Flow maps are a type of thematic map used in cartography to show the movement of objects between different areas. These types of maps can show things like the movement of goods across space, the number of animal species in a specific migration pattern, as well as traffic volume and stream flow. They can also show both qualitative and quantitative data. Flow maps usually represent the movement of goods, weather phenomena, people and other living things with line symbols of different widths. Thus, the use of lines on a flow map is similar to the use of graduated symbols on other types of thematic maps (Chang, 2012).
When properly designed, flow maps are beneficial because they allow cartographers, GIS analysts and map users alike to easily see the differences in magnitude of a wide variety of items across space with very little map clutter (Phan, et al). This in turn allows businesses to see where the majority of their products are going, commuters to see traffic patterns, and meteorologists to see wind patterns.
This article provides a basic overview of flow maps and a description of how they work. It also describes the three different types of flow maps and explains the characteristics and components of a good flow map. Finally it presents several examples of different types of flow maps.
How Flow Maps Work
Flow maps typically use lines to show the movement of people and goods between various locations. The lines are varied in width to represent the quantity of flow (Sathyaprasad). Therefore if there is a very wide line showing traffic on one California roadway and a very thin line showing traffic on another, the road with the wider line is generally the one that contains more traffic.
Because the movement of goods and people are usually shown lines, most flow maps are created with vector, instead of raster, based data so that they can show movement continuously over the Earth’s surface (Buckley). In vector based flow maps, the vectors are points or lines that hold information about the direction and magnitude of item that is moving (Buckley). The points and lines can then be overlaid onto a map to show the movement throughout a given area.
Vectors can be symbolized in a flow map with different orientations, point size, and line length or width to show direction and magnitude. For example, flow maps showing global wind patterns often have lines with arrows on them. The point of the arrow shows the direction of movement (straight, circular, curved, etc.), while the width of the line shows the wind’s intensity.
Although most flow maps use vector data, ArcGIS has recently introduced a tool for distributive flow maps (discussed in the next section) that uses raster data. The Distributive Flow Lines tool (DFLT) is a spatial analyst tool in ArcGIS that generates distributive flow lines from one source to many different destination points. The DFLT is completely raster based until the end when it creates a vector based flow line feature class to be used on flow maps (Bgerit). Some ArcGIS users say this raster based tool is optimal because it allows for more control over the flow lines and it decreases processing time (Bgerit).
Another important thing to note about flow maps is that they can use and display both qualitative and quantitative data. For qualitative data the maps usually display symbols of uniform width that just show movement with arrows (McGraw-Hill). This data is a connection of some sort and it is not based on magnitude. Quantitative flow mapping uses lines and symbols of different widths and sizes to show changes in magnitude between areas.
Types of Flow Maps
When looking at and creating flow maps it is important to note that there are three basic categories for the maps. These are radial, network and distributive. Radial flow maps show relationships between one source and many destinations and use separate lines radiating out from a starting point to show movement. Network flow maps show the quantity of flow over an existing network (Sathyaprasad). These types of flow maps most frequently show transportation and communication networks. Distributive flow maps are maps that show relationships between a single source and many destinations like a radial flow map. These maps are different however, because they often have a large, single line produced from one source and that forks into many smaller lines once they reach their destination.
In addition to these three map categories, flow maps can also be still or animated. The traditional printed wind, traffic and other maps showing the movement of goods across space are still but computers now allow interactive, animated flow maps that can show things like wind speed during and after a hurricane for instance (Buckley).
What Makes a Good Flow Map?
Whether radial, network or distributive all good flow maps should have the following characteristics and components as explained by Doantam Phan et al. in their paper, “Flow Map Layout”:
- Intelligent Distortion: Some flow maps feature distortion to show the movement of goods. Therefore it is important that any intended distortion not change the meaning of the map.
- Merging of Edges that Share Destinations: If there are many lines going to the same destination it is important that their edges be combined to reduce map clutter.
- Intelligent Edge Routing: In some cases branches or lines on flow maps will route themselves through the center of the map. This can obscure the other lines so they can be routed to the edge of the map so that all of the data can be easily seen.
- Layering and Branching Structure: Some flow maps have a common set of nodes. In these cases layering of their lines works well to reduce map clutter.
- Linear or Logarithmic Display Widths: Flow maps can use both linear and logarithmic display widths. It is important to choose the correct one to best show the data.
Example of Flow Mapping
Because flow maps can use a wide variety of data there are also many different projects in a plethora of fields that can use this mapping technique. Business geographers can for example, use flow mapping to examine the amount of coal exports from a specific country. Hydrologists can monitor stream flow for a particular state or region. City planners and transportation geographers can use flow mapping to examine traffic patterns and the volume of cars on specific roadways to determine the best places for new businesses and/or residential developments.
To learn more about flow maps and to practice making them with a variety exercises for ArcGIS, visit the ArcGIS – Flow Maps page on ArcGIS.com.
ArcGIS. (n.d.). ArcGIS – Flow Maps. Retrieved from: http://www.arcgis.com/home/group.html?id=62918569d92344efa8b50bf3df5e8f25 (21 March 2014).
Bgerit. (26 August 2013). “Distributive Flow Maps – More Raster, More Faster.” ESRI Applications Prototype Lab. Retrieved from: http://blogs.esri.com/esri/apl/2013/08/26/flow-map-version-2/ (21 March 2014).
Buckley, Aileen. (23 April 2013). “AAG Flow Mapping Presentation Available for Download.” ArcGIS Resources. Retrieved from: http://blogs.esri.com/esri/arcgis/2013/04/23/aag-flow-mapping-presentation-available-for-download/ (22 March 2014).
Buckley, Aileen. (April 2013). “Go With the Flow.” ArcWatch: GIS News, Views and Insights. Retrieved from: http://www.esri.com/esri-news/arcwatch/0413/go-with-the-flow (21 March 2014).
Chang, Kang-tsung. (2012). Introduction to Geographic Information Systems. McGraw-Hill: New York, 6th Edition.
McGraw-Hill. (n.d.). Mapping Exercise – Thematic Mapper: The Flow Map. Retrieved from: http://highered.mcgraw-hill.com/sites/dl/free/0072943823/599982/Flow_Mapping.pdf (21 March 2014).
Phan, Doantam, Ling Xiao, Ron Yeh, Pat Hanrahan, and Terry Winograd. (n.d.). “Flow Map Layout.” Stanford University. Retrieved from: http://graphics.stanford.edu/papers/flow_map_layout/flow_map_layout.pdf (21 March 2014).
Sathyaprasad. (12 September 2012). “Generating Distributive Flow Maps with ArcGIS.” ESRI Applications Prototype Lab. Retrieved from: http://blogs.esri.com/esri/apl/2012/09/12/generating-distributive-flow-maps-with-arcgis/ (21 March 2014).
Wikipedia.org. (6 November 2013). Flow Map – Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/Flow_map (21 March 2014).