Contour plot obtained with a generalized additive model.

The contour plot shows a regression surface of pronunciation distance as a function of longitude and latitude obtained with a generalized additive model using a thin plate regression spline. The (black) contour lines represent distance isoglosses, darker shades of gray indicate smaller distances closer to the standard language, lighter shades of gray represent greater distances. Note that the empty square indicates the location of the IJsselmeer, a large lake in the Netherlands.