A carrier, such as an autonomous surface treatment device, e.g., a robotic vacuum cleaner, has motive units, such as wheels, for transporting the carrier over a field of operation. The state of the carrier, which can be its position on the field of operation, is determined based, at least in part, on the actual track width of the motive units which will change depending on the carrier's driving mode. For example, when the carrier is operating in a driving mode where it is turning, the actual track width of the motive units will be different from the actual track width of those units when the carrier is proceeding straight ahead. A determination is made of the driving mode in which the carrier is operating at the time its state is to be determined, and the actual track width of the motive units that corresponds to that driving mode is employed in determining the state of the carrier.