The goal of a sustainable land use and landscape development can only be reached by re-establishing regulation functions. That is why it is necessary to assess such a function besides others (production, carrier, information) to derive different potentials for various parts of landscape properties and to promote these potentials by introducing relevant measures. A first step in this direction is to assess profoundly how capable landscape units are. Regarding landscape units as areas of assessment they have to be constant for a long time, especially along their boundaries. As land use changes profoundly on a large scale, landscape units have to be separated on a regional scale. Thinking in geographic dimensions it is the chronological level we have to work on. Unfortunately, there are no assessment methods for landscape functions in this dimension. Due to the fact that all traditional assessment procedures are only valid for small areas it is necessary to look for methods that are of value for larger areas too. The use of fuzzy set methods and fuzzy logic seems to be promising. Traditional assessment methods are taken and combined with fuzzy set theory to get results for larger landscape units. Taking the function of ground water protection as an example the method is exemplarily described and applied on landscape units in the agglomeration area of Leipzig-Halle in the central part of Eastern Germany. Finally, a suggestion for the cartographic implementation is given.