13.) Flow maps

Flow Map:

http://www.mpassociates.gr/software/distrib/science/golden/PrismMap.gif

Cartographers have long used flow maps to show 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. The advantage of flow maps is that they reduce visual clutter by merging edges. Most flow maps are drawn by hand and there are few computer algorithms available. Some generate flow maps using hierarchical clustering given a set of nodes, positions, and flow data between the nodes. These techniques are inspired by graph layout algorithms that minimize edge crossings and distort node positions while maintaining relative position to one another. It demonstrates technique by producing flow maps for network traffic, census data, and trade data. ect.