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Empirical correction of low Sun angle images in steeply sloping terrain: a slope‐matching technique

A technique based on slope matching is presented, which corrects the topographic effect on images with very low illumination due to very steep terrain, very low Sun angle, or both. The technique is a modification of Civco's (1989) two‐stage normalization correction. The modified correction arose from the failure of existing correction methods to eliminate dark shadows on wintertime IKONOS images, in the steeply sloping terrain of our study area. The shadows, which were particularly severe in the near‐infrared (NIR) band preclude accurate habitat classification due to the importance of this band on IKONOS multispectral images: the remaining three bands being highly correlated. Since the objective of topographic correction is to equalize the radiance between shady and sunny slopes, the slope‐matching technique normalizes the radiance values to the mean of the sunny slope, rather than to the overall mean, as in the two‐stage normalization correction of Civco (1989). This enables a more appropriate correction factor to be computed, suitable for the wide range of values encountered for the incident angle of illumination. The slope‐matching correction was able to reduce intra‐class variance significantly more than the two‐stage normalization correction, as well as increase classification accuracy by 7%.