Modeling illumination effects to retrieve the true reflectance in rugged terrain

Topographic effects are the main obstacles to further analysis of satellite spectral data in mountainous area, especially to quantitative remote sensing. To obtain the true reflectance of the land surface, we must remove the topographic effects first. A physical model for rugged terrain is developed in this paper. Both atmospheric and topographic effects are considered in the model. 3 illumination sources are expressed analytically which include: 1)direct solar irradiance; 2) diffuse sky irradiance; 3) reflected irradiance from the adjacent terrain. Based on a quick searching algorithm for local horizon, the most complex part in the model-the reflected radiation can be calculated throughout the whole image in an economic computation time. In stead of using the field measured data or standard atmospheric condition obtained from the commercial software, we invert the atmospheric parameters from the image itself based on stochastic programming theory, a priori knowledge is used in the inversion process. To test the model, a Landsat TM-scene is matched to a digital elevation model(DEM) which has a resolution of 1m for elevation. The true reflectance map is obtained from the model. It is found that most of the topographic effects are removed in the map.