How do you map the invisible movement of ideas, trade, and wind?

Countries & Cities · Published 30 September 2026 Swipe more like this →

In short: A flow map is a hybrid of a map and a flow diagram, using linear symbols to track movement between locations. From the migration of people to the flow of telecommunications data, these maps transform abstract networks into visible, intuitive structures that reveal how our world actually moves.

At its core, a flow map uses line width to represent volume, making it a cousin to proportional symbol maps. By scaling the thickness of a line in direct proportion to a value—such as vehicles per day or cubic feet per second of water—cartographers allow readers to make intuitive judgments. For instance, a line twice as thick as another visually represents twice the value. However, this becomes problematic when variation exceeds a 25:1 ratio, resulting in lines that are either overwhelmingly thick or nearly invisible. In such extreme cases, designers often use linear interpolation between a set minimum and maximum weight to maintain legibility.

The history of this technique spans nearly two centuries. The earliest known visual representations of flow volume appeared in 1838, when engineer Henry Drury Harness published two maps for a report on Irish railroad construction, showing cargo traffic via road and canal. The field was later mastered by Charles Joseph Minard, who published forty-two flow maps during the 1850s and 1860s. His 1869 map of the French invasion of Russia (1812–1813) is famously regarded as "the best statistical graphic ever drawn."

Modern cartography has expanded these principles into several specialized categories. Origin-destination maps focus on the connection between two points, often using curved arcs to avoid the visual clutter of straight lines. Distribution maps, like those showing shipping lanes between port cities, show paths that divide as they move toward various destinations. For continuous movements like ocean currents or wind, cartographers use vector fields or streamlines. While early computer-generated maps in the 1980s struggled to meet the standards set by Minard, modern GIS and graphics software now allow for sophisticated, automated generation of these complex networks.

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