Hill-shading Techniques to Enhance Terrain Maps

Maps shading terrain as if illuminated by a point light source (i.e. hill-shading) are commonly used in cartographic displays. These hill-shaded maps are especially important for representing mountainous areas, due to the rapid variations in the orientation of terrain elements. Although these hill-shaded maps reveal the coarser landforms and the finer texture of the terrain, they can be enhanced to reveal additional detail with numerous methods. This paper discusses three methods to enhance such maps. The first technique varies the luminosity of colors based on the aspect of the terrain. Colors are selected in an elliptical pattern in hue-saturation-value (HSV) color space such that terrain facing opposite directions will show variations in luminosity. These colors are then combined with shading from a point illumination source. This illumination model is designed to complement the colors of varying luminosity. Resulting displays reveal subtle terrain features not apparent with hill-shading alone. This is especially apparent for terrain elements oriented parallel to the direction of illumination. The second methodology illuminates terrain with multiple light sources. These illuminants are from numerous directions and are of varying intensity. In this work, the lighting model approximates diffuse illumination distributed across a virtual sky. The resultant renderings contain soft shading and shadows which provide useful visual cues to the relative height of terrain. With this model, the terrain appears as if viewed on a cloudy day. Details are also revealed beneath what otherwise would be harsh, black shadows that occur with default geographic information system (GIS)-based shading and shadowing algorithms. The third technique captures local variations in curvature. Maps of curvature can be displayed in grayscale in a manner consistent with diffuse illumination models. These maps can then be displayed over hill-shaded maps to highlight trends associate with rapid changes in curvature. The resulting maps can highlight areas of plan or profile curvature, related to areas of rapid change in aspect and slope, respectively. Examples of the use of such maps would be for detailing trends in geomorphologic features such as drainages (with planimetric curvature) and more resistant rock layers (with profile curvature). In addition to revealing or highlighting more detail, all of these techniques represent alternatives to common hill-shaded maps. Such techniques provide the map designer more options for creating visually interesting maps based on physical properties of the sky and terrain.