By plotting cholera deaths street by street across London, Snow traced the outbreak to a single contaminated water pump, laying early groundwork for modern epidemiology.
In late August 1854, a cholera outbreak in London’s Soho district killed more than 500 people within ten days. The dominant medical theory of the time blamed "miasma," or bad air, rather than anything in the water supply.
Physician John Snow took a different approach: he plotted each death directly onto a street map of the neighbourhood, marking a bar at every address where someone had died. The marks formed a dense cluster around a single water pump on Broad Street, while households served by other pumps were largely unaffected — including, notably, a nearby workhouse and brewery with their own independent water sources.
Snow presented the map to local officials and persuaded them to remove the Broad Street pump handle. The outbreak subsided soon after. The map became a founding example of spatial epidemiology — using geography itself as evidence — and helped shift medical opinion toward the germ theory of disease decades before germs could be directly observed.
It is, in effect, an early geospatial analytics project: location data turned into a decisive, actionable finding. That same discipline — finding the pattern hiding in where things happen, not just what happens — underpins Vis Lab’s geospatial and environmental monitoring work.
Featured in Vis Lab's overview of landmark visualizations that shaped the field.
