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

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.

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)

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

5  Shading = density of cattle holdings Application 1: Salmonella surveillance - without cartogram  Dot density = cases  Dot density = cases

6  Shading = cattle per holding Application 1: Salmonella surveillance Birch, C.P.D. (2008) - using cartogram  Area = number of cattle holdings  Dot density = cases

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

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

9 Cartogram methods and tools are readily available  Downloads for ArcGIS  ESRI Cartogram and Geoprocessing Tool (Tom Gross,  Cartogram Creator – used here (Eric Wolf,  Method  Gastner & Newman (2004) Proc. NAS 101:  Website 

10 Other references  Birch, C.P.D., 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., 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., Maps and cartograms of the 2004 US presidential election results. Advances in Complex Systems 8,

11 Visualization of epidemiological information using area cartograms or density-equalized maps Colin Birch  To start presentation click arrow at bottom right.