Characteristics of shadow and removal of its effects for remote sensing imagery

The effects of shadow in remote sensing imagery are investigated. The measurement of radiance in sunlit and shadowed areas was carried out to investigate the spectral characteristics of sunlight. Based on this observation, it is found that the radiance ratio (shadow/sunlit) increases as the sunlight gets weaker and the ratio is dependent on the wavelength of sunlight. The darkness of shadow is also found to vary depending on the surrounding condition. Thus the condition to restore a shadow-free image depends on the spectral bands and the location even in one image. A QuickBird image is then introduced and the spectral characteristics of sunlit and shadowed areas are investigated. Based on these observations, a method to detect shadowed areas and restore the shadow-free radiance for the multi-spectral bands is proposed. The effectiveness of the shadow correction method is demonstrated for the QuickBird image.