An Exploration of conifer canopy anisotropy: 3D modeling versus airborne imaging spectrometer and multispectral scanner imagery
Date
1999
Authors
Burnett, Charles Nils
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Abstract
Canopy biophysical data measured in the field was compared to modeled simulations of bidirectional reflectance (BR) and to airborne hyper-spectral and multi-spectral scanner imagery. The simulations were produced using a 3D computer graphics implementation of the geometric-optical model of Li and Strahler (1984). Two independent experiments were undertaken. In the first experiment, a solar position change of7° in zenith and 68° in azimuth produced changes in reflectance from Douglas-fir (Pseudotsuga menzeseii) forest stands on Vancouver Island. A VIRIS-measured change and 3D-modeled reflectance change were compared for 5 conifer stands ranging in age from 20 to 150 years. Sites 8, 9 and 10 contained mature (>30m in height) Douglas fir stands, while site 7 and 6 were populated by 30 year old and IO year old trees respectively. Change in values recorded by the spectrometer varied with wavelength. In the 700-800nm range, a grouping pattern of sites 8, 9, and IO separate from sites 7 and 6 was discernible, suggesting even with only a 7°/68° change in solar position, canopy shape may have a measurable effect on reflectance. Topography was incorporated into a second iteration of the first experiment, resulting in change estimates closer to the measured reflectance. Finn relationships between change and biophysical parameters were neither found nor expected with so few BR measurements and with the use of endmember reflectance values that were extracted from the literature rather than in situ.
In the second experiment, semi-hemispheric bidirectional reflectance functions were calculated at two spectral wavelengths for two stands (aged 40 and 150 years) using the 3D graphical modeling method described above, but with modifications to the input variables. These functions were compared against each other and then compared to canopy measurements from multi-spectral airborne imagery.
Ray-traced renderings demonstrate very accurate calculation of the areal extent of each of the four Li and Strahler geometric-optical model (GOM) components; however, the model is too simplistic for accurate BR modeling. For example, the differences in the BR change due to wavelength suggest the importance of introducing spectral parameters to the GOM. If the spectral information is incorporated into a 30 graphics/ray-tracing approach to modeling, the method could provide a powerful tool for the estimation of BR corrections for remotely sensed imagery acquired over topographically complex areas. This implementation of a simple version of the Li and Strahler model has demonstrated some of the strengths and weaknesses of using models in forest canopy research.