Interpretation Of Topographic Relief From Digital Multispectral Imagery

Abstract

Oblique illumination of irregular topography generates a pattern of highlighting and shadow as slopes facing the solar beam receive direct illumination, and as those on opposite sides of ridges are shadowed. On remotely sensed images these patterns appear as alternating dark and bright regions that reveal approximate positions of ridges and valleys. Knowledge of scene-specific variables (such as sun angle and elevation), general knowledge of geomorphology, atmospheric scattering, and spectral characteristics of landscapes permits reconstruction of the topography from its manifestation on the image. Raw image data record combined effects of topography, atmosphere, and varied spectral reflections of surface materials as a single image. Our interpretation procedure isolates brightnesses caused by direct and indirect illumination, varied material reflectances, and topographic modulation. From varied brightnesses caused by direct and indirect illumination, positions of ridges and valleys can be approximated. From variations in material reflectance, large rivers (channels with large areas of open water) can be detected. Finally, relative elevations can be estimated from analysis of drainage and ridge patterns using a strategy of growing that assigns increasing elevation values to pixels as they are positioned at greater distances from rivers or other valley pixels already assigned elevations. Elevations along directions perpendicular to valleys increase as they climb towards the ridges interpreted from the shadow pattern. The result is an image that approximates the patterns of relief of the original topography.