G. T. A. Hof, R. Reiling, E. Zilverberg, and Th. W. M. Levelt. 1981. “An Areal Classification by Means of Cluster Analysis Using Abiotic Landscape Data”. Netherlands Journal of Geosciences, January, 427-32. https://njgjournal.nl/index.php/njg/article/view/13649.