An alternative to John Snow’s Cholera map

Hi there! I am Defne, a journalist from Deutsche Welle’s traineeship programme. I am spending this month with the Datawrapper team to learn more about creating simple and effective charts.

This week, I am imitating John Snow. And nope. Not that John Snow, dear Game of Thrones fans. I am talking about someone else. He is the man who created one of the most celebrated examples of data visualization at London in 1854. Yet, apparently, his life revolved around death almost as much as that John Snow:

John Snow mapped the locations of the deaths caused by Cholera disease and water pumps in Soho, London. He didn’t know that the pumps were connected to the disease. But his map made it clear: Most of the death cases were clustered around one specific waterpump on Broad street (now Broadwick). This pump was contaminated and therefore deadly!

John Snow MapJohn Snow’s original Cholera outbreak map, found on Wikimedia Commons, a little bit chopped. The black circles show the pumps and the stacked black rectangles show the deaths at each address.

Chart creation

As a journalist, I was intrigued to replicate a map that is considered as one of the most inspirational examples of data journalism. So I wanted to give it a try and map an alternative version with Datawrapper. Thanks to Robin Wilson’s digitization, the Cholera map data is available online.

Thanks to Elana’s and Lisa’s feedback, I first calculated the distances between every death cases and every water pumps on Snow’s map[1]. Now I could figure out which water pump was surrounded with the highest number of deaths. With Elana’s suggestion, I measured the area where the closest 50 deaths happened around each water pump and marked those areas on a locator map. It’s not possible to create GeoJSON circles directly within locator maps and not even with online tools like, so Hans (who built the Datawrapper locator maps) and his patience helped me to navigate through the QGIS software. We used it to calculate the circles with the correct radiuses for each pump and uploaded these circles to Datawrapper as markers. They all cover 50 deaths around each pump. (I decided to make the most important pump the least transparent, to make it stand out more.)

A map as a chart

Then I went one step further. I wanted to show a different view on the data – a chart that disintegrates the information on the map. The chart shows how many people died every 5 meters within 100 meters around each pump. Out of 8 pumps, only 5 are surrounded with Cholera deaths within the reach of 100 meters. And the closer other pumps were to Pump No.1, the more people died within their periphery.

Again, it is so visible: Most people died around Pump No.1.

I hope you enjoyed this new take on some old map. If you have any questions or feedback, you can find me on Twitter at @AltiokDefne or via email ([email protected]). You will hear from me again next week for my second Weekly Chart. See you then!

  1. Here is the sweet little formula to calculate distances between GPS positions on Excel: =ACOS(COS(RADIANS(90-Lat1)) *COS(RADIANS(90-Lat2)) +SIN(RADIANS(90-Lat1)) *SIN(RADIANS(90-Lat2)) *COS(RADIANS(Long1-Long2))) *6371. I found it in this old blog post, but please don’t ask me to explain it.

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