One of my first recollections of being interested in maps and mapping was at school when I watched a TV series about Victorian London. The series was concerned with the social conditions of London and one programme was about the frequent cholera epidemics of the mid-nineteenth century. It was this programme that grabbed my attention since it told a very interesting story of how the link was made between cholera and dirty drinking water by the use of a map. At the time, it was generally accepted that cholera was caused by miasmas - noxious gases and smells emanating from London's sewers and spoil heaps. However, during a major cholera epidemic in 1854, the programme described how Dr John Snow, an eminent London physician, had isolated the real source of the epidemic. He did this by drawing a map of deaths from cholera that had occurred in a small number of streets in the vicinity of Golden Square in Soho and saw that they centred upon a particular water pump in Broad Street (Figure 11.1). When the handle was removed from the pump, deaths from cholera abated. This was regarded by Dr Snow as proof that cholera was being spread, not by air-borne gases as was generally accepted, but through contaminated drinking water although it would not be until 1884 before the actual pathogen was discovered (the cholera bacteria Vibrio Cholarae).
Looking back, this story of how Dr Snow used a map to discover the cause of cholera is interesting for two reasons. First, because it provides a powerful illustration of how maps can be used as tools for undertaking research and for knowledge discovery. By plotting deaths from cholera on to a map and revealing the geographical relationship between deaths and the location of the Broad Street pump, Dr Snow had showed how maps could provide a unique insight into the patterns, processes and relationships of spatial phenomena. The relationship between polluted
drinking water and cholera was not self-evident and had to be graphically displayed before the connection could be made. The second reason why this famous story is interesting is because it is not true!
Contrary to popular belief, Dr Snow did not discover that cholera was spread by contaminated drinking water by drawing a map of cholera deaths clustered around the Broad Street pump (Brody et al., 2000). He had hypothesized that cholera was transmitted through dirty drinking water six years earlier, following the south London cholera epidemic of 1848. With the arrival of the 1854 epidemic, Dr Snow had decided to test his controversial hypothesis and was in the process of undertaking a large-scale study of the relationship between cholera deaths and the water supply in south London when Broad Street witnessed its first death. Due to the severity of the outbreak (more than 500 people died in a 10-day period) and also its localized nature, Dr Snow realized that the Soho epidemic would also be a good place to test his polluted water supply hypothesis. By enquiring where the people who had died had obtained their drinking water, he quickly isolated the Broad Street pump as the likely source of the outbreak. With this information and with other anecdotal evidence, he got the handle of the pump removed. So where did his now famous map fit into the story?
Dr Snow actually drew his map of cholera some time after the epidemic had abated and definitely after the handle of the pump had been removed. The map formed part of a report written by Dr Snow on the Soho epidemic that had been commissioned by the officials of St James's parish where Broad Street was located. The map was drawn purely as an illustrative device to show the spatial correlation between the cholera deaths and the water pump. The majority of the report focused upon other evidence that linked the Broad Street pump to the cholera deaths. The map was used simply to add weight to this evidence rather than being the crux of the argument. In actual fact, it has been suggested (Brody et al., 2000) that Dr Snow may have been inspired to use a map to illustrate his report by a map in Shapter's (1849) earlier study on cholera in Exeter, which Dr Snow had cited in his own work. Even after the publication of the map and the report, not everybody was convinced by Dr Snow's claims and his hypothesis remained controversial for some time.
So what does the story of Dr Snow's map tell us, apart from not believing everything we are told? First, it demonstrates the different uses that maps can have. From Dr Snow's point of view, the map was used purely as an illustrative device to present what he already knew about the spread of cholera through drinking water to a wider, lay audience - in this case, the officials on the parish committee. The map was used to communicate information in a visually striking - and at the time innovative - way. However, from the point of view of the programme makers over one hundred years later, the map had taken on a different role. The re-telling of the story has it that Dr Snow had generated his contaminated water hypothesis from examining the cluster of cholera deaths on his map and their relationship to the Broad Street pump. Rather than a communicator of knowledge, Dr Snow's map had become a tool for constructing knowledge in an exploratory and highly inductive way. The main role of the map had been switched from a communicatory to an exploratory device.
The second thing the story tells us is something about the power of maps. Dr Snow must have realized the utility of drawing a map of cholera deaths in Soho to have included it in his report. In fact, evidence suggests that the map was re-drawn several times to include additional information such as a boundary indicating the homes that were closest to the pump in terms of walking distance. The time and effort devoted to redrafting the map by hand implies that Dr Snow must have appreciated how the map could help legitimate his controversial claims on the spread of cholera to a then sceptical audience. Although he had not needed to draw a map to convince himself of the validity of his hypothesis, the evidence that had persuaded him of the link between cholera and dirty water was obviously not enough to convince other people. A visual aid was needed to coerce others into accepting his argument. And this visual aid was so powerful that over one hundred years later it is the map that people remember and not the real chain of events.
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