The invention provides a method of generating data relating to a vertical structure (20) of a built environment comprising a plurality of floors (24, 26, 28), the method comprising: receiving altitude change data representing changes in altitude within the built environment of a plurality of mobile devices (1a, 1b, 1c, 1d) or from which changes in altitude of a plurality of mobile devices within a built environment comprising a plurality of floors can be determined; determining a parameter relating to an estimated storey height of the building from received said altitude change data; and processing received said altitude change data and the said parameter relating to the estimated storey height to thereby generate data relating to the vertical structure of the multi-storey building.