Flow trees for exploring spatial trajectories

A trajectory is a directed path that defines a link between two spatial locations. That path may be as simple as the Euclidean shortest distance between a start and end point, or may involve a more complex traversal through time and space to travel from start to end. Within GI analysis, trajectories are used to represent phenomena such as movement of people as migration and commuting, goods and information. Trajectories are commonly represented as Origin-Destination Matrices (OD matrices) (Voorhees, 1955) that record the numbers of directed links between a set of start and end points (see Figure 1).