John Snow’s Maps of the Broad Street Cholera Outbreak

In September 1854, central London suffered an outbreak of cholera.¹ To stop that outbreak, Dr. John Snow made a map. By seeing, visually, where the cholera deaths were clustered, Snow showed that the water from a pump on Broad Street was to blame. His work addressed an ongoing medical debate — in what is widely regarded as one of the most important early examples of epidemiology, he clearly linked cholera’s spread to water instead of air.

But Snow’s map is not just famous in epidemiological circles. To us infographic geeks, it is also one of the most important early examples of data visualization.

In his 1855 text On the Mode of Communication of Cholera, Snow included a rendition of his Broad Street map (though it seems not to have scanned in the Google books version). The deaths at each address are indicated by the little horizontal lines, stacked up from the street like a pile of little corpses.

Snow has made the map big enough to include the locations of the other water pumps in the vicinity, clearly showing a drop-off in cholera deaths where houses are nearer to a different pump.

Visualizing his data allowed Snow to investigate abnormalities in the outbreak. He learned, for example, that the brewery and the workhouse, both near the Broad Street pump, had their own independent water sources and so escaped the outbreak largely unscathed.

Edward Tufte, a godfather of information display, analyzes this map in his 1997 book, Visual Explanations. Tufte points out that as a dot map,² Snow’s visualization is implicitly assuming that the population in the area is uniformly distributed. In other words, a house without any deaths means that the inhabitants of that house successfully avoided cholera, not that the house sits empty. But even with that shortcoming, Tufte praises the map’s usefulness.

What I find particularly striking about this example is that Snow is clearly using his map to test a clear hypothesis: cholera is a water-borne disease that is originating at the Broad Street pump. The map allowed him to focus his investigative efforts and reach his conclusion — that his hypothesis was correct — quickly.³ The visualization was not an end unto itself, but just one tool in Snow’s science.

Notes.

¹Cholera is a bacterial infestation of the small intestine that causes its victims to suffer from such severe gastrointestinal distress that they can die in two hours. It’s fatal in half of untreated cases. The bacteria infect the victim through water or food that’s been tainted with the Vibrio cholerae bacterium, usually through the feces of someone else with cholera. Read the World Health Organization cholera fact sheet for more.

²Dot maps are representations in which each data point is individually placed on a map. They’re useful when there’s enough data to show a trend at a glance, but not if there’s so much data that the map is too crowded to be legible. They’re problematic in epidemiology because just showing the cases themselves, without also showing the underlying population distribution, can be misleading. Consider the alligators example from UC Santa Barbara.

³For more on Snow’s work, explore a thorough, if très 1998, web site at the UCLA epidemiology center.