Non-visible objects which differ in their physical properties from their surroundings by association with a variable strength magnetic field may be detected by a suitable array of sensors which can be moved relative to the object in question. By analyzing the signals from the plurality of the sensors in the array, the position of the object can be deduced relative to the array and the array moved to enable a machining guide thereon to be aligned with the non-visible object. The system is of particular value in locating apertures in wing spars when attempting to fix the skin of the wing on to them where it is important to be able to locate the correct point at which to drill a hole through the skin to coincide with the hole in the spar. By defining a magnetic field in the vicinity of the hole to identify the hole magnetically and using an array of magnetic field sensors in a base with an aperture, it is possible to shift the array so that the aperture is precisely aligned with the non-visible hole.