Assessment, limitations, and problems

The Boolean overlay works best for suitability analysis with sharp-edged and clear exclusionary criteria. If settlement area and farmland can be excluded as wolf habitat from the beginning, the potentially suitable area can be narrowed down easily with a simple Boolean overlay. However, for many questions the division of the world into only two categories is unsatisfactory. Additionally, criteria for the natural environment can seldom be set selectively. In the next unit you will learn about more precise approaches to suitability analysis with GIS.

Raster or vector overlay in suitability analysis?

Overlays, and in particular Boolean overlays, are possible using both vector and raster models. In general, vector overlay is more complex, time-consuming, and computationally intensive than raster overlay which is easier, faster, and more efficient. Nevertheless, before a raster overlay can be performed, a set of elaborate preprocessing steps must be conducted to approximate the cost for the two methods. There are other reasons to opt for one or the other of the two data models. The following list shows when each model should be considered.

Advantages of vector overlay:

• Clear, sharp-edged criteria, e.g. no-building zones in a hazard map where you are not allowed to build under any circumstances;
• Clear, sharp-edged input data;
• Clear distance criteria, e.g. maintaining a minimum distance, "at least 100 meters away from the settlement area."

Advantages of raster overlay:

• Fluffy or fuzzy criteria, e.g. the criterion "steep" is difficult to clearly delineate;
• Continuous and arguable data;
• The creation of cost surfaces.