“Thematic map” and “thematic cartography” are phrases that get tossed around a bit casually. To put things drily, though, a thematic map uses design elements to depict the distribution of one or more phenomena across a geographic area.  Despite the dry definition, though, thematic maps are one of the most interesting emerging areas of map collecting.

Thematic mapping dates back at least to the late 18th century, as with this early economic map of Europe. Arguably it goes back even further, to the earliest sea charts that showed phenomena such as depth soundings.  But the field really took off in the 19th century, when the mutually-reinforcing requirements of growing government bureaucracies and science–including the social sciences–stimulated both demand for data and the development of means for gathering, collating and depicting it.

Pretty much anything can be the subject of a thematic map: Here at Boston Rare Maps, for example, we have handled thematic maps of weather in the Indian Ocean, the spread of cholera, the Antebellum cotton trade, and camps of the Soviet Gulag. Despite the obvious differences, what all have in common is that they depict some phenomenon that can be both located and counted.

Moving from the relatively straightforward depiction of place names, topographical features, roads and so on to the presentation of thematic data can present the mapmaker with all sorts of graphic design challenges.  The best thematic maps combine intellectual clarity, visual efficiency and aesthetic appeal to create compelling, even unforgettable images such as this map using simple geometric figures to encodes data for the color, range, frequency and sweep of dozens of British lighthouses.