Abstract
IMPLICITY is a fundamental goal of cartography. An overly complicated map can impose severe perceptual difficulties on a user interested primarily in general spatial trends. With many types of maps, complexity can be minimized if the cartographer employs relevant principles of graphic design. However, in the case of choropleth maps the selection of class intervals is an additional determinant of a map's effectiveness. The traditional objectives of compact, homogeneous classes, and the de-emphasis of relatively large. variations in the tails of a numerical distribution, often do little to promote visual simplicity. These criteria for class-interval selection can often be satisfied by a spatially fragmented pattern of shaded area symbols. Although polynomial trend surfaces might provide a partial solution to this problem of spatial generalization, the frequent inability of low-order trend surfaces to account for a high percentage of the spatial variation of a quantitative variable may render this approach unacceptable. Furthermore, in some cases where isopleths are substituted for area shadings severe conceptual difficulties can arise when the value for an area is assigned to a mere point within that area. An ideal solution would retain the boundaries of individual enumeration areas and at the same time allow proximity a role in the selection of class intervals. The purpose of this study is to develop a procedure for basing class intervals for choropleth maps not only on the numerical distribution of the mapping units but also on their spatial contiguity.
Published Version
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