Saturday, May 11, 2013


Kernel Density Estimation (KDE), also known as a "heatmap," is a wonderful way to graphically convey epidemiological data for outbreaks of infectious disease. At a glance, one can discern likely points of origin, routes of transmission, and density by area. KDE maps are attractive and fairly easy to render in QGIS, but it can be exceedingly difficult to obtain spatial point data, which is necessary for creating the nodes used by the software to model density. For this KDE, the underlying raster image is John Snow's original 1854 map, with vector overlays of spatial point data showing water pumps and cholera deaths. The proportional mortality is most densely concentrated around the Broad Street pump (the source of infection as deduced by John Snow). For proper viewing, click HERE to enlarge.

Density radius: 30m
Decay rate: 0
Cell size X,Y: ~1,1
Global transparency: 20%

Snow's original map is available in a high resolution format here:

Unfortunately I did not bookmark the site which offered the spatial point data for download, but if I can locate it again, I'll be sure to give appropriate credit here.


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