Wednesday, July 10, 2024

John Snow's Proximity Mapping

I've probably seen over 100 modern interactive visualizations of John Snow's famous map of cholera victims during the 1854 cholera outbreak in Soho. John Snow's map shows the locations of cholera deaths clustered around a water pump in Broad Street. The map helped to disprove the prevailing miasma theory on the spread of diseases and establish that cholera was actually spread by contaminated water.

Despite the central importance of proximity to John Snow's interpretation of the Soho cholera outbreak I think that Carto's new 1854 Soho cholera outbreak map is the first of the 100+ reinterpretations of the data to actually visualize which water pump the victims lived closest to. 

On the Carto map straight lines are drawn from John Snow's recorded victim's homes to the nearest pump. The purple lines indicate all those victims which lived closest to the Broad Street Pump. As you can see a number of victims actually lived closer to other pumps. John Snow wrote in a letter to the 'Medical Times and Gazette', "In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street, as they preferred the water to that of the pumps which were nearer. In three other cases, the deceased were children who went to school near the pump in Broad Street."

John Snow's original Broadstreet Map showing cholera cases clustered around the Broad Street water pump

You can view an interactive version of John Snow's original map on the Wellcome website. The map, was originally published in 1855 in Dr. John Snow's 'Report on the cholera outbreak in the Parish of St. James, Westminster, during the autumn of 1854'. 

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