Scene Shadow Effects on Multispectral Response

The behavior of canopy reflectance and spectral bandtransformations, normalized difference (ND), and greenness (GR) dueto the amount and location of scene shadows was examined. Live balsamfir (Abies balsamea L. (Mill.)) trees were arranged in an equidistantpattern on a platform. Spectral reflectances were acquired forthree canopies (44-, 70-,and 90-pereent cover) and three backgroundtypes (grass-sod, black-,and white-painted boards) for a range of solarzenith angles. Rotating the platform produced variations in the locationof shadows cast by trees in the scene.Red and near-IR reflectance factors decreased as the amount of sceneshadows increased while ND increased with increasing scene shadows.However, when large anmounts of green vegetation were present, NDsaturated (i.e., approached an asymptote) and the observed increaseswere reduced. GR was least affected by changing shadow amounts.Spectral response variations due to shadows decreased as canopy coverincreased.A simple model was used to examine the effects of shadows in "pure"pixels of backgrounds representing vegetation, bare soil, and snow.Red and near-IR reflectance decreased and ND increased as the proprotionof shadows increased. These results suggest that green vegetationphytomass may be overestimated by ND when shadows cast byvegetation are present in a scene.