Landsat TM-based forest damage assessment: Correction for topographic effects

Detection of forest damage is one of the various remote sensing applications complicated by topographic effects. Different vegetation types are known to respond differently to slope and illumination effects. This paper describes the response of Landsat Thematic Mapper data to the topography in Norway spruce forest, and the possibility to assess forest damage in rugged terrain. The effect at the examined medium and low solar elevations was non-Lambertian. Minnaert corrections and other empirical functions proposed for different cover types were found to be inadequate. Two new models were developed; one based on Minnaert constants changing with the cosine of the incidence angle, and the other based on an empirical relationship. Both models gave satisfactory results although the empirical model performed better for nearly shadowed northern slopes. With a model accounting for terrain and canopy inhomogeneity effects using digitized stand data and digital elevation models, healthy to slightly defoliated spruce forest could be separated from moderately defoliated forest. The method enables an improvement of the earlier documented Landsat TM capability to detect severely damaged forest.