Map a historic cholera outbreak
In 1854, London was in the middle of a cholera pandemic. The Broad Street outbreak in the Soho district was particularly severe, killing hundreds of people. At the time, most people believed that cholera was spread through the air. You'll explore the data collected by John Snow with pop-ups and heat map symbology to see if you can prove otherwise.
Explore cholera data
GIS allows you to compare geographic data as layers. You'll start by exploring layers related to the 1854 cholera outbreak in ArcGIS Online.
- Open the Cholera Outbreak, 1854 map.
- At the top of the page, Click Sign In and sign in to your ArcGIS account.
- In the side pane, click Content to view the layers in the map.
Light Gray Canvas is the basemap layer, providing context for the other data. The only other layer that is turned on and visible is Snow's Cholera Map. This is the map John Snow drew and published to document the data he collected during the outbreak. Each hash mark represents a death that occurred at that address due to cholera.
- Explore the map to see where cholera was most prevalent in the Soho district.
- In the Contents pane, check the box for the Cholera Cases layer to turn it on.
These are the same addresses that Snow mapped.
- Click one of the points on the map.
A pop-up appears, telling you the address and the number of deaths that occurred there. A GIS can store much more information than you can draw or label on a map. You can access this information in pop-ups or in the layer's attribute table.
- In the Contents pane, point to Cholera Cases. Click the Show Table button that appears beneath the layer name.
An attribute table appears, listing the number of deaths that occurred at each address.
- Close the attribute table.
- Above the map, click Save and click Save As.
- For Title, replace the word Copy with your name or initials. Click Save Map.
Use heat map symbology
The attribute table provides all of the geographic information that you need. However, it is much easier to detect patterns when the data is viewed on a map. Next, you'll draw the data in a different way to make patterns even clearer.
- In the Contents pane, point to Cholera Cases and click the Change Style button.
- For Choose an attribute to show, choose Deaths.
The map updates to show circles of different sizes. Instead of showing only the locations of addresses, the map now shows how many deaths occurred at each one: larger circles indicate addresses with more cases of cholera. This symbology is closer to the original map with the hash marks.
- For Select a drawing style, choose Heat Map. Click Done.
Heat map symbology is helpful for quickly visualizing the density of a dataset over space. Purple and red colors represent areas where there was a greater density of cholera cases. Yellow represents the densest concentration. The density takes into account how close locations are to one another, and the number of deaths at each location.
- In the Contents pane, turn on the Water Pumps layer.
There is a water pump within the yellow zone of the heat map.
- Click the pump symbol to view its pop-up. Close the pop-up
This is the Broad Street Pump, made famous by Snow's discovery. (Broad Street has since been renamed Broadwick Street.) Your map may lead you to the same conclusion that Snow drew: that the Broad Street pump should be investigated as a possible source of the Soho outbreak.
- Close the pop-up and save the map.
Thanks to Snow's investigation, the handle of the water pump was removed. When people were no longer able to access the contaminated pump, the cholera outbreak in Soho came to an end. Lives were saved and cholera was confirmed as a waterborne disease. Snow did further work to improve water and waste services in London. His research helped to improve sanitation and public health around the world.
Create a story map
John Snow's spatial analysis was able to help people because he shared his findings. You'll complete your GIS project by sharing it in a story map. ArcGIS StoryMaps allow you to combine maps, text, images, and other media to tell a geographic story.
Share the map
Before you can build a story map, you need to share your map to make it public.
- Click the About button and click More Details.
The overview page for your map appears. This is where you can manage the map and add metadata.
- Click Share.
- Click Everyone (public) and click Save.
- Click Create Web App and choose StoryMaps.
The StoryMaps builder appears with placeholder text.
Configure the story map
To finish, you'll make a few changes to the story map to best present your analysis.
- For Title your story, type London Cholera Outbreak, 1854.
- For a subtitle, type Heat map of the cholera outbreak in the Soho district of London in 1854.
- Scroll down.
Your map is already embedded into the story map. However, it may not be zoomed to the best scale.
- Hover over the top of the map to expose a toolbar. Click the Edit button.
The Adjust web map appearance window appears.
- Zoom in on the map to see the data in more detail. Click Place map.
The current story map design is better suited for a longer story with multiple maps. Next, you'll change the layout.
- On the top toolbar, click Design.
- In the Design pane, under Cover, click the Minimal button.
- Close the Design pane.
- Point to the area beneath the map. Click the Add content block button and choose Paragraph.
- Write a description of your map to help other people better understand it. You can copy and paste this example:
In 1854, a severe cholera outbreak struck the Soho district of London. At the time, most people believed that cholera was spread through the air. Dr. John Snow's map was able to spatially associate cholera cases with a single contaminated water pump. This led to three positive changes: the water pump was disabled, preventing further deaths, cholera was identified as a waterborne disease, and efforts began to improve water and waste systems in London.
Shown here is the map created by Snow in 1854, as well as the same data represented with heat map symbology. The yellow part of the map represents the highest density of cholera cases and coincides with the Broad Street water pump.
- Click Publish and choose Everyone (Public).
- Click Publish story.
In this lesson, you reproduced the analysis done by Dr. John Snow to find the source of a cholera outbreak. You explored the relationships between different layers of data using attribute tables, pop-ups, and symbology. You also presented your findings in a story map.
In this example, the datasets were small, and the relationship between them was relatively easy to see. Often, geographic data is larger and more complex. Having a system for organizing, analyzing, and sharing geographic information (a GIS) quickly becomes necessary to answer spatial questions and solve problems.