GIS Analyses of Snow's Map
Snow's map, demonstrating the spatial clustering of
cholera deaths around the Broad Street well,
provided strong evidence in support
of his theory that cholera was a water-borne disease.
Snow used some proto-GIS methods to buttress his argument:
first he drew
Thiessen polygons around the wells, defining straight-line least-distance
service areas for each.
A large majority of the
cholera deaths fell within the Thiessen polygon surrounding the Broad
Street pump, amd a large portion of the remaining deaths were on the
Broad Street side of
the polygon surrouding the bad-tasting Carnaby Street
Next, using a pencil and string, Snow redrew the service area polygons to
reflect shortest routes along streets to wells.
An even larger proportion of the cholera deaths
fell within the shortest-travel-distance area around the Broad Street
Try replicating and extending Snow's analysis with ArcGIS.
The pumps and deaths
datapoints were digitized by Rusty Dodson at the
National Center for Geographic Information & Analysis (NCGIA)
at UC Santa Barbara, using an arbitrary (not geo-referenced) scan of
These data locate 578 cholera deaths and the 13 public wells in an
coordinate system. I edited these plain text files so they are
directly importable to Arc: deaths.txt
I edited a
high-resolution JPEG-format scan of Snow's map,
correcting some broken lines and
converting it to a large PNG image:
Click and download the full-size image of this map along with the
pumps and deaths
datafiles, add them into a blank Arc map session.
Use File--Add Data--Add XY Data to display the pumps and deaths as
points on your map.
Georeference the map image to the well points, and
save a rectified version of the map image.
Use Arc's Euclidean Allocation tool to
define zones of cells (Theissen polygons) closest to each
pump. What percent of cholera deaths fall within the Broad Street
well's Theissen polygon?
Use Arc's Kernel Density tool to calculate the spatial densities
of deaths around each of the wells. What are the density
measures at each pump?
The Euclidean allocation implies travel through walls and
buildings rather than only on streets. Use an image editor program
(Paint or GIMP) fill the just streets in the map image with a distinct
(If the color leaks into any blocks, you will have to edit the image
to close line breaks.)
Add this revised image to your map.
Use the Raster Calculator on this image (or one of its
bands) to create a raster in which street cells have a
value of 1 (a low travel cost), and "not street" cells
have a high travel cost like 100.
Then use the Cost Allocation tool to identify the area
served by each well according to shortest travel cost
distance (via streets).
What percent of the cholera deaths occurred in the Broad
Street well's shortest-travel cost area?
The very first cholera death was an infant in the house nearest
the Broad Street pump; the house's cesspool actually leaked
directly into the well.
But once the epidemic got going, there may have been
secondary sources of infection.
Some of the subsequent deaths were not directly
traceable to the Broad Street pump.
Unfortunately we don't know the dates of the deaths, just their
locations, so we need some forensic analysis of the spatial
clustering of deaths to infer the presence or absence of
secondary infection points.
Assuming the population density was uniform
across the map, use Arc's Spatial Statistics tools to
see if you can identify secondary clusters indicating likely
secondary infection points.
Google Earth image of the area today:
Try georeferencing a large version of this image
to your cholera map.
How much alteration have the streets undergone in 150 years?
The Broad Street well used to
be at what is now the intersection of Broadwick and Lexington Streets.
The building at that corner is now a pub named The John Snow.
back to Intro