A thematic map is a type of map that portrays the geographic pattern of a particular subject matter (theme) in a geographic area. This usually involves the use of map symbols to visualize selected properties of geographic features that are not naturally visible, such as temperature, language, or population. In this, they contrast with general reference maps, which focus on the location (more than the properties) of a diverse set of physical features, such as rivers, roads, and buildings. Alternative names have been suggested for this class, such as special-subject or special-purpose maps, statistical maps, or distribution maps, but these have generally fallen out of common usage. Thematic mapping is closely allied with the field of Geovisualization.
Several types of thematic maps have been invented, starting in the 18th and 19th centuries, as large amounts of statistical data began to be collected and published, such as national censuses. These types, such as choropleth maps, isarithmic maps, and chorochromatic maps, use very different strategies for representing the location and attributes of geographic phenomena, such that each is preferable for different forms of phenomena and different forms of available data. A wide variety of phenomena and data can thus be visualized using thematic maps, including those from the natural world (e.g., climate, soils) and the human world (e.g., demographics, public health)
History
According to Arthur Robinson, thematic maps were largely an Industrial Age innovation, with some Enlightenment-era roots; almost all of the modern graphical techniques were invented between 1700 and 1850. Prior to this, the most important cartographic development was the production of accurate general base maps. Their accuracy improved slowly, and even in the mid-17th century, they were usually of poor quality; but they were good enough to display basic information, allowing the production of the first thematic maps.
One of the earliest thematic maps was one entitled Designatio orbis christiani (1607) by Jodocus Hondius, showing the dispersion of major religions using map symbols, in the French edition of his Atlas Minor (1607). This was soon followed by a thematic globe (in the form of a six-gore map) showing the same subject, using Hondius' symbols, by Franciscus Haraeus, entitled Novus typus orbis ipsus globus, ex Analemmate Ptolomaei diductus (1614)
An early contributor to thematic mapping in England was the English astronomer Edmond Halley (1656–1742), who introduced the Enlightenment conception of the thematic map as a tool for scientific thinking. His first significant cartographic contribution was a star chart of the constellations of the Southern Hemisphere, made during his stay on St. Helena and published in 1686. In that same year he also published his first terrestrial map in an article about trade winds, and this map is called the first meteorological chart. In 1701 he published the "New and Correct Chart Shewing the Variations of the Compass", see first image, the first chart to show lines of equal magnetic variation and possibly the first isarithmic map. Early chorochromatic (nominal area-class) maps also appeared in the late 18th century as scientific instruments for exploring geographic phenomena such as geology and language.
The early to middle 19th century could be considered, as Robinson called it, a "golden age" of thematic mapping, when many of the current techniques were invented or further developed. For example, the earliest known choropleth map was created in 1826 by Charles Dupin. Based on this work Louis-Léger Vauthier (1815–1901) developed the population contour map, a map that shows the population density of Paris in 1874 by isolines.
One of the most influential early works of thematic cartography was a small booklet of five maps produced in 1837 by Henry Drury Harness as part of a government report on the potential for construction of railroads in Ireland. Included were early chorochromatic and flow maps, and possibly the first proportional point symbol and dasymetric maps.