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A variable observed at a set of geographic localities may often be profitably viewed as being measured at discrete points on an underlying continuous surface. Wombling, a method first developed by Womble (1951) for gene frequencies and morphological measurements, and later refined by Barbujani, Oden, and Sokal (1989), is a technique for detecting zones of rapid spatial change in a collection of such surfaces. The method rests upon the notion of a "systemic function" defined over the geographic space. At any locality, the systemic function is equal to the average of the absolute values of the gradients of the separate continuous surfaces defined at that locality. Mapping the systemic function over geographic space reveals zones of rapid change. Related ideas have been developed by Monmonier (1973) and Adams (1970).

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