Map-making and myth-making in Broad Street: the London cholera epidemic, 1854

Brody, H., Rip, M. R., Vinten-Johansen, P., Paneth, N., & Rachman, S. (2000). Map-making and myth-making in Broad Street: the London cholera epidemic, 1854. Lancet (London, England), 356(9223), 64–68. 

(http://www.ph.ucla.edu/epi/snow/mapmyth/mapmyth_fig1.html)

Image from eagereyes.org

In a few select circles, John Snow is a well-known name.

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Image from Historyday.coldray.com

No, not that one.  This John Snow (right-most image).  The man who is often referred to as one of the fathers of modern epidemiology.  The work he did mapping and charting the outbreak of cholera in London during the summer of 1854 is commonly studied as the beginning of the use of geographical tools and mapmaking as a method of understanding public health concerns.  Having previously observed the effects of cholera, John Snow had developed a hypothesis as to the method in which cholera was spread through large geographic areas.  When a cholera outbreak caused more than 500 deaths in a 10-day period within a few blocks of Broad Street, an area named Golden Square in London, Snow saw it as an ideal opportunity to test his hypothesis that cholera was spread through contaminated water sources.

Dr. Snow proceeded to tabulate all of the individuals who had died of cholera within the 10-day period and marked them on a "spot map" according to where they resided.  The geographical representation of the deaths by proximity to the Broad Street water pump proved that there was in fact a major correlation between an individual's consumption of the pump's water and their illness and consequent death.  Snow, while publishing his findings, however, made clear that the spot map did not lead to his hypothesis, but rather that the map was simply a logical method of visually presenting data.

Snow's map of cholera deaths in the Broad Street area, Dec 1854
Image from Brody H et al. The Lancet 356(9223), 64-68, 2000.

Earlier that same year, another man by the name of Edmund Cooper had produced a spot map representing the same cholera outbreak.  In response to rumors floating around that "gully holes", or storm drains in American vocabulary, and sewers were to blame for the cholera outbreak, Cooper had been commissioned by the Metropolitan Commission of Sewers to draw a spot map disproving the association between cholera fatalities and proximity to storm drains.

Edmund Cooper's map for the Metropolitan Commission of Sewers, Sept 1854
Image from Brody H et al. The Lancet 356(9223), 64-68, 2000.

Along with John Snow and Edmund Cooper, the Committee of Scientific Inquiries of the General Board of Health also produced a spot map of the outbreak.  This map was equally detailed as Cooper's, but the board was hesitant to accept Snow's theory and instead was inclined to believe that cholera was spread atmospherically.  They drew a circle around the entire affected area instead of the area within walking distance of the pump, as in the map Snow had produced.

Board of Health ("government") map, from General Board of Health, 1854
Image from Brody H et al. The Lancet 356(9223), 64-68, 2000. 

Ultimately, John Snow was just as convinced of his hypothesis on the spread of cholera and disease as both Edmund Cooper and the Board of Health were of theirs.  However, Snow was unique in his method of using his spot map as a way of backing up an already well-researched hypothesis instead of using it to entirely inform his hypothesis.  While using GIS technology, we must consider that the risk of stumbling upon associative data is a real one.  We must use scientific methodology in conjunction with geographical representation.  "Associative data, plotted in the form of a highly sophisticated and accurate map, may easily seduce us into concluding that we have learned something", be it true or not (Brody).  If this were not the case, we would be publishing Edmund Cooper's methodology and maps into textbooks instead of Snow's.