Abstract: A computer implemented method (400) of determining a location of one or more transitions (5a-e) on a map and/or the time at which one or more transitions (5a-e) occurs, the one or more transitions (5a-e) made by a set of mobile computing devices (13a-c), from a first zone (2) to a second zone (4), the method (400) comprising: obtaining (402) trajectory data representing a plurality of trajectories (302 a, b) collected from one or more mobile computing devices (13 a-c), at least some of the trajectories (302 a, b) passing through the first zone (2) and/or the second zone (4); positioning (404) the trajectories (302 a, b) in a frame of reference defined relative to the map, wherein at least part of at least some of the trajectories (302 a, b) are positioned based on correspondence with other trajectories (302 a, b); and processing the plurality of trajectories (302 a, b) positioned in the frame of reference defined relative to the map to determine the location and/or time.