Wednesday, August 14, 2019

John Snow's Cholera Map in 3D

John Snow's map of cholera victims during the 1854 cholera outbreak in London is one of the most famous examples of effective data visualization. By plotting the homes of cholera victims on a map Snow was able to identify a water pump in Broad Street as the cause of all the local cases of cholera. Snow's map essentially proved that cholera was spread by contaminated water and disproved the prevailing miasma theory, which believed that diseases like cholera were transmitted by bad air.

Creating an interactive version of John Snow's original map has become something of a rite of passage in the field of cartography. However it is rare for any of these attempts to bring anything essentially new to John Snow's visualization. John Snow Cholera Map 2D-3D does offer something different. This map allows you to switch between a digitized version of John Snow's original map and a 3D version of the map. On this 3D view Snow's black dots (indicating where people have died from cholera) become vertical stacks. These vertical stacks provide a clearer picture of the exact location of the households that experienced multiple deaths.

Cholera & Elevation

In developing his theory that cholera was transmitted by water rather than air Snow was able to use the detailed statistics of William Farr. In 1838 Farr, a qualified doctor, was appointed to the General Register Office. This was the government department responsible for recording births, deaths and marriages. In his role at the General Register Office Farr was able to introduce a system which recorded causes of death. This data could then be used to look for geographical, environmental and occupational patterns in death rates and different diseases.

It was partly Snow's use of these death rate statistics which led him to believe that cholera was caused by germs which were transmitted by water. William Farr was impressed with Snow's germ theory of cholera being transmitted by water. However Farr himself believed that cholera was more commonly transmitted by air (the miasma theory). He even developed his own theory based on the idea that deadly miasmata are greater at lower than higher elevations. In his 'Report on the mortality of cholera in England 1848-49' Farr's detailed analysis of the distribution of cholera deaths in London actually established an apparent link between the rate of cholera deaths and elevation.

In this map from the report the red numbers 'denote the elevation in feet above the Trinity Highwater Mark' (image from the Wellcome Collection). Farr believed that the link between elevation and cholera was further evidence for the miasma theory. In 1854 Farr was a member of the Scientific Committee for Scientific Enquiries in Relation to the Cholera Epidemic of 1854. A committee which rejected John Snow's Broad Street pump analysis. The report concluded that "on the whole of evidence, it seems impossible to doubt that the influences, which determine in mass the geographical distribution of cholera in London, belong less to the water than to the air."

William Farr however was finally persuaded of Snow's germ theory of cholera and its waterborne transmission. In 1866 Farr himself wrote a report, which included detailed analysis of death statistics, to show that water and not air transmission was the most important cause of cholera.

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