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Flow map

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Charles Joseph Minard’s map of French wine exports for 1864.

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, or even telecommunications data.[1] 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

1838 map of pre-railroad cargo traffic in Ireland, one of the first thematic maps to use proportional 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.[2][3] In subsequent years, others experimented with the technique in Europe, until it was mastered by Charles Joseph Minard.[4]

Charles Joseph Minard map of the 1812 French invasion of Russia.

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."[5][6] 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.[7][8] 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:[9]: 94–95 

  • Origin and Destination: "from where and to where is something moving?"
  • Route: "Along which line does the the movement take place?" This may be represented precisely, generalized, or only in the schematic form of straight lines[10]
  • 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 arrow.s[11]
  • 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.[12]

Origin-destination maps

In this type, the primary intent is to show the existence of a connection between two places, often accompanied by a representation of the volume of flow and/or direction. The route is generally not important to the audience, so connecting lines are often straight or slightly curved. A common example of this form is the airline route map. Parks distinguished radial maps (emanating from a single origin or destination) from network maps (interconnecting a number of nodes), but this is a difference in the geographic pattern; the design is very similar for both types.


Vector fields and streamline maps

A 1943 map of ocean currents using the streamline technique. Note the use of color to distinguish hot and cold currents.

Not all flow occurs along linear networks; two- and three-dimensional masses can also flow, especially water (e.g., ocean current) and air (wind). Their movement can be modeled as a vector field, in which the magnitude and direction of movement could be measured at any point in space. In Imhof's list above, a vector field focuses on direction and speed of flow, while other aspects such as origin/destination and route of travel are largely meaningless.

In 1688, Edmund Halley mapped the trade winds using an array of short line segments oriented in the direction of air flow.[4]: 69  In early 20th Century weather maps, this technique was elaborated into wind barbs, complex symbols used to indicate wind speed as well as direction. Unit vectors and streamlets have been proposed as a general term for point-based flow symbols, but neither is in wide usage.[13] These can be readily generated by computer algorithms, especially using raster GIS data or sample point data (e.g., weather stations).[14] This approach is more akin to proportional point symbols than a typical flow map.

Another visualization option for vector fields is to draw streamlines, which connect points that would flow into each other; this has been commonly used to represent ocean currents since the early 19th Century.[4]: 82  Among Tobler's 1981 computer algorithms were models of vector fields to generate "streaklines," and subsequent work has improved results.[7][13]

Other types of flow diagrams

Main article: Flow diagram
A non-cartographic flow map showing the relative percentages of cardiac output delivered to major organ systems

Beside the flow maps in cartography there are several other methods for visualizing non-geographic flow:

See also

References

  1. ^ Phan, Doantam; Xiao, Ling; Yeh, Ron; Hanrahan, Pat; Winograd, Terry (2005). "Flow Map Layout". Proceedings of the 2005 IEEE Symposium on Information Visualization (InfoVis '05): 219–224. doi:10.1109/INFVIS.2005.1532150.
  2. ^ Robinson, Arthur H. (Dec 1955). "The 1837 Maps of Henry Drury Harness". The Geographical Journal. 121 (4): 440–450. doi:10.2307/1791753. JSTOR 1791753.
  3. ^ Griffith, Richard John; Harness, Henry Drury (1838). Atlas to Accompany 2nd Report of the Railway Commissioners. Ireland.
  4. ^ a b c Robinson, Arthur H. (1982). Early Thematic Mapping in the History of Cartography. University of Chicago Press. pp. 147–154.
  5. ^ Tufte, Edward (2006). Beautiful Evidence. Graphics Press.
  6. ^ Jacobs, Frank. "The Minard Map - "The best statistical graphic ever drawn"". Big Think. Retrieved 13 November 2020.
  7. ^ a b Tobler, Waldo R. (January 1981). "A Model of Geographic Movement". Geographical Analysis. 13 (1): 1–20. doi:10.1111/j.1538-4632.1981.tb00711.x.
  8. ^ Tobler, Waldo R. (1987). "Experiments in Migration Mapping by Computer". The American Cartographer. 14 (2): 155–163. doi:10.1559/152304087783875273.
  9. ^ Imhof, Eduard (1972). Thematische Kartographie. Berlin: De Gruyter.
  10. ^ Robinson, Arthur H., Elements of Cartography, 2nd Edition, New York: Wiley, 1960.
  11. ^ Dent, Borden D.; Torguson, Jeffrey S.; Hodler, Thomas W. (2009). Cartography: Thematic Map Design (6th ed.). McGraw-Hill. pp. 188–201.
  12. ^ Parks, Mary J. (1987). American flow mapping: A survey of the flow maps found in twentieth century geography textbooks, including a classification of the various flow map designs. Georgia State University: Unpublished M.A. thesis.
  13. ^ a b Turk, G.; Banks, D. (August 1996). "Image-guided streamline placement". SIGGRAPH '96: Proceedings of the 23rd annual conference on Computer graphics and interactive techniques: 453–460. doi:10.1145/237170.237285.
  14. ^ Lavin, S.J.; Cerveny, R.S. (1987). "Unit-vector density mapping". The Cartographic Journal. 24 (2): 131–141. doi:10.1179/caj.1987.24.2.131.


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