Floor assignment

A method of automatically assigning segments of trajectories 110, 120, 130 to corresponding floors of a building 100, comprising: receiving height (i.e. pressure, inertial, GPS data) and ambient data (i.e. Wi-Fi (RTM) data) corresponding to a trajectory from a mobile device 152; and segmenting the trajectory using the height data, such that a change in the height data greater than a threshold marks an end region of one segment and a start region of an adjacent segment, and wherein a change in the height data between adjacent segments of the trajectory is referred to as a height link. Similarity values for pairs of segments are then calculated based on the ambient signal data, and the segments are grouped using the similarity values. A check for errors in the grouping is then carried out using the height links, and the grouping is accepted if there are no errors, or if there are errors, the grouping is re-run and/or one or more segments are removed from the grouping process. Once the grouping is accepted, they are assigned to floor identifiers.