When digital stereo correlation methods are used to generate elevation data from stereo images of the terrain, the likelihood that the correlation process will be successful throughout the whole of the stereo model can be enhanced significantly by using hierarchical, multi-scale, pyramidal, coarse-to-fine, etc., techniques. One main advantage of these techniques is that the correlation data obtained at a lower resolution in the hierarchical procedure is used to guide and control the correlation process at the next higher resolution and, consequently, the user can be reasonably assured that the correlation process will not get 'lost'. Unfortunately, constraints imposed by results obtained at the lower resolution can also lead to generation of erroneous elevation data in some circumstances. For example, if the terrain is spotted with individual trees, buildings or other structures, the correlation process, when performed on the low resolution images, will include approximations to the heights of these features in the resulting elevation data. These are erroneous elevations compared to the 'bald earth' and their influence will be carried from one step to the next in the hierarchical procedure. The result will be a 'noisy' digital elevation model (DEM) containing approximations to the elevations of trees and structures on the terrain and not just the terrain itself.