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Integration of digital elevation model and spectral radiance for topographic normalization of Landsat TM imagery

The so-called topographic effect in remote sensed images is caused by the different orientation of the terrain surface relative to the light source and the sensor position. It produces a variation in the spectral radiances associated with a given cover type and causes problems in image classification and interpretation due to the increased in-band variance for theoretically homogeneous classes. Different techniques have been proposed in the literature to reduce the topographic effect. The method presented here is based on the modeling of the radiances reflected by a Lambertian surface. A normalization coefficient is derived from a simplified radiative transfer equation and is used to reduce, for each pixel of the image, the reflected radiances to the values they would have on a horizontal surface; the unknown atmospheric parameters are empirically evaluated by an iterative process for variance minimization of radiance values in a few significant sites. We apply the method to a Landsat TM image for which we obtained an accurate DEM. To define the value of the unknown parameters, the correction is first applied to a number of training areas, characterized by both homogeneous cover and rugged topography. Using these values the correction can be finally extended to the entire image. Misregistration of DEM and TM images and local changes in atmospheric parameters appear to be the major sources of error, as well as the deviation of the surface response from a purely Lambertian model.