Geographic Information Systems: An Introduction, 3rd Edition

5.1 Terrain Surface Representation

5.1.1 General

The models discussed in Chapter 4 describe limited parts of a two-dimensional real world. Several other data models used in GIS can extend the real world to include the terrain surface, the time factor, and movable objects. The digital representation of a terrain surface is called either a digital terrain model (DTM) or a digital elevation model (DEM). In GIS disciplines, the term DTM is often used not just for the model itself, but also for the software used to manipulate the relevant data. Only the model is discussed in this chapter; data manipulation is discussed in detail in Chapter 15.

The terrain surface can be described as comprising two basically different elements—random and systematic elements. The random (stochastic) elements are the continuous surfaces with continuously varying relief. It would take an endless number of points to describe exactly the random terrain shapes, but these can be described in practice with a network of points. It is usual to use a network that creates sloping triangles or regular quadrants.

The systematic part of the terrain surface is characterized either by sharp cracks in the terrain, such as the top or bottom of a road cut, or by characteristic points such as spot depression and spot height. The systematic part is thus best represented by lines and typical single points. Prominent terrain features can be verbally described using many terms, such as smooth slope, cliff, saddle, and so on. ...