An Investigation of Terrain Effects on the Inversion of a Forest Reflectance Model

The effects of terrain on conifer stand reflectance were estimated using sensitivity analysis of the Li and Strahler bidirectional reflectance (BRDF) model, and model inversions with both simulated and real Landsat Thematic Mapper (TM) reflectance data. Emphasis was placed on surface characteristics, and atmospheric correction was assumed. The results of sensitivity analysis indicated that strong interactive effects between nadir coverage of crowns, stand structure, and the slope and aspect of the stand occur in the reflectance data. This was supported by the results of model inversions using simulated reflectance data of sloping stands. It was found that, by neglecting slope in the inversion process, significant errors occurred in the retrieved values of forest coverage. Finally, inversion was tested using real TM reflectance data from a mountainous site in British Columbia, with a digital terrain model (DTM), forest attribute data, and the 6S atmospheric code. The results indicate that the effects of sloping terrain would preclude the retrieval of forest coverage unless terrain effects were specifically included in the inversion process.