Transition Detection

A computer-implemented method 400 relates to determining a location of one or more transitions on a map and/or the time at which a transition occurs, the transitions being made by a set of mobile computing devices moving between a first zone (4, figure 1, not shown) and a second zone (2). The method includes obtaining trajectory data representing a plurality of trajectories collected from the mobile devices 402, at least some of the trajectories passing through the first and/or second zone. The trajectories are positioned in a frame of reference defined relative to a map 404, wherein at least part of some of the trajectories are positioned based on their correspondence with other trajectories. The trajectories are then processed so as to determine the location and/or time of transitions from first zones to second zones of the map 408. Candidate transitions may be identified in each trajectory and the transitions may be determined based on groupings of candidate transitions.