POG construction. Symbols inside the circles are the aligned node IDs. The table associated with each node encodes the set of points aligned to it. In particular, each row represents a point with its trajectory ID (T column) and its index along the trajectory (S column). For example, the entry (1, 2) associated with node b in (a) means that the aligned node b currently include the point , the second point from trajectory-1. In (a), a POG is initialized by the trajectory T1. An example of a POG after aligning a few trajectories is shown in (b). Note that a new node/branch is created when a point cannot be aligned to any existing nodes. For example, node e was created when (i.e, the 3rd point of T2) was inserted. (c) shows the POG after merging point from the node b to the node e constrained by the distance threshold ε.