The effects of relief variation and the spectral contrast are not as straightforward to quantify. We note qualitatively here
that surface relief is an issue. If a feature in the nadir image looks very different in the aft image due to the effect of
parallax, then a good stereo match will be difficult to obtain. Spectral contrast is an issue because in relatively homogeneous regions of spectral reflectance, there are few or no features to match from one scene to the other, as is
demonstrated by the correlation distributions in Fig. 4 for land cover types over the ASTER-derived Taranaki DEM.
Areas in relief, shadow or with excessively high and low albedo (e.g. snow, salt lakes, some types of forest) may also
become problematic during the stereo matching process.