Introduction Uncertainty has been an important topic in geographical information science for over a decade (e.g. Goodchild and Gopal, 1989; Veregin, 1996; Kiiveri, 1997; Heu-velink, 1998; Zhang and Goodchild, 2002). This focus has led researchers to recommend over many years that the spatial output of a geographical information system (GIS) should be (at least) twofold: […]