Flow map

. A flow map is a type of thematic map that uses linear symbols to represent movement between locations. 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.

.]] A flow map is a type of thematic map that uses linear symbols to represent movement between locations. 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. === Origin-destination map=== 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. Origin-destination maps have a unique design focus on the schematic shape of the connecting lines. Straight lines are easy to draw, but can cause issues, especially when longer lines and shorter lines are collinear, obscuring each other and their destinations. They can also have a haphazard look. For these reasons, curved lines, typically circular arcs, are preferred as more aesthetically pleasing. They also have the ability to be adjusted to avoid intervening lines and points. Early automated line generation algorithms were typically straight lines, but recent algorithms have been successful at creating curved lines. \frac{w}{w_0} = \frac{v}{v_0} This helps readers make intuitive judgements about relative value ratios based on relative weights; a line that is twice as thick as another represents twice the value. However, when there is a very high level of variation between the highest and lowest values (generally, more than 25:1, although this depends on the geography of the flow network and the design), the resultant map can be problematic, with overwhelming thick lines and almost invisible thin lines. An alternative in this case is to set the minimum and maximum weights (w max and w min ) and perform linear interpolation between them, even though this eliminates the ability to intuitively judge weight ratios: \frac{w - w_{min}}{w_{max} - w_{min}} = \frac{v - v_{min}}{v_{max} - v_{min}} A third alternative is to simply use ordinal weights (thick, medium, thin, etc.) to represent an ordinal variable or a quantitative variable that has been classified (often called range grading in this context). This also loses the ability of readers to intuitively judge value differences, but has the advantage of overall simplicity. For all of these types of scaling, the legend typically shows a sample set of line weights with their respective values, in a fashion similar to the legend of a proportional symbol map.

Text derived from https://en.wikipedia.org/w/index.php?oldid=1359051791 under CC BY-SA 4.0; modifications have been made.