Deriving vegetation metrics using lidar
NASA's Experimental Advanced Airborne Research Lidar (EAARL) is a raster-scanning, temporal-waveform-resolving, green-wavelength lidar designed to map nearshore bathymetry, topography, and vegetative structure simultaneously. The EAARL sensor records the time history of the return waveform within a small footprint (15-20 cm at nominal flying altitude of 300 m) for each laser pulse, enabling characterization of canopy structure and 'bare Earth' under a variety of vegetation types. EAARL data acquired over the coastal vegetation at Assateague Island National Seashore (ASIS) in Maryland, and Terra Ceia Preserve at the southeast coast of Tampa Bay, Florida, were used to evaluate the capability of lidar data to determine canopy and sub-canopy vertical characteristics across a diverse set of vegetation classes. A collection of individual waveforms combined within a synthesized large footprint was used to define three metrics: canopy height, 'bare-Earth' elevation (BEE), canopy reflection ratio, and height of median energy. The metrics derived from these composite waveforms were tested for accuracy and reproducibility. BEE values were derived from the individual waveforms to limit the spreading of the ground return on steep slopes and to distinguish between 'bare Earth' and low shrubs. Results show that combining several individual small-footprint laser pulses to define a composite "large-footprint" waveform is a promising method for describing the vertical structure of vegetated canopies.
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Above left: Schematic showing the composite footprint waveform principle. [larger version]
Above right: Sample Experimental Advanced Airborne Research Lidar composite-footprint waveform and description of the derived canopy metrics. CHT = Canopy Height; BEE = Bare Earth Elevation (derived from individual small-footprint waveforms); HOME = Height of Median Energy, CR = amount of energy from canopy; GR = amount of energy from ground; CRR = Canopy Return Ratio. [larger version] |
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