Thematic map

A thematic map shows how a specific topic—like temperature, language, or population—is spread across a geographic area. It uses symbols to make invisible patterns visible. This sets it apart from general reference maps, which mainly show where physical features like rivers, roads, and buildings are located. Thematic mapping is closely related to the field of geovisualization.

History

The idea of the thematic map emerged largely during the Industrial Age, with roots in the Enlightenment. Almost all modern techniques were developed between 1700 and 1850. Before that, cartographers focused on creating accurate general base maps, which were often poor in quality even by the mid-17th century. Still, these base maps were good enough to support the first thematic maps.

One of the earliest known thematic maps was created in 1607 by Jodocus Hondius, showing the spread of major religions using symbols. A similar globe map followed in 1614 by Franciscus Haraeus. In England, astronomer Edmond Halley advanced thematic mapping in the late 1600s, publishing a star chart, a meteorological map of trade winds in 1686, and in 1701 a chart of magnetic variation—possibly the first isarithmic map. Late 18th-century maps also began using chorochromatic (nominal area-class) techniques for subjects like geology and language.

The early to mid-19th century is often called a "golden age" of thematic mapping. Many current techniques were invented or refined during this period.

The earliest known choropleth map was made in 1826 by Charles Dupin. Louis-Léger Vauthier later used isolines to map population density in Paris in 1874. In 1837, Henry Drury Harness produced a small booklet of five maps for a railroad report in Ireland, including early chorochromatic, flow, proportional point symbol, and dasymetric maps.

London physician John Snow used thematic mapping for analysis in 1854. By plotting cholera deaths on a base map of a London neighborhood that included streets and water pumps, he identified a pattern centered on one pump.

After the pump handle was removed, new cases stopped. Charles Joseph Minard is considered an early master of thematic mapping and information visualization. In the 1850s and 1860s, he combined thematic maps—especially flow maps—with statistical charts to tell visual stories, most famously his 1869 map of Napoleon’s 1812 invasion.

By the early 1900s, manual methods for drafting thematic maps were well established, but these maps were still far less common than general reference maps and received little attention in cartographic education. Their popularity surged in the second half of the 20th century, driven by the quantitative revolution in geography, the rise of cartography as an academic discipline, new technologies like personal computers, GIS, graphics software, and the Internet, and the growing availability of large datasets.