Issues related to delineation of forest boundaries on Landsat Thematic Mapper winter images

This paper describes an approach for estimating the effect of factors influencing the determination of forest boundaries on medium-resolution satellite images. Forest edges were delineated on a Landsat Thematic Mapper (TM) image made in late winter under plain snow cover conditions. The study investigated the best Landsat TM spectral band and threshold value for the detection of forest edges in winter images. It was hypothesized that shadows cast by trees on forest edges on the bright snow of the surrounding open area make north- or north-west-facing forest edges less sharp than edges facing in other directions. If this holds true for medium-resolution Landsat TM satellite images, forest area change studies should carefully consider images taken under different atmospheric and solar elevation conditions in order to distinguish real changes at forest edges from those stemming from different conditions of solar illumination. The results of the study show that there is no significant shadow effect, as the reflectance contrast at forest edges exposed in different azimuthal directions does not differ on Landsat TM winter images under plain snow cover conditions. Landsat TM bands 2-4 are all equally good for the detection of the forest edge location at an average value of reflectance. These results are valid for forest edges that have remained stable for several decades.