Thematic Maps

Mapping Disease (1 & 2)

Thematic Maps –

“Clothing the Dry Bones of Statistics in Flesh and Blood”

Thematic maps – maps that reach beyond a simple pictorial representation of geography to incorporate statistical data – are a relatively new phenomena and an important intellectual leap. While the history of mapping can be traced back 8,000 years, perhaps as long as 15,000 years [1], thematic maps likely did not evolve until the mid 17th century. In his book Visual Explanations (1997), Edward Tufte explains that:

Despite their quantifying scales and grids, maps resemble miniature pictorial representations of the physical world. To depict relations between any measured quantities, however, requires replacing the map’s natural spatial scales with abstract scales of measurement not based on the geographic analogy. To go from maps of existing scenery to graphs of newly measured and collated data was an enormous conceptual step.

Dr. Edmund Halley, the English astronomer famous for his work on the orbits of comets, is credited with what is perhaps the first thematic map (see figure). In a 1686 issue of Philosophical Transactions, Halley used small dashed lines in the shape of narrow tear drops on a world map to represent the location and direction of trade winds (Halley 1686).

Following on the heels of Dr. Halley’s innovations, the 18th century gave rise to some of the first maps of geology and medicine and the development of contours and isolines. But it was the 19th century that witnessed an explosion in proficiency and creativity related to thematic maps (Friendly and Denis 2003). In particular, it was the period 1835-1855, that one historian called a “golden age” in thematic cartography (Robinson 1955).

For example, in 1837, one series of maps constructed to accompany a report by the Irish Railway Commissioners, was the first to use several important cartographic techniques. Henry Drury Harness, a Lieutenant in the Royal Engineers and creator of the railway maps, was the first to use graduated circles for city population, shading to represent population density and flow lines to show movement patterns (line thickness represented number of travelers) (Robinson 1955).

The portrayal of data in such a fashion – on maps filled with detail and color – was still a novelty, however, even by the late 19th century. After attending the Paris Geographical Congress, E.G. Ravenstein, a geographer and fellow of the Royal Geographical Society, could barely contain his enthusiasm for the “new” thematic map, what he refers to as a “statistical map,” technique. In his report back to the Statistical Society of London he explained:

A statistical map is intended to present graphically the numerical results obtained by statistical inquiry, and you will agree with me that, as respects the geographical or topographical distribution of phenomena, such a map, judiciously elaborated, conveys far more accurate notions than are to be gathered from tabular statements. No method for recording almost every kind of statistical information can compare with it… Mr. Walker’s statistical atlas of the United States… is one of the most valuable publications of its kind; and if you spend an hour in its examination, you will admit that during that hour you will have gained more knowledge about the natural resources of the United States and their geographical distribution than it would be possible to obtain by studying a number of volumes. It is, I may say, the concentration or essence of statistical information which is laid down in these maps (Ravenstein 1875).

And the authors of the 1883 Scribner’s Statistical Atlas of the United States, one of the first of its kind, were similarly impressed by the visualization techniques. They explain:

The study of statistics, with the means ordinarily at hand, is a dry and difficult one. It should not, therefore, be a matter of surprise that comparatively little information regarding the condition and resources of a country is to be found among its people. Such information is, for the most part, hidden away in long and forbidding columns of figures, and dispersed through many volumes more or less inaccessible....Let these facts be expressed not alone in figures, but graphically, by means of maps and diagrams, appealing to a quick sense of form and color and “clothing the dry bones of statistics in flesh and blood,” and their study becomes a delight rather than a task. The density of settlement, the illiteracy of the people, the wealth or poverty of different sections, and many other features of great importance, hitherto but vaguely comprehended, are made to appear at a glance, and are so vividly impressed as not to be easily forgotten. By such aids not only the statistician and political economist, but the masses of the people, who make public sentiment and shape public policy, may acquire that knowledge of the country and its resources which is essential to intelligent and successful government (Hewes, Fletche and Gannett 1883).

[1] Peter Gould, in his book The Geographer at Work, argues that the world’s first map was carved on mammoth bone 15,000 years ago on the river Dneper in what is today the Ukraine (Gould 1985).

© 2006 ZevRoss Spatial Analysis