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Harvard Forest Symposium Abstract 2013

• Title: Separating Leaves from Trunks and Branches with Dual-Wavelength Terrestrial Lidar Scanning
• Primary Author: Alan Strahler (Boston University)
• Additional Authors: Supriya Chakrabarti (University of Massachusetts - Lowell); Timothy Cook (University of Massachusetts - Lowell); Darius Culvenor (CSIRO Sustainable Ecosystems, Australia); Ewan Douglas (Boston University); Glenn Howe (University of Massachusetts - Lowell); David Jupp (CSIRO Marine and Atmospheric Research, Australia); Zhan Li (Boston University); Jenny Lovell (CSIRO Marine and Atmospheric Research, Australia); Jason Martel (University of Massachusetts - Lowell); Glenn Newnham (CSIRO Sustainable Ecosystems, Australia); Ian Paynter (UMass Boston); Edward Saenz (University of Massachusetts - Boston); Crystal Schaaf (Boston University); Zhuosen Wang (University of Massachusetts - Boston); Curtis Woodcock (Boston University); Xiaoyuan Yang (Boston University); Tian Yao (Boston University); Feng Zhao (Boston University)
• Abstract:

  First scans from the Dual Wavelength Echidna® Lidar (DWEL), merged with scans from the Echidna Validation Instrument (EVI), demonstrate the separation of leaves from trunks in early trials in a deciduous hardwood stand at Harvard Forest acquired in August, 2012. Power returned from trunk hits is very similar to power returned from leaves at the NIR wavelength of 1064 nm, whereas returned power from leaves is only about half as large at the SWIR wavelength of 1548 nm. At that wavelength, leaf scattering is strongly attenuated by liquid water absorption, which reaches a maximum at about 1450 nm. SWIR scans were obtained in August, 2012, from the DWEL instrument in engineering mode using the 1548-nm laser, while NIR scans were obtained in June, 2010, using the Echidna Validation Instrument (EVI), a heritage instrument to the DWEL.
  
  The DWEL and EVI instruments share many similarities in design. Both use a rotating mirror to scan in a vertical circle as the instrument revolves in azimuth on its base, thus providing angular coverage of the upper hemisphere and much of the lower hemisphere near the instrument. Laser pulses are long and sharply peaked, allowing both full sampling of the return waveform shape and easy interpolation for accurate distance retrievals. Beam divergence is adjustable, ranging from 1.25 to 5 mrad. Scattering events as far as 100 m from the instruments and beyond can be detected and digitized with 8- (EVI) or 10-bit (DWEL) resolution.
  
  DWEL data were acquired at Harvard Forest, Petersham, MA, during August 1, 2012. The location was the center point of a 1-ha deciduous hardwood plot (100 m by 100 m) that has been scanned by the EVI at intervals since 2007. The DWEL instrument scanned in engineering mode, acquiring data with the 1548 nm laser only, as the 1064 nm laser was inoperable at the time. Scanning used an unexpanded collimated laser beam of 6-mm cross section, with data recorded at 2 mrad angular intervals in zenith and azimuth. EVI data at 1064 nm were acquired at the same location in June, 2010, using a divergent beam of 5 mrad, sampled at 4 mrad resolution.
  
  Figure 1 shows images from the two scans at the two different wavelengths. The comparison readily shows leaves to be darker at 1548 nm than at 1064 nm. Inset images show regions of interest for a trunk (red) crossed by a leafy branch (green) as seen in each scan. In the 1064 nm scan, the trunk/leaf ratio is 0.95; in the 1548 scan, the ratio is 2.0. The finer scan resolution of the DWEL image (2 mrad) as compared to the EVI image (4 mrad), provides a more detailed image. In the two years between the scans, a leafy branch has grown across the prominent trunk in the foreground.
  
  This work clearly demonstrates the potential for effective discrimination between leaves and woody trunk and branch stems with a dual-wavelength terrestrial lidar scanner. We have also demonstrated the ability of the DWEL to scan effectively and provide data that can be converted to point cloud format and registered to the heritage EVI instrument. The DWEL is now scanning in the laboratory and we anticipate first full field trials in the March-April 2013 time frame.
  
  Figure 1. Left, EVI scan at 1064 nm. Right, DWEL scan at 1548 nm. Image brightness is proportional to returned power.
  
  

• Research Category: Ecological Informatics and Modelling


• Figures:
EVI-DWEL_Pair.jpg