An Interactive Approach to Analytical Relief Shading

The software currently available for analytical relief shading does not generally permit local adaptations of the light direction, the simulation of aerial perspective, and other necessary techniques developed for manual relief shading. To remedy this deficiency, a program for computer-assisted relief shading has been developed that allows users to locally adapt shading characteristics, permitting seamless interactive control over the entire process. The grey values of the image are determined by a combination of aspect-based shading for steep regions, diffuse reflection for lowlands, and a bright grey tone for flat areas. Furthermore, an algorithm for the simulation of aerial perspective is presented. Tests with the program have shown that, with minimal investment of time, the quality of analytically produced shaded relief can be improved significantly. Using the proposed techniques and software presented herein, experienced cartographers can transfer their manual relief-shading knowledge and experience to the digital realm. Introduction elief shading is used in cartography to depict topography. Historically, it was produced by traditional manual methods based mainly on the interpretation of contour lines and hydrographic networks, and, more recently, by computer calculations based on digital elevation models. Different attempts with analytical relief shading have been undertaken, and the associated algorithms have been improved. If digital elevation models are available, analytical shaded relief can be produced much more quickly and thus less expensively than with manual relief shading. Unfortunately, the currently available computer programs largely ignore the specialized rules and guidelines that have been developed for manual relief shading. Among mountain cartographers, the legibility and aesthetic R quality of a well-executed hand-shaded relief is generally considered to be superior to that of a computer-generated relief, and therefore it is preferred for demanding applications. To overcome the limitations associated with digital relief, a prototype program for computer-aided relief shading was developed. The goal was to simulate the techniques used for manual relief shading with an interactive digital approach. The program enables the cartographer to locally adapt relief shading to enhance topographic features that defy clear depiction, and it uses a new method to simulate aerial perspective. To determine the grey values of the shaded relief, diffuse reflection is combined with aspect-based shading and a tone for flat areas. A series of tests in collaboration with the Swiss Federal Office of Topography demonstrated that, with a small amount of invested time, the overall quality of analytical relief shading can be improved significantly, approaching the standard of well-executed manual shading. This paper provides a detailed description of interactive analytical relief shading. A more conceptual presentation can be found in Hurni and others (2001). To illustrate the discussed methods, a series of figures show the sequential improvement of relief shading generated from a digital elevation model of Mount Rigi (including three surrounding lakes) near Lucerne, Switzerland. Manual Relief Shading The manual depiction of terrain according to a specific illumination model (northwest lighting) is a time-consuming task. Topography is not generally represented in a mathematically correct form. Instead, the shaded relief is slightly altered to make it more intuitive for readers. This goal is achieved by applying the following techniques (Imhof 1982): adjustments of the light source; placement of bright grey tones in flat areas; adjustments of brightness; aerial perspective; and the use of colour. In traditional relief shading, a fictional light direction is chosen, usually from the northwest, to globally illuminate the relief. In addition, highly skilled cartographers will turn the main light direction slightly in order to emphasize and clarify selected topographic features, such as ridges, valleys, and watersheds. Bernhard Jenny, Institute of Cartography, Swiss Federal Institute of Technology (ETH), ETH Hönggerberg CH-8093 Zurich, Switzerland. Tel.: ++411 633 30 36. E-mail: [email protected]