Abstract: A magnetic field sensor for an automated guided vehicle is disclosed which includes a magnetisable rod (62) located in proximity to sensor element (60) so that the rod (62) becomes magnetised to create a secondary magnetic field which is detected by the sensor element (60) to thereby increase the localised magnetic flux density produced by a magnet located in the roadway. The sensor also includes a power switching circuit (90, 93) which selectively powers on and off the sensor element (60) to perform a power reset to the sensor element (60) to eliminate hysteresis within the element (60). The sensor (60) is preferably included in an array which comprises a central section having sensor element (60) spaced by a first distance and side section on each side section on each side of the central section in which the sensor elements (60) are spaced apart by a second distance larger than the first distance.