Assessment of Bidirectional Effects over Aquatic Macrophyte Vegetation in CIR Aerial Photographs

Radiometric effects hamper the use of airborne remote sensing data for quantitative studies of biophysical environments. The effect of bidirectional reflectance is evaluated in four areal samples from three lakes in the Vuoksi drainage basin in Finland. It is shown statistically that the relationship between measured upwelling radiation and the sun-target-sensor geometry is weak for an aquatic macrophyte species, the common reed (Phragmites australis), but strong for birch (Betula pubescens) and mixed birch-spruce (Picea abies) forest. The density and phenotype of the common reed vegetation seem to have an effect on the strength of the bidirectional reflectance in the images. When the effect of bidirectional reflectance over common reed vegetation is statistically significant, an empirical normalization method can be used, after which the brightness of the test areas is no longer statistically dependent on the suntarget-sensor geometry.