Discovering how two datasets spatially relate to each other when they are placed over one another is called overlay analysis. An overlay can be compared to a sheet of tracing paper. For example, you could overlay the tracing paper on top of your base map and see what areas overlap each other. This process is and was a game changer in spatial analysis and modeling. Computer-aided GIS computations can therefor automatically identify where two geometry sets spatially touch for example.

The goal of this chapter is to give you a feel for the most common overlay analysis functions, such as unions, intersects, and symmetrical differences. These are based on the Dimensionally Extended nine intersection model (DE-9IM), which can be found at http://en.wikipedia.org/wiki/DE-9IM, and describes our list of possible overlays. All processes that we use or name here are derived using a combination of these nine predicates.

We will explore these topology rules in depth in Chapter 9, Topology Checking...