Value‐by‐alpha Maps: An Alternative Technique to the Cartogram

Authors: Roth, Robert E; Woodruff, Andrew W; Johnson, Zachary F

Source: Cartographic Journal, The, Volume 47, Number 2, May 2010 , pp. 130-140(11)

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Abstract:

The cartogram, or value‐by‐area map, is a popular technique for cartographically representing social data. Such maps visually equalize a basemap before mapping a social variable by adjusting the size of each enumeration unit by a second, related variable. However, to scale the basemap units according to an equalizing variable, cartograms must distort the shape and/or topology of the original geography. Such compromises reduce the effectiveness of the visualisation for elemental and general map‐reading tasks. Here we describe a new kind of representation, termed a value‐by‐alpha map, which visually equalizes the basemap by adjusting the alpha channel, rather than the size, of each enumeration unit. Although not without its own limitations, the value‐by‐alpha map is able to circumvent the compromise inherent to the cartogram form, perfectly equalizing the basemap while preserving both shape and topology.

Keywords: value?by?area map; representing uncertainty; visual equalisation; Cartogram3; representing social data; cartogram; value?by?alpha map; colour perception; transparency

Document Type: Research Article

DOI: http://dx.doi.org/10.1179/000870409X12488753453372

Publication date: 2010-05-01

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