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Lidar Mapping of Vegetation at Assateague Island National Seashore—a First Look

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graphic showing lidar collection
LIDAR Collection: The NASA Experimental Advanced Airborne Research Lidar (EAARL) collected georectified digital aerial photographs and high-resolution lidar data over Assateague Island. The instrument uses a green laser and a raster scanning mechanism to acquire lidar data. Researchers set up a GPS base station to precisely locate the position of the aircraft and the data it collected.
Barrier islands, such as those in Assateague Island National Seashore (in Maryland), are dynamic, responsive entities that are important and fascinating in terms of their intertwined geologic and ecologic processes. As part of its resource-management program, the National Park Service (NPS) at Assateague Island requires accurate and detailed topographic maps to study beach dynamics and vegetation communities at periodic intervals.

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 where—in the space between the ground and the treetops—the 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.

USS Arizona memorial
Melanie Harris conducts field observations.
discussing the second deployment dive
Tom Smith looks over a camera with a "fisheye" lens that was used to acquire hemispherical canopy photographs from beneath a canopy looking upward.

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.

Related Sound Waves Stories
NASA EAARL Lidar Test at Wallops Flight Facility
April 2001

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in this issue: Fieldwork cover story:
USS Arizona

Adriatic Sea Sediment-Transport Cruise

Bear Lake Sea-Floor Mapping

Assateague Island Vegetation Mapping

Field-Testing New Portable Drilling System

Research Diamondback Terrapin

Outreach Transoceanic Dust Impacts

Woods Hole Field Center Open House

St. Petersburg Field Center Open House

Great American Teach-In

Fourth-Graders Tour St. Petersburg Field Center

Girl Scouts 90th Anniversary


Meetings Effects of Fishing Activities on Benthic Habitats

Planning Gas-Hydrates Research

Science and Politics in Ecosystem Decisions

Sea-Floor Mapping Techniques

Staff & Center News GHASTLI Lab Visitors

Science Museum Board

Two New Scientists

Louisiana Coastal-Restoration Advisory Board

Air Medical Transport Center Tour

MRIB Programmer

New Webmistress

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