A method (20) is described for optically measuring the three-dimensional location of one or more wires W, in a group of wires W1-Wn, such a overhead power cables in an electric rail system. A first step (22) comprises obtaining stereoscopic image data for each of the wires W from the first and second spaced apart stereoscopic camera pairs 10a and 10b which lie in the common plane P1. At step (24), image data from the first and second stereoscopic camera pairs 10a and 10b is processed to identify each of the wires W in the region of interest (12). At step (26), a determination is made of the location in 3D space of selected identified wires W using image data from one of the cameras C1 or C2; and, C3 or C4 in each of the first and second camera pairs 10a and 10b.