@article {Hengl2004IJGIS,
	title = {{A double continuous approach to visualisation and analysis of categorical maps}},
	journal = {International Journal of Geographical Information Science},
	volume = {18},
	number = {2},
	year = {2004},
	pages = {183-202},
	author = {Hengl, T. and Walvoort, D.~J.~J. and Brown, A. and Rossiter, D.~G.}
}