The topographic correction of spot imagery for mountainous terrain

In general, the topographic effect is particularly evident for steep sloped mountainous terrain. Despite the same land-cover, significant variations between the phototropic and apheliotropic areas can be clearly detected from satellite imagery. Therefore, the consideration of the topographic effect is essential when satellite data is applied in retrieving the vegetation index or classifying the mountainous terrain. In this study, multi-temporal SPOT datasets coupled with the digital terrain model (DTM) are used to analyze and rectify the topographic effect of bamboo, coniferous and hardwood canopy forests in phototropic and apheliotropic areas. As the satellite sensors can take slanted shots, the satellite view angle is also taken into account during the topographic correction. The results show that the topographic effect in the phototropic and apheliotropic areas can indeed be effectively corrected. For example, the difference in the NDVI (Normalized Difference Vegetation Index) value between phototropic and apheliotropic areas of hardwood forest were significantly reduced from 25% to 13%. Meanwhile, the topographic effect in NIR (near-infrared) spectral band is more significant than in Red band due to its high reflectivity of vegetated canopy over mountainous terrain.