Abstract: Corrections to be applied to the motion of a tunnelling shield to maintain or restore motion of the shield along a predetermined path are determined by directing a beam of collimated light in a direction parallel to or at a fixed angle with the axis of said predetermined path; focussing the beam on to the input end of an optical cable carried by the shield and comprising a plurality of optical fibres, the input ends of the fibres being arranged in an array in which the position of the input end of each guide relative to that of another is fixed; and displaying light transmitted through the optical cable or guides on to a screen at which the output ends of the guides are so arranged as to facilitate ready determination of any correction required to be applied to the motion of the shield. The method may be employed to indicate which propulsion rams of a tunnelling shield should be operated to apply the necessary correction to the motion of the shield.