Zachary Forest Johnson provides an overview on Indiemaps.com on the first published instances of each of the six most commonly used thematic mapping techniques: choropleth, proportional symbol, dot density, flow, isarithmic, and cartogram. Each technique’s first mapping instance is described along with a scanned sample of the map. In conclusion, Johnson writes:
Four of the six classic thematic cartography symbologies — choropleth, dot density, proportional symbol, and flow — originated between 1826 and 1837. Two of them — proportional symbol and flow — were initially produced by one man (Harness), and appeared in the same obscure railway atlas. All were refined in the 19th century, and only one (isolines) predate the century.
Read more: the first thematic maps