ANINTRODUCTIONTOGIS


The Cholera Map


The Cholera Map by Dr. John Snow is one of the first known works of cartographic analysis. Snow’s map helped stop the 1854 cholera epidemic in the district of Soho (London) by showing that the disease was spread by water rather than by air (which was the prevailing opinion at the time).

In this tutorial, we will repeat part of Snow’s analysis, trying to locate the water pump responsible for the outbreak.

To do so, we will:

  1. create a shapefile with the location of the water pumps;
  2. compare the number of deaths around each pump.


I. Create a shapefile with the location of water pumps

On the map, start by projecting the raster cholera_map.tif, which is available in the folder with the same name.

As Coordinates Reference System (CRS), we will use the British National Grid (EPSG:27700).

We will create a new shapefile to locate the pumps. To do so:

  • Layer > Create Layer > New Shapefile Layer
  • Fill the new window as follows:

Figure 1 Create a new point shapefile layer


Now look for the black dots representing the pumps on the map. Once a pump has been located, we will add it to the new shapefile using the following procedure:

  • Activate the editing mode (right click on the vector name then click on Toggle Editing)
  • In the tops menu: Edit > Add Point Feature
  • Click on the pump and then fill the table of attributes (by giving the name of the street to the pump)
  • Once finished, do not forget to exit the editing mode and save the changes

Figure 2 Point vector and attribute table


Practise: Repeat the procedure to identify the 13 water pumps in the neighborhood


II. Identify the origin of the outbreak

To identify the origin of the outbreak, we will compare the number of deaths around each pump. On the original map, you can see that John Snow had identified each death from the disease with a black line in the corresponding house. For your convenience, this data has already been digitized into a vector, Cholera_Deaths.shp, that summarizes the number of deaths per house (note that this is a slightly different representation than that chosen by John Snow).

Practise: To facilitate the visualization of the information, create a map with the size of the points varying according to the number of deaths recorded


To analyze the number of deaths per pump in more detail, we will start by creating a positive buffer of 100 meters around each pump (Vector > Geoprocessing Tools > Buffer).

Figure 3 Create a buffer around each pump


To link each death to a geographical area around a pump, we will then make an intersection between the layer listing the number of deaths and the newly created buffers (Vector > Geoprocessing Tools > Intersection).

Figure 4 Intersection between the number of deaths and the buffers around the water pumps


Unsing GroupStats extension (once loaded in the Plugins menu, you may find it at in the Vector menu), we may then calculate the number of deaths per pump.W can now determine the pump responsible for the epidemic!


Figure 5 How to calculate the number of deaths per pump


Practise: What improvements could be made to the procedure performed in this tutorial?