THE HISTORY OF CHOROPLETH MAPPING

J.E. Mersey

University of Guelph

jmersey@uoguelph.ca

Since their initial appearance in the early 19^th century, choropleth maps have become the most widely used form of thematic map. This report traces the history of this ubiquitous method focusing on the societal influences, technical constraints and opportunities, and cartographic paradigms that have shaped its adoption and design. The surge of thematic mapping activity around 1840-1860 was influenced by innovations in both the social sciences and printing technology, as well as an increased government concern with economic, demographic, and health conditions. It was then that principal European countries began to conduct their first national censuses, providing a wealth of statistical information related to defined administrative areas on the ground. Once basic ideas concerning data processing and classification were worked out, choropleth mapping continued essentially unchanged up to the present day, although the term “choropleth” to describe these area symbol maps was not adopted until 1938.

Choropleth maps were one of the first types of thematic maps to be automated around 1960-1970. Not only were new production technologies emerging at this time, but a paradigm shift was occurring within the scholarly discipline. The new research paradigm envisioned mapping not as an end in itself with the map as a static end-product, but as part of a dynamic communication process whereby spatial information was transmitted from the map maker to the map user via the map. With the communication model came the focus on the map user, and interest shifted toward understanding the tasks for which choropleth maps were used, and how to effectively design maps to facilitate these tasks. Research in the last several decades of the 20th century delved into every aspect of the use and design of choropleth mapping.

By 2000 choropleth mapping could be readily automated with excellent results. In fact, with vector-based geographic information systems (GIS) which store attribute data in a spreadsheet format keyed to map areas, choropleth mapping became more pervasive than ever. By this time the communication model evolved into a paradigm of geographic visualization which recognized the important role of mapping in interactive data exploration. GIS facilitated this direct interaction between the user and the spatial information which was increasingly available in digital format from government agencies. The incredible rise in popularity of the Internet at the turn of the century led to the creation of Web-based tools for creating choropleth maps.