John Snow: The Myth of the Map

John Snow destroys the Pump handle

I have admired Jon Snow’s map of the 1854 Broad Street Cholera Epidemic since the first time I saw it, flicked up on a presentation about the power of GIS and described as the beginnings of spatial data techniques:

“John Snow mapped the Cholera deaths, and used the map as an argument to have the Handle removed, so stopping the epidemic. He is the father of modern GIS spatial analysis!”

It’s an oft repeated tale that resonates with us , a scientist as hero, the mapmaker as saviour.

With the recent bicentenary, Jon Snow’s map of cholera has been celebrated widely anew, with new exhibitions and even a GIS data package from a Southampton University postgraduate researcher, displayed to great effect by the Guardian.

It’s a great advert for GIS, or Spatial IT.  So much so that some spatial professionals groan when it pops up in a presentation, even playing ‘Snow Map bingo’ over Twitter.

The trouble is that this is all a myth.

Snow apparently drew the map months after the outbreak. I’d seen this stated as fact before, but in the wealth of publicity telling me otherwise decided to re-investigate.

I decided to actually read John Snows book, On the Mode of Communication of Cholera. (Available as an ebook from Google)

I began to seriously doubt he would have had the time to produce his famous spot map from becoming involved to getting the pump closed.

He becomes involved on September the 3rd,  after the horror of 70 depths on the 1st  and 127 by the 2nd.  The local inhabitants are fleeing the scene, Snow arrives and examines the water from the local Pumps, already suspecting contamination of the Broad Street pump.

He examines the Broad Street Pump water and having found:

“so little impurity in it of an organic nature, I hesitated to come to a conclusion” 

He appears to discout the pump as the root cause.Then on the 5th, he re-examines water collected over 2 days from the Broad Street Pump and discovers:

“an amount of organic impurity, visible to the naked eye”

By the 6th he requests permission to gather a list of the deaths of Cholera from the week ending 2nd September, tabulates this to conclude the outbreak began on the Thursday and then concentrates on gathering as much information as he can for the 83 deaths registered for the last 3 days of the week

He does this by physically returning to the Broad Street pump, and interviewing people :

“On proceeding to the spot, I found that nearly all the deaths had taken place within a short distance of the pump”

By the evening of the 7th, he has had an interview with the board of Guardians of St, James parish.

“ In consequence of what I said, the handle of the pump was removed on the following day.”

He never mentions mapping the locations at this stage, In fact, it’s rather more a frantic dash around the Streets of London, interviewing surviving relatives  shopkeepers, cafe owners etc, all based on his textual research. He’s aware of the spatial link, but he doesn’t need a physical map to display it.

In fact he  gathers information that would have been unobtainable from just looking at the locations of deaths on a map, finding spatial links in the day to day narratives of people at work, drinking from infected water supplies and returning home to die, or having water brought from Broad Street to their new homes miles away.

John Snow is no less the hero, he just used different tools to get the pump closed.

Snow first presented a Cholera Spot Map at a meeting of the Epidemiological Society of London on Dec 4, 1854.  Probably because, then, as now, it immediately leads the viewer to decide the pump must have been responsible.

Not convinced?  Still want to believe?

Then check out this 2000 Lancet article, Map-Making and Myth-making in Broad Street: The London Cholera Epidemic, 1854

The authors detail the evidence, including maps that actually predate Snow’s, by Edmund Cooper, an engineer for the Metropolitan Commission of Sewers, drafted to counter claims that the sewers were to blame for the out break.

More, the authors’ warn of the possibility of GIS to mislead and seduce, stating:

“If the methodological assumptions underlying these uses of GIS are correct, Edmund Cooper and not John Snow would have been credited with unravelling the Broad Street cholera outbreak”

Furthermore, a cartographer’s study of the  modern reworkings of John Snow’s map, Tom Koch’s 2004 paper provides sobering reading:

“…dangers occur when professional cartographers and geographers attempt to map data from fields in which they are ignorant”

The following words from his conclusion, echo my view.  That Maps aren’t a depiction of reality, they are powerful memes that deliver their desired message to viewers.

“…map-making is mired in the myths and assumptions of the individuals who promote this or that map within the culture(s) the map-makers serve.”

Perhaps now is the time to finally stop promoting the myth of John Snow’s map within Spatial IT.

 


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