Production of Shaded Relief with Digital Tools
Bernhard Jenny email@example.com
Stefan Raeber firstname.lastname@example.org
Institute of Cartography
Manual relief shading is a time consuming task. Experienced cartographers are needed to analyze and interpret topography and to derive a cartographic relief shading there from. Traditional manual shadings are produced with simple pencils or airbrushes. Naturally, production time is of paramount importance for the realization of any cartographic project. Hence, the Institute of Cartography of ETH Zurich has been researching alternative, computer-based ways to produce high quality relief shading. In addition to the desired high quality, two supplemental conditions have to be taken into account: Experienced cartographers must be able to transfer their knowledge to the new system, and the software must be easy to learn and use.
Experimenting with relief shadings of different scales showed that the most effective way to produce shaded reliefs consists in two steps:
Step1. Derivation of a shaded relief from a digital elevation model (DEM) using specialized software (analytical relief shading).
Step 2. Final touch and corrections of the image of step 1 using a standard raster graphic software.
Analytical relief shading
A prototype of a specialized software (called Shadow) for interactive, analytical relief shading was developed. A DEM is used in combination with an illumination model to derive shading. The software allows for the adaptation of the relief shading at global and local scale. Global settings are established for the complete map sheet or a series of map sheets, whereas a local adjustment is specific for a single topographic feature. The adjustable parameters are light direction (Figure 1), vertical exaggeration, brightness, illumination model, aerial perspective, tone for flat areas, and contrast. The relief can be adjusted to the desired scale of the map by filtering the DEM. Based on the DEM, the relief can be vertically exaggerated, the effect of aerial perspective can be simulated (in function of elevation and slope lines), and a bright gray tone can be applied to flat areas.
Hypsometric colors are easily derived from a DEM. Colors, in function of exposition and elevation as used by the "Swiss style" of relief shading, can also be computed. The developed prototype offers the possibility to assign a color to each pixel in the shaded relief (Figure 2).
A so-called yellow "light tone" can alternatively be used to further emphasize and accentuate the depiction of the relief (Figure 3). This is a more simple and economic technique than the method of Figure 2, since only one additional color (usually yellow) is needed. The "light tone" can be simply derived from the original relief shading by inverting the gray values and masking out flat areas, glaciers, and cliff areas.
Treatment with Raster Software
A raster software (such as Adobe Photoshop) is used in this subsequent step to correct undesired artifacts in the relief shading. These artifacts usually originate from erroneous DEMs. Photoshop is also used to draw flat areas especially in large-scale maps and to accentuate and correct topographic features.
In the final step, Photoshop is used to adjust the gradation curve of the relief shading and to mask cliff areas.
To illustrate the proposed method, we will present an example of Macedonia (Figure 4, Figure 5, and Figure 6). A relief shading at the scale of 1:750.000 for thematic maps was produced using the described prototype for analytical relief shading and Photoshop for necessary corrections. Analytical shading and the work done with Photoshop are both based on contour lines, hydrography (rivers and lakes), and administrative boundaries.
The shading relief resulting from Shadow needed to be revised in the following points:
--Overemphasized riverbeds had to be softened.
--False terraces and other artifacts in the DEM had to be eliminated.
--In the central plain of Macedonia, relief shading had to be redesigned completely.
--Certain small landforms had to be eliminated, whereas others had to be emphasized.
--Light direction had to be further adapted in some areas.