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1 Visualization of epidemiological information using area cartograms or density-equalized maps C.P.D. Birch Centre for Epidemiology & Risk Analysis, Veterinary Laboratories Agency – Weybridge, Surrey, UK
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2 Introduction Land area is irrelevant to many epidemiological presentations. So why use it to determine the size of mapping units? Area cartograms provide an alternative. Base the size of mapping units on measures that matter.
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3 US Presidential Election 2004 Unit = area This may be how ex-President Bush would like us to see the 2004 US Presidential Election result. Unit = voters This is a better representation of how The US population voted. M. T. Gastner, C. R. Shalizi, and M. E. J. Newman (2004)
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4 Application in epidemiology Area of mapping units = target population. Hence automatic visual weighting based on population distribution. Shading = Case density Sampling distribution or Other relevant factors Dot density = cases
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5 Shading = density of cattle holdings Application 1: Salmonella surveillance - without cartogram Dot density = cases Dot density = cases
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6 Shading = cattle per holding Application 1: Salmonella surveillance Birch, C.P.D. (2008) - using cartogram Area = number of cattle holdings Dot density = cases
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7 Area = number of sheep holdings Application 2: Scrapie surveillance 5000 Birch, C.P.D., Chikukwa, A.C., Hyder, K., Del Rio Vilas, V.J. (2009) Shading = % holdings sampled Dot density = cases
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8 Benefits of area cartograms in epidemiology Better presentation of cases vs. denominator population Direct visualization showing whether cases are uniformly or unevenly distributed. Visual impression more accurate. Richer data presentation More data can be presented. More space on the map for areas likely to contain more information. Precise locations obscured (confidentiality) Maps that emphasize features that matter
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9 Cartogram methods and tools are readily available Downloads for ArcGIS ESRI Cartogram and Geoprocessing Tool (Tom Gross, http://arcscripts.esri.com) Cartogram Creator – used here (Eric Wolf, http://arcscripts.esri.com) Method Gastner & Newman (2004) Proc. NAS 101: 7499-7504 Website www.worldmapper.org
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10 Other references Birch, C.P.D., 2008. Methods for spatial visualization by using maps in animal health surveillance. Veterinary Laboratories Agency, Report to Defra pp. 35. Birch, C.P.D., Chikukwa, A.C., Hyder, K., Del Rio Vilas, V.J., 2009. Spatial distribution of the active surveillance of sheep scrapie in Great Britain: an exploratory analysis. BMC Veterinary Research 5, 23. Gastner, M.T., Shalizi, C.R., Newman, M.E.J., 2005. Maps and cartograms of the 2004 US presidential election results. Advances in Complex Systems 8, 117-123.
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11 Visualization of epidemiological information using area cartograms or density-equalized maps Colin Birch To start presentation click arrow at bottom right.
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