Revisiting John Snow's map: network-based spatial demarcation of cholera area

Abstract

Dr. John Snow's cholera map is known as one of the pioneering examples of an epidemiology map, illustrating the spatial distribution of the victims from the cholera outbreak. This article revisits his map and expands on his attempt at visualizing the distribution of the victims by focusing on spatial demarcation using the sphere of influence along the street network by applying two analytical methods that are designed for analysis of network space. First, the article generates a network-based Voronoi diagram of the water pumps in the map that encompasses Snow's original version of the equidistance line that was drawn around a single pump. The article then presents a new, revised equidistance line that better reflects the circumstances around the time of the outbreak. This is followed by the construction of another set of boundaries derived by the application of a network-based clumping method. Comparing the demarcation lines produced with the two methods shows a sphere of influence which was unclear when using the network Voronoi diagram alone. Results from the analysis using the clumping method also confirm some of Snow's observations on the spatial distribution of the victims.

Keywords:

• John Snow
• network
• spatial demarcation
• clumping
• Voronoi diagram

Acknowledgments

The author greatly appreciates valuable comments from Professor Robert Haining, Dr. Narushige Shiode, Professor Atsuyuki Okabe, Professor Tatsuhiko Kawashima, and the anonymous reviewers for this article. This research was funded in part by the International House of Japan.