E. Oele, W. Apon, M. M. Fischer, R. Hoogendoorn, C. S. Mesdag, E. F. J. de Mulder, B. Overzee, A. Sesoren, and W. E. Westerhoff. 1983. “Surveying the Netherlands: Sampling Techniques, Maps and Their Application”. Netherlands Journal of Geosciences, January, 355-72. https://njgjournal.nl/index.php/njg/article/view/13396.