Techniques for Highlighting Relief on Orthoimaginery

In cartographic products such as shaded relief maps or orthoimagery, the direction of illumination can cause a perceptual phenomenon called terrain reversal effect where the depressions appear as peaks and vice versa. This is the case for predominantly north-oriented shaded relief maps and geographic imagery in the northern hemisphere. The problem is a troubling one for distinguishing three-dimensional spatial relationships between the landforms, photo interpretation, and possibly for image classification tasks. Rotating the image or creating a negative of it seems to reverse the spatial relationships back to the intended representation, however these are poor solutions. The former discards the familiarity of the north-orientation and is prone to confuse the viewer, and latter modifies the color information in such a fundamental way that it is nearly impossible to tell the natural features apart. Currently there are only a few technical solutions that address this issue. With this paper, we contribute a range of techniques that explore the possibility of merging a perceptually-driven adjustment of a hill shading model with the orthoimagery to visualize the depth information in a perceptually correct manner.