Birkmann, J. and Finke, L. (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), pp. 276–286. doi:10.1007/BF03183027.