In rugged areas

topographic effects severely influence the spectrum characters of remote sensing image. Topographic correction is a necessary step to estimate real land surface situation from remote sensing images. As a kind of topographic correction methods

empirical approaches can hardly achieve desirable results. Existing physical topographic correction methods which are based on radiant transfer models require a considerable number of input parameters. Furthermore

the topographic correction and atmospheric correction are mixed

which cause inconvenience and instability. A simplified physical algorithm is proposed in this paper to overcome the disadvantages of existing algorithms. A ratio factor of diffuse irradiance on horizontal surface to solar direct irradiance is introduced. The algorithm only need solar geometry information and atmosphere model parameters as its input data

and is mainly concentrated on the removal of topographic effects. It can correct the apparent radiance of optical remote sensing images over rugged terrain to the apparent radiance without topographic effects. The algorithm was illustrated using a Landsat TM image.