A flow map is a type of thematic map that uses linear symbols to represent movement. It may thus be considered a hybrid of a map and a flow diagram. The movement being mapped may be that of anything, including people, highway traffic, trade goods, water, ideas, telecommunications data, etc. The wide variety of moving material, and the variety of geographic networks through they move, has led to many different design strategies. Some cartographers have expanded this term to any thematic map of a linear network, while others restrict its use to maps that specifically show movement of some kind.
Many flow maps use line width proportional to the amount of flow, making them similar to other maps that use proportional size, including cartograms (altering region area), and proportional point symbols.
History
The earliest known map to visually represent the volume of flow were two maps by engineer Henry Drury Harness, published in 1838 as part of a report on the potential for railroad construction in Ireland, showing the quantity of cargo traffic by road and canal. In subsequent years, others experimented with the technique in Europe, until it was mastered by Charles Joseph Minard.
During the 1850s and 1860s, Minard published forty-two flow maps on a wide variety of topics among his cartes figuratives. Among these is his 1869 map of the French invasion of Russia in 1812-1813, which has been called "the best statistical graphic ever drawn." Many of Minard's maps use design techniques that have yet to be improved upon, even in the age of computer graphics.
During the 1980s, Waldo Tobler conducted experiments to create flow maps by computer. Early computer-generated maps were not up to Minard standards, but Geographic information system (GIS) and graphics software has improved in the ability to design flow maps.
Flow phenomena
A wide variety of flow maps have been created since the 1830s, showing movement in many forms. According to Eduard Imhof, a flow map may represent several different aspects of the phenomena that are moving and the networks along which they move; he listed the following: Origin and Destination: "from where and to where is something moving?" Route: "Along which line does the movement take place?" This may be represented precisely, generalized, or only in the schematic form of straight lines Type of movement: "what is moved?" This nominal category could include variables such as the product being shipped from one point to another, the vehicular mode of transport, or the water temperature in an ocean current. Amount of movement: "how much is moved? Is the quantity of the transported item changing steadily (gradually) or in stages?" This will be a non-negative ratio variable, such as vehicles per day, the total number of migrants, or water flow in cubic feet per second. Some flow maps represent flow capacity rather than actual movement volume, such as telecommunications bandwidth. Direction of movement: "in which direction, or in both directions, is something moved?" This is often shown by arrows. Flow velocity: "At what speed or in how much time is something moved?" This is a ratio variable that is similar to, but distinct from the amount of movement. An example would be representing highway traffic levels using mean vehicle speed, or wind velocity.
These are not distinct types of maps; a flow map may portray any of all of these aspects simultaneously.
The types of phenomena that have been the subject of flow maps are varied. Topics relating to human geography include: migration, travel, international trade, logistics, public utilities (water, sewer, electricity, telecommunications), and traffic, among others. Others relate to physical geography: streamflow, wind, wildlife migration, etc.
Types of flow maps
The variance of flow maps in subject matter, and the relative importance of Imhof's aspects of flow, has led to a number of design strategies. In a 1987 thesis, Mary Parks identified several distinct types of flow maps, which has been widely cited although her list was not comprehensive and more are included here. These types are prototypical; actual maps can combine some aspects of several types.