Weighting-by-choosing: a weight elicitation method for map overlays

In land suitability assessment, the map overlay technique is often used in conjunction with a weighting scheme. A person first determines parent maps’ weights by his perceptions about the importance or relative importance of these maps to land suitability. These weight values are then incorporated into the map overlay process. On the resultant overlaid maps, the higher suitability scores are always assigned to those sites that have better conditions on the more important parent maps. Of the two approaches that one can take in determining maps’ weights, tradeoff weighting is more precise than direct assessment, but also more difficult to use because it requires greater cognitive efforts from the users. This article presents a weighting-by-choosing method that facilitates the process of making tradeoffs through a series of site selection exercises. By using hypothetical reference sites as tangible manipulatives, it transforms an otherwise difficult cognitive task into a simple selection exercise. At present, the method applies to two maps at a time, but could potentially be extended to multiple maps.