What thematic maps do is to give the viewer a unique way to look at quantitative information, which can provide insight not obtainable directly from the raw data itself. It is the nexus of the data and the geographic base which presents new patterns, which in turn can lead to a new understanding of the data and, in some case, important discoveries.




Thematic maps use various techniques, such as the marks used by Snow, or applying different colors to represent different densities of the theme in question (choropleth maps), or the use of proportional symbols on a map, or isarithmic or contour maps, where a fixed data point of, say, precipitation or temperature, is mapped in a continuous line on the map (reminds me of what a map of Alcatraz by an ex-prisoner is called, a Con Tour map…).
While few thematic maps have had quite the impact of Dr. Snow’s, there have been many instances of important and fascinating thematic maps over the years. A nice book on the topic is Arthur H. Robinson’s 1982 Early Thematic Mapping in the History of Cartography.
The subject of this blog was inspired by a fascinating talk on thematic maps by Dr. Susan Schulten of the University of Denver. Her talk, “Disease Mapping and the Advent of Meteorology in America,” was presented as part of the Arts in Medicine program at the University of Colorado medical school. Susan looked at how the attempt to understand the nineteenth century epidemics of cholera and yellow fever involved the use of maps (the Snow map, obviously, featuring prominently), and how the success of thematic mapping in this arena extended its use to other areas of science, such as meteorology.
Susan (another example of the terrific network of map scholars who are members of the Rocky Mountain Map Society) is nearing completion of a book on thematic mapping. This is what she says about her project.... “The book I'm writing is a history of thematic mapping in the nineteenth century, looking at maps of the present but also the rising interest in old maps and maps of the past at that time. I'm closing in on a draft of the entire manuscript…. It will come out with Chicago, and will be accompanied by a website where high resolution versions of the maps will be available for close inspection.” I am looking forward to its publication and I’ll make sure to announce it in this blog when it makes its appearance.
Halley's map is also one of the very first uses of contours, or isolines, on a map. It's interesting that his use of it to map something invisible, magnetic declination, preceded its acceptance for depicting terrain by at least 150 years!
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