Thematic Maps and Choropleth Design

Unlike general-reference maps, which show a broad mix of natural and built features such as roads, rivers, and settlements, thematic maps are built around a single subject or dataset, deliberately subordinating other geographic detail to highlight the spatial pattern of that one variable, whether population density, rainfall, election results, or disease incidence. Among the earliest and most celebrated examples is the physician John Snow's 1854 map of a cholera outbreak in London's Soho district, which plotted individual cholera deaths as dots clustered tightly around a single water pump on Broad Street, providing strong visual and statistical evidence, well before the germ theory of disease was established, that contaminated water rather than "miasma," or bad air, was spreading the disease.