Computer-based image analysis requires explicit models of the image-forming process in order to deal with the effects of variations in viewing direction, incident illumination, surface slope and surface material. A fixed illumination, surface material and imaging geometry is incorporated into a single model, called a reflectance map, that allows observed brightness to be written as a function of surface orientation. The reflectance map is used to generate synthetic images from digital terrain models. Synthetic images are used to predict properties of real images. This technique is illustrated using Landsat imagery. Accurate shadow regions are determined from a digital terrain model by calculating which surface elements are visible from the light source. Once shadows are determined, the effect of sky illumination and atmospheric haze is estimated.