Shortest path trees and effective distance among districts in Germany.

Each column depicts the shortest path tree for a sample root node (red), from left to right districts Uelzen, Göttingen, and Oberalbkreis. The top row depicts embedded in the conventional geographic representation, the bottom illustrates the shortest path tree in a layout such that the radial distance is proportional to the effective distance from the root node in the same way as in Fig. 3. The shortest path tree represents the most probable path that a contagion process takes with initial outbreak in node .