A METHOLOGY FOR CREATING ANALYTICAL HILL-SHADING BY COMBINING DIFFERENT LIGHTING DIRECTIONS
D. Loisios1, N. Tzelepis2, B. Nakos2
1 - Hellenic Military Geographical Service, Athens, Greece
2 - National Technical University of Athens, Greece
Hill-shading images for cartographic use are mostly produced considering the light coming from north-west, as this has been visually approved to help all map users, experienced or not, to perceive the forms of earth’s surface relief immediately and accurately. This condition is deeply rooted in the hill-shading process, due to subconscious, positive understanding of graphic elements under upper-left light in our everyday experience. In some cases, when slopes that face to opposite north and south directions or to northwest and southeast are dominating, the acceptance of alternative lighting directions (usually from west or south) might be carefully utilized for better shading results. Also, the idea of south lighting on three-dimensional shaded maps of areas in the northern hemisphere had been strongly supported by some cartographers, more concerned for the clear presentation of earth’s surface covering, than for the correct sight of the relief.
Scepticism arises from the fact that, a strict adherence to only one, optimal light direction for the whole map, would be rather difficult to accentuate all the relief forms that exist in the portrayed area, having several, different main directions. The well-known method of local adjustment of light direction has been developed to deal with this problem, but it works in a rather local way by adapting within a limited range the azimuth and slope angles of light around the local terrain. The problem still remains for entire sides of ground formations that are not properly lightened. What is more, steep slopes that don’t face the light, are shaded with dark grey tones that create optical confusion, finally leading to the encryption of underlying relief information or any other superimposed cartographic symbol. In larger scales with more detailed relief, the above phenomena acquire more attention, because steeper slopes occur, variability of aspect is higher and local adaptation is less effective.
This study experiments with ways to create an analytical hill-shading image where the above deficiencies are possibly omitted. Two images of shaded relief are produced, an initial one based on a standard light direction, and an enhanced mixture of shading images derived from different lighting directions, where each one participates with a specific weight, either calculated as a function of aspect or even empirically defined in order to give bonus to the traditional directions. The cells of the initial shading that correspond to steep slopes are partly replaced by the values of the enhanced shading, and the amount of partial replacement depends on slope value. The final combined hill-shading image lacks of highly dark tones in steep slopes and allows the relief details in these areas to be revealed, still keeping the beneficial influence of north-west or other optimal standard lighting.