Locating Shadows in Aerial Photographs Using Imprecise Elevation Data

Realistic images of terrain can be created by texture mapping aerial photographs onto elevation data. How- ever, lighting and shadows in the original photographs are a problem when multiple aerial photos are merged, or when the sun direction used for the generated image is dif- ferent from the original aerial photograph. Our goal is to generate a terrain surface reflectance map from an aerial photograph. This will be used to simulate a new aerial photograph with different lighting and viewing directions. We are concentrating on forest and mountain areas for use in ecosystem management and modeling applications. One key to generating realistic new images is finding shadows cast by the terrain in the original aerial pho- tographs. Digital elevation values are uncertain due to ver- tical imprecision in the measurement process and coarse sample spacing. This uncertainty can result in substantial differences between calculated shadow positions and ac- tual shadow locations in photographs when conventional shadow location methods are used. We have developed a technique which takes into account this uncertainty to determine the probability that any given point on the ter- rain is in sunlight or shadow in the aerial photograph. This shadow probability is crucial to accurate calculation of the reflectance of terrain points corresponding to the pixels of the original aerial photograph.