Flow map

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. 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. A wide variety of flow maps have been created since the 1830s, showing movement in many forms.