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.
Next up: Types of Flow Maps, What Makes a Good Flow Map?, and Examples of Flow Maps