Abstract: A method of determining a mode of transport for a portion of a user's journey is described. The method comprises: receiving, at a portable device being carried by a user, position data for current locations of the user over a period of time; determining, from the position data, a characteristic of the speed for the journey portion; identifying possible modes of transport on the basis of pre-established ranges of speed characteristics for particular modes of transport; and selecting a most likely mode of transport from the identified possible modes of transport based on the proximity of the user's position data to known public transport nodes. The method can also be used to determine the carbon footprint of a user using a portable device.