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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Introduction
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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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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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Application in epidemiology
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Area of mapping units = target population.
Hence automatic visual weighting based on
population distribution.
Shading =
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Case density
Sampling distribution
or Other relevant factors
Dot density = cases
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X
Inverness
Inverness
X
Aberdeen
Aberdeen
Application 1:
Salmonella
surveillance
Perth
Perth
X
X
Edinburgh "" Lasswade
Lasswade
Edinburgh
X
Ayr
Ayr
- without cartogram

X
St Boswells
Boswells
St
X
Dumfries
Dumfries
Newcastle
"" Newcastle
Penrith
"" Penrith
Shading = density of cattle
holdings
Thirsk
"" Thirsk
Preston
"" Preston
)"")
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Dot density = cases
Liverpool
Liverpool
Shrewsbury
"" Shrewsbury
Sutton Bonington
Bonington
"" Sutton
Aberystwyth
"" Aberystwyth
Bury St
St Edmunds
Edmunds
"" Bury
Luddington
"" Luddington
Carmarthen
"" Carmarthen
)"")
Langford
"" Langford
London
London
Weybridge
"" Weybridge
Winchester
"" Winchester
""
""
Truro
Truro
Starcross
Starcross
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Inverness
Inverness
X
Aberdeen
Aberdeen
X
Application 1:
Salmonella
surveillance
- using cartogram
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Area = number of cattle
holdings
Salmonella Incidents
Perth
Perth
X
Ayr
Ayr
X
!
Lasswade
X"" Lasswade
!
")
St
St Boswells
Boswells
")
X
Number of Cattle / Holding
Dumfries
Dumfries
! !
X
! !
2 - 30
! !
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Newcastle
Newcastle
!
31 - 60
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Penrith
Penrith
!!
!!
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61 - 90
!
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!!
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!! !
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!!
91 - 120
!
!!
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!!
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121 - 150
!
!! !
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Thirsk
Thirsk! !
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151 - 250
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VLA Regional Laboratory
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Preston
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! !Preston
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VLA Surveillance Centre
!
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SAC Disease Surveillance
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Shrewsbury
Shrewsbury
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Sutton
Bonington
Sutton
Bonington
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Aberystwyth !!
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! Aberystwyth
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Bury
Bury St
St Edmunds
Edmunds
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Luddington
Luddington
!
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Carmarthen
Carmarthen
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Weybridge
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! ! ! Weybridge
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Langford
Langford
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Winchester
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! Winchester
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!Starcross
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Starcross
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"" Penrith
!!
"" Newcastle
"" Thirsk
"" Preston
)"
")
Liverpool
Liverpool
X
"" Shrewsbury
Shading = cattle per holding
"" Sutton Bonington
"" Aberystwyth
"" Bury St Edmunds
"" Luddington
"" Carmarthen
)"
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Dot density = cases
Locations randomized within counties
Area Representing 1000 Holdings
Edinburgh
Edinburgh
!
!
1 Dot = 1 incident
!
"" Langford
London
London
"" Weybridge
"" Winchester
"" Starcross
Truro
"" Truro
!
Birch, C.P.D. (2008)
6
Centre
Scrapie cases detected
Classical
Application 2:
Scrapie surveillance
Atypical
% Holdings Sampled
0.0% - 1.5%
1.6% - 5.0%
5.1% - 20.0%
20.1% - 50.0%
> 50.0%
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Area = number of sheep
holdings
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Shading = % holdings sampled
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Dot density = cases
5000
Birch, C.P.D., Chikukwa, A.C., Hyder, K., Del Rio Vilas, V.J. (2009)
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Benefits of area cartograms in
epidemiology
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Better presentation of cases vs. denominator
population
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Richer data presentation
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Direct visualization showing whether cases are
uniformly or unevenly distributed.
Visual impression more accurate.
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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Cartogram methods and tools are
readily available
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Downloads for ArcGIS
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Method
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ESRI Cartogram and Geoprocessing Tool (Tom Gross,
http://arcscripts.esri.com)
Cartogram Creator – used here (Eric Wolf,
http://arcscripts.esri.com)
Gastner & Newman (2004) Proc. NAS 101: 7499-7504
Website
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www.worldmapper.org
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Other references
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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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Visualization of epidemiological
information using area cartograms or
density-equalized maps
Colin Birch
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To start presentation click arrow at bottom right.
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