A Brilliant Question and Two Map Layers

Maps and data sets that have made longstanding impacts on society are often the most simple in design. In 1854 a massive cholera epidemic swept through the Soho district in London England, claiming the lives of 616 residents and bringing the area’s mortality rate to a staggering 12.8 percent. At the time, physicians and scientists subscribed to the theory that the cholera’s rapid spread was attributed to noxious or “bad air,” however, with a brilliant theory, two data sets, and a map, John Snow refuted the miasma theory and created what is now modern day epidemiology.

Snow’s approach to identifying the source of the Soho cholera outbreak is one that will always resonate. Prior to collecting vast amounts of data or creating maps, he started with a single hypothesis that could be validated with the ongoing deaths occurring in the district. John Snow believed that cholera was transmitted through ingestion not inhalation and it was this ember of an idea that sparked effective data collection and later a map that would lead him to the root cause of the epidemic. John Snow’s hypothesis was tested by comparing data on death count locations to proximity of the district’s water pumps. Death count sharply spiked in residences closest to the broad street pump. Efforts to connect the incidence of cholera with potential geographic sources centered on creating what is now known as a Voronoi diagram.

He mapped out the locations of individual water pumps and generated cells which represented all the points on his map which were closest to each pump. The section of Snow’s map representing areas in the city where the closest available source of water was the Broad Street pump circumscribed most cases of cholera. In the end the Soho restricted access to the pump by removing the handle and the cholera deaths dramatically decreased. It was later found that this public well had been dug only 3 feet (0.9 m) from an old cesspit that had begun to leak fecal bacteria. A baby who had contracted cholera from another source had its nappies (diapers) washed into this cesspit, the opening of which was under a nearby house that had been rebuilt farther away after a fire had destroyed the previous structure, and the street was widened by the city.

Solving the cholera epidemic is an example that underscores the true value in creating analysis and visualizations that are clear, well thought out, and grounded in the issue at hand. Taking a step back, analysts should always be able to state the problem they are solving in a few sentences. Snow leveraged basic statistics, had no access to modern day mapping software, and yet used his data visualization to give birth to an entirely new scientific field. In an era where analysts are constantly keeping up with the latest feature list and software release it’s critical that we remember why these tools have been developed in the first place. Combining good questions with a clear analytical approach will lead to groundbreaking visualizations but more importantly new information that moves our environment and society forward.


Originally published at chesapeakecommons.org on October 26, 2015.