Birkmann, Jörn, and Lothar Finke. 2001. “Indicators for the Evaluation and Guidance of Sustainable Spatial Development in North Rhine-Westphalia: Purposes, Potentials and Problems Associated With the Use of Sustainability Indicators in Area Development Planning in North Rhine-Westphalia”. Raumforschung Und Raumordnung | Spatial Research and Planning 59 (4): 276–286. https://doi.org/10.1007/BF03183027.