Difference boundaries: Appear as lines or zones indicating where the variables change rapidly through space. Usually open. Interpretation: values of variable(s) on one side of the line boundary differ significantly from values on the other side.
Areal boundaries: Appear as polygons, each of which encloses an area of relative homogeneity. Always closed. Interpretation: The values of variables inside each polygon vary little compared with the values of variables across adjacent polygon borders. (Alternative interpretation: the space inside each polygon is somewhat uniform (in terms of the variables) and differs from areas inside adjacent polygons.)
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