Flow map



A Flow map is a type of Thematic map that hybridizes maps and flow charts, showing the movement of objects from one location to another such as the number of people in a migration, the amount of goods being traded, or the number of packets in a network. Flow maps are useful when one needs to show any of the following: a) the presence or absence of a connection between places; b) the route taken from one place to another; c) the (qualitative) kind of movement taking place; d) the direction of movement; or e) the (quantitative) amount of movement taking place.

History of Flow Maps
One of the most famous flow maps is Minard's Map of Napoleon's Invasion of Russia. Drawn in 1869 by French engineer Charles Joseph Minard, it shows the route, and size of Napoleon's army as they attempted to invade Moscow before the battle of Waterloo. The line gets thinner as the number of French troops diminishes and eventually retreats to Poland. The chart at the bottom coincides with the black retreat line and graphs the cold winter temperatures the French soldiers endured throughout their homeward journey. This is one of the earliest examples of flow maps, and is described by French scientist E.J. Marey as "seeming to defy the pen of the historian by its brutal eloquence."

In 1987, Waldo Tobler developed a piece of software for displaying migration flows. One of the simpler options in his software was the depiction of one-way migration to or from a particular state. It was possible to show the migration between each state and a selected state by using arrows of different width. Today, most Flow maps are produced using standard GIS or graphics design software.

Types of Flow Maps
There are a variety of kinds of flow maps, depending on the kind of information that is being portrayed. The different aspects of movement that may or may not be represented include:


 * Route of movement, which could be represented precisely, generalized, or not at all (i.e., schematic lines between points)
 * Type of movement, typically nominal, such as the category of product being shipped from one point to another, or the mode of transport. Typically shown using nominal graphic variables such as hue or pattern.
 * Direction of movement, often shown by arrowheads on the flow lines
 * Amount of movement, a non-negative ratio variable, such as the available bandwidth on a telecommunications line or the total cars that travel a road segment each day. Typically shown using ordinal or quantitative graphic variables such as line thickness or color value. This type of flow map could be considered a type of Proportional symbol map.

These are not distinct types of maps; a flow map may portray any or all of these aspects simultaneously.

Design Guidelines
Lines on quantitative flow maps can be scaled so that their widths are proportional to the amount they represent. Flow maps without quantitative data often use lines that are unscaled and are of uniformed thickness. Direction of movement is frequently important, so arrowheads are a likely part of the symbolization as well.

The principles of scale and generalization are of particular importance when creating and reading line flow maps. Flow maps usually require a large amount of generalization due to the fact that they often include many traffic routes which, if not generalized to the proper degree, will likely overcome the map and cause a high degree of clutter.

Other important cartographic principles that affect flow maps include the type of projection used, the colors employed, and the line styles. Cartographers should remember these cartographic principles when designing flow maps, so that the map reader can clearly understand each map and its purpose. Several studies have discussed the importance of cartographic principles in the creation of flow maps.

Examples
Flow maps can be used to show movement of:

Tangible variables
 * People
 * Products
 * Natural Resources
 * Weather
 * Currency

Intangible variables
 * Know-how
 * Talent
 * Credit of Goodwill

Flow Maps are one of the ten diagrams defined in Thinking Maps.

Other Types of Flow Maps
Beside the flow maps in cartography there are several other kinds of flow maps:
 * Baker flow map of fluid flows
 * Blood flow maps, see history of neuroimaging
 * Flow map or solution operator
 * Process flow map of a manufacturing process
 * Sankey diagram in petroleum engineering
 * Transportation flow maps
 * XSL flow maps, see XSL Formatting Objects
 * Continuous vector-based flows. These are composed of the variable magnitude and direction that can change at any point. For example, wind is a continuous vector-based flow because at any point in the atmosphere we can consider the speed and direction at which it flows.