ABSTRACT
Geographical information systems or GIS (Burrough and McDonnell, 1998) have been used since the late 1960s (Coppock and Rhind, 1991) for automating map making and using digital spatial data. Today we see that in many cases, particularly in environmental studies, the paper map has been replaced by the much more flexible electronic database. Digital GIS provide powerful spatial databases with many applications (see Longley et al., 2001) ranging from land registration, marketing and in-car navigation systems to the dynamic modelling of environmental processes such as soil erosion and land degradation (de Roo and Jetten, 1999) or nutrient transport of over hydrological catchments (de Wit, 1999).
