Abstract: To determine the track of an element in terms of the components of its displacement parallel to a predetermined axis, the element is arranged to be movable relative to a pattern having pattern features which considered in directions parallel to said predetermined axis form a series exhibiting a windowing property. By sensing the pattern features traversed by the element and comparing window-length portions of this succession of features with a stored representation of the overall series of features, it is possible eventually to detect an initial location of the element along this series. Further movement of the element can then be tracked incrementally; however, there will generally be a substantial initial track portion preceding the first location fix.