A previous investigation into the utility of Landsat multispectral scanner (MSS) data for detecting gypsy moth defoliation has met with limited success. The ability to separate healthy and moderate defoliation is confounded by spectral similarity and topographic effects. In order to better understand the physical processes involved in defoliation assessment from remotely sensed data, a field study was designed to investigate the effect of forest canopy closure and other environmental variables on in· coming solar radiation. Diffuse radiation measurements were recorded in red, infrared, and middle infrared wavelengths using the Mark II Three Band Field Radiometer. Results to date indicate that the percent canopy closure is the single most important variable affecting incoming solar radiation. In the visible and near infrared regions, interaction between time of day and date (defined later as solar zenith angle) also affect radiometric response. Aspect has only limited influence on radiance response. These same variables do not influence middle infrared response, however. Uniformity of the forest canopy appears to be more important. These results are compared to Landsat MSS classification results of gypsy moth defoliation.
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