability of a "multiple reflection" lidar instrument to study plant communities and extract "bald earth" elevations in moderately dense vegetated areas. A multiple-reflection lidar records the full "waveform" of the returned signal. When such a lidar is flown over a vegetated area, the unique shape of the waveform can reveal wherein the space between the ground and the treetopsthe foliage, trunk, and branches are concentrated. Depending on the density of vegetation, some of the laser light will penetrate to the ground, and the reflected waveform will provide a measure of the ground elevation as well.
In September, a team of researchers led by USGS scientist John Brock (St. Petersburg, FL) departed for a 5-day field effort at Assateague Island, MD, to assess and validate the performance of a new lidar instrument, the National Aeronautics and Space Administration (NASA)'s Experimental Advanced Airborne Research Lidar (EAARL). (See related story "NASA EAARL Lidar Test at Wallops Flight Facility.")
Wayne Wright (NASA) developed the EAARL instrument, which was flown on a Cessna 310 by pilot Virgil Rabine and copilot Wayne. The purpose of this mission was to acquire lidar data and digital camera photography over the sandy beach and various vegetated communities at Assateague Island. Amar Nayegandhi (USGS) and Mark Duffy (NPS) operated a survey-grade global-positioning-system (GPS) base station in a parking lot at Assateague Island National Seashore. EAARL flights were conducted from Salisbury, MD, over 2 days to acquire 7 gigabytes (GB) of lidar data. Amar also assisted in real-time programming and aircraft data processing at Salisbury.
To test and validate the lidar instrument, John Brock, Tom Smith, Melanie Harris
(all USGS), Helen Hamilton, Mark Sturm, and Mike O'Connell (all NPS) braved the mosquito-infested grasslands and forests of Assateague Island to conduct field observations coincident with the lidar overflights. Six 10-m-radius plots covering various plant communities were surveyed, and the diameter at breast height (DBH) and locations of all stems within each plot were measured. A handheld laser rangefinder was used to determine the height of each stem in the plot, and a "fisheye"-lens camera, provided by Tom, was used to acquire hemispherical canopy photographs from beneath a canopy looking upward. The NPS crew, with their indepth local knowledge, were instrumental in identifying plant species and accessing hard-to-reach spots in the park.
This trip served as an initial assessment of the NASA EAARL instrument to characterize vegetation communities across Assateague Island National Seashore. Data acquired from the aircraft and the on-the-ground measurements are being used to develop methods to extract "bald earth" topography and enable the estimation of vegetation-canopy metrics based on temporally resolved airborne lidar waveforms.
in this issue:
Assateague Island Vegetation Mapping
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