Compact Representation of GPS Trajectories over Vectorial Road Networks

Many devices nowadays record traveling routes, of users, as sequences of GPS locations. With the growing popularity of smartphones, millions of such routes are generated each day, and many routes have to be stored locally on the device or transmitted to a remote database. It is, thus, essential to encode the sequences, to decrease the volume of the stored or transmitted data. In this paper we study the problem of coding routes over a vectorial road network (map), where GPS locations can be associated with vertices or with road segments. We consider a three-step process of dilution, map-matching and coding. We present two methods to code routes. The first method represents the given route as a sequence of greedy paths. We provide two algorithms to generate a greedy-path code for a sequence of n vertices on the map. The first algorithm has O(n) time complexity, and the second one has O(n) time complexity, but it is optimal, meaning that it generates the shortest possible greedypath code. Decoding a greedy-path code can be done in O(n) time. The second method codes a route as a sequence of shortest paths. We provide a simple algorithm to generate a shortest-path code in O(kn logn) time, where k is the length of the produced code, and we prove that this code is optimal. Decoding a shortest-path code also requires O(kn logn) time. Our experimental evaluation shows that shortest-path codes are more compact than greedy-path codes, justifying the larger time complexity.