Floor assignment

A method 400 of automatically assigning segments of trajectories 110, 120, 130 to floors of a building 100 comprises receiving 801 data concerning a trajectory from a mobile device152, the data comprising height data and ambient signal data; segmenting 802 the trajectory using the height data, such that a change in the height data marks an end region of a segment. A change in the height data between adjacent segments is referred to as a height link. The method further comprises calculating 803 similarity values for pairs of segments based on the ambient signal data; using the similarity values, grouping 804 the segments into a plurality of groups based on the ambient signal data; checking 805 for errors in the grouping using the height links; accepting 808a the grouping if the checking does not identify any errors, or re-running 807 the grouping if it does; and, once the grouping is accepted, assigning 809 the groups to corresponding floors.