Summary

Suitability analysis with GIS is an evaluation of a location or area for certain use. The evaluation is usually done by intersecting social, ecological, economic, physical, biological, and other criteria. The result of a suitability analysis is usually a suitability or hazard map. It must be distinguished – depending on the selection of criteria for a purpose – if several criteria apply (Multi Criteria Evaluation MCE), or if exclusionary land use claims exist that must be balanced (Multi Objective Evaluation MOE). Overlay refers to the digital integration of location and attribute information of several spatial layers. Boolean overlay is based on the assumptions that different combinations can be either "true" or "false", but never both (e.g. we are looking for areas for which the combination "forest" and "eastern slope" are "true"). Boolean overlays available in common GIS software are intersection (AND), union (OR), exclusive union (XOR), and negation (NOT). A distinction in categories of only "true" and "false" is rarely realistic. Therefore, layers based on interval and ratio scales and weighted decision criteria are introduced. The weak spot of weighted overlay is the fair allocation of weights. It can vary considerably depending on the interests of the experts who are assigning the weights. It can also noticeably influence the results. Rules for weighting are defined by different methods to avoid arbitrariness. The simplest weighting is based on ranking the criteria: the higher the rank of a layer the larger is its weight. Another method is rating: a part of the set overall score is assigned to the layers according to their importance. Furthermore, weighting can be done using pairwise comparison: the essence of this approach is that two criteria are evaluated at a time to determine their relative importance. The weight of each layer is calculated in a comparison matrix. All three methods involve the risk of producing results of limited significance by careless and untrustworthy weighting.