Decoupling Slope and Aspect Vectors to Generalize Relief Shading

Relief shading is designed to vary the brightness of terrain elements on a two-dimensional map to create a three-dimensional effect. One concern for cartographers is how this layer can be generalized for use in multi-scale mapping. We propose a methodology that calculates relief shading from slope and aspect vectors, allowing map users to recognize characteristics of the terrain and show consistent trends in spatial autocorrelation with generalization. To demonstrate its use, we show two examples of generalizing and effectively smoothing detailed relief shading and compare results to relief shading of the next coarser scale of DEM data available. In both cases the generalized maps remove or smooth out minor landforms while preserving larger or more prominent landforms. The authors predict methods to generalize shaded relief will gain in importance as elevation data are collected at the finest resolutions, with limited effort put towards collecting multi-scale elevation data at coarser resolutions.