How would you show the movement of people, money, energy or almost anything from one location to another? The best answer is using flow maps.
Flow maps denote the direction a phenomenon is being moved from one location to another.
Generally speaking, flow maps can be divided as radial, network and distributive.
Let’s give you some insight on how to produce flow maps of your own.
1 Radial Flow Maps
In radial flow maps, lines radiate from an origin node to a single or multiple destination nodes. Each hub links that movement of phenomenon with a directional flow line, often in a radial pattern.
In this example, we use the United States foreign trade census data. Essentially, this displays what, where and how much does the United States export to other countries.
The FlowMapper plugin for QGIS is a quick and easy way to generate flow maps of your own. When you download the plugin, it comes with test data that you can use as a template. But essentially, it boils down to these two text files as inputs:
- A list of nodes with their latitude and longitude coordinates
- A square matrix of magnitude values with each row corresponds to a ‘from’ node and each column corresponds to a ‘to’ node.
After running the plugin, the results look good. Note, how line thickness depicts magnitude of that movement. This means that the United States biggest trading partners are its neighbors in the north (Canada) and south (Mexico).
2 Distributive Flow Maps
Although similar to radial flow maps, distributive flow maps branch off to their destination. It’s not the necessary route that is being traveled. But it’s an abstract way how this commodity or information travels from an origin to multiple destinations.
The famous example is the English Coal Exports 1864. Coal exports diffuses from its origin in England to multiple destination almost like curled fingers.
The tool you want to download to create this type of map is a customized Distributive Flow Lines tool (DFLT) in ArcGIS. Given an origin and multiple destinations, this tool uses cost distance and impedance to generate a distributive flow map for you.
3 Network Flow Maps
Network flow maps show movement along an established network usually based on transportation or communication interconnectivity. It uses the network information and attributues to delineate the amount of flow in the map.
For example, HERE maps shows the average daily traffic in a given road network. These linkages can use line thickness and color for magnitude.
Another example is a network flow map that you see for airline companies. Heathrow Airport serves flights to 84 different countries in the world. With the focal point centered at Heathrow, 84 flow lines would radiate to each possible destination.
To show the flow of goods, cartographers use lines.
To show the magnitude, they change the width of the lines
Waldo Tobler once said: “Geographical movement is critically important. This is because much change in the world is due to geographical movement. Movement of people, ideas, money, energy or materiel”.
And there’s no better way than to denote this movement than with flow maps.