A very commonly used type of data in GIS are topographic surfaces, which typically take the form of raster maps known as digital elevation models (DEMs) or digital terrain models (DTMs).
Although clearly not everything that happens on Earth can be seen from the surface, topography tells much about where water goes, hence where water-borne contaminants may be transported. Also, topography strongly affects a number of ecological and physical processes, including the redistribution of sediments, the formation of soils, shading and the incidence of solar radiation, and temperature.
DEMs are available globally with increasing resolution (see Chapter 12), currently down to a cell size of 30 m. For several regions of the world, high resolution DEMs are available with higher standards of accuracy than global products. In this chapter, we introduce the most commonly used tools for the analysis of DEMs and we show examples of how these can be used in chemical modeling.
A topographic surface is a function that can be treated mathematically with standard calculus. In particular, its derivatives can be computed, which represent the slope and curvature of terrain. Another important feature of a surface is the direction of slope, or aspect. Most GIS packages with raster functionalities ...