Journal
CARTOGRAPHIC JOURNAL
Volume 47, Issue 2, Pages 130-140Publisher
TAYLOR & FRANCIS LTD
DOI: 10.1179/000870409X12488753453372
Keywords
value-by-alpha map; value-by-area map; cartogram; Cartogram(3); visual equalisation; transparency; colour perception; representing social data; representing uncertainty
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Funding
- NCI NIH HHS [R01 CA095949, R01 CA095949-05] Funding Source: Medline
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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.
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