Flow map
Thematic map using linear symbols to show movement between locations.
A flow map is a type of thematic map that uses linear symbols to represent movement between locations, such as people, highway traffic, trade goods, water, ideas, or telecommunications data. It may be considered a hybrid of a map and a flow diagram, and its design varies widely depending on the moving material and geographic networks involved. Many flow maps employ line width proportional to the amount of flow, a technique similar to that used in cartograms and proportional point symbols. The earliest known examples of this technique were two maps created in 1838 by engineer Henry Drury Harness, which depicted cargo traffic volumes on roads and canals in Ireland as part of a report on potential railroad construction. Subsequent European experimentation led to mastery by Charles Joseph Minard, who produced forty-two flow maps during the 1850s and 1860s. His 1869 map of the French invasion of Russia is often regarded as the finest statistical graphic ever drawn, employing design techniques that remain unsurpassed even in the computer age. In the 1980s, Waldo Tobler began experiments in computer-generated flow maps, though early results fell short of Minard’s standards; later improvements in GIS and graphics software have enhanced automated flow map design. Flow maps can represent several aspects of movement, including origin and destination, route, type of movement, amount of movement, direction, and velocity. These aspects are not mutually exclusive; a single map may depict several simultaneously. Subjects range from human geography—migration, trade, traffic, and public utilities—to physical geography, such as streamflow, wind, and wildlife migration. Design strategies vary accordingly, with origin-destination maps often using curved lines for aesthetic clarity, while distribution maps balance focus between nodes and routes.
- field
- Cartography
- known_for
- Visualizing movement and flow volume using proportional line width
- key_figure
- Charles Joseph Minard (1850s–1860s)
Lore & Background
A flow map is a thematic map that uses linear symbols to depict movement between locations, functioning as a hybrid of a map and a flow diagram. The movement represented can involve people, highway traffic, trade goods, water, ideas, or telecommunications data. The wide variety of moving materials and geographic networks has led to many design strategies. Some cartographers expand the term to any thematic map of a linear network, while others restrict it to maps specifically showing movement. Many flow maps use line width proportional to the amount of flow, similar to cartograms and proportional point symbols. The earliest known map to visually represent flow volume was created by engineer Henry Drury Harness in 1838, showing cargo traffic by road and canal in Ireland. Charles Joseph Minard later mastered the technique, producing forty-two flow maps during the 1850s and 1860s, including his renowned 1869 map of the French invasion of Russia. Flow maps can represent several aspects of movement: origin and destination, route, type of movement, amount of movement, direction, and flow velocity. These aspects are not mutually exclusive; a single map may portray several simultaneously. Topics include human geography (migration, travel, trade, traffic) and physical geography (streamflow, wind, wildlife migration). Types include origin-destination maps, which focus on connections between places with straight or curved lines, and distribution maps, which balance focus on origin-destination nodes.
Reader's Guide
Flow maps are significant for their ability to convey complex movement data—origin, destination, route, type, amount, direction, and velocity—in a single visual representation. According to Eduard Imhof, a flow map may represent several aspects of moving phenomena and the networks along which they travel. Types include origin-destination maps, distribution maps, network route maps, and continuous/mass flow maps. The most common technique to visualize amount or speed is through line weight proportional to value, similar to scaling methods for proportional symbol maps. During the 1980s, Waldo Tobler conducted experiments to create flow maps by computer, and while early results were not up to Minard's standards, subsequent improvements in GIS and graphics software have enhanced automated design capabilities. Flow maps remain essential in human geography (migration, trade, traffic) and physical geography (streamflow, wind, wildlife migration).
Did You Know?
- Charles Joseph Minard produced forty-two flow maps during the 1850s and 1860s.
- Waldo Tobler conducted experiments to create flow maps by computer during the 1980s.
Frequently Asked Questions
What is a flow map in cartography?
A flow map is a thematic map that depicts movement between geographic locations using linear symbols, where the width of each line is proportional to the volume being transported. It functions as a hybrid between a conventional map and a flow diagram, with its visual design shifting based on the type of material and the network involved.
Why is Charles Joseph Minard considered a key figure in flow-map history?
Minard, active in the 1850s and 1860s, elevated the flow map from a simple trade illustration to a powerful analytical tool for visualizing complex movements such as military campaigns and commodity flows. His work demonstrated that proportional line width could encode both direction and magnitude simultaneously, a principle that remains central to the technique.
What kinds of movement can a flow map represent?
Flow maps are not limited to a single subject; they can chart the movement of people, highway traffic, trade goods, water, ideas, and telecommunications data. The specific network and material being tracked determine how the linear symbols are styled and scaled.
How does a flow map differ from a standard choropleth or dot-density thematic map?
Where choropleth and dot-density maps emphasize static quantities tied to regions, a flow map encodes dynamic transfer between locations using directional lines. The proportional width of each line conveys volume, and the overall layout reads more like a diagram of connections than a shaded area map.
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