Many land use decision problems are sufficiently complex, controversial and non-routine in nature that their resolution requires not only knowledge, expertise and data drawn from several distinct domains, but also debate of multiple and often conflicting viewpoints, objectives and solution strategies. Despite the promise of Geographic Information Systems (GIS) technology for examining a wide variety of land use issues, it has been criticised for its lack of decision support capabilities in this and other application areas (Hendriks and Vriens, 2000; Towers, 1997; Heywood et al., 1994). In particular, the capacity of commercial GIS to facilitate debate and achieve some measure of balance among different viewpoints has been identified as a major weakness. This capability is constrained by several factors in commercial software, foremost of which is an intrinsic single-user perspective that disregards the multi-interest character of the decision making process and the socially constructed nature of data and analytical methods (Pickles, 1999; Flowerdew, 1998; Jones et al., 1997; Goodchild, 1995).