Abstract: A survey system for determining the shape of a fixed path along which there may be no line of sight from end to end. The system comprises a mobile optical detector adapted to travel along the path and a plurality of optical sources which can be arranged along the path. As it moves along, the detector receives optical signals from consecutive groups of the sources and processing circuitry in the system deduces the angular positions of the sources and hence the shape of the path. In a coal mine, the optical sources are positioned on mine roof supports and the detector on a mining machine which traverses to and fro along the coalface. The system then determines the shape of the coalface. The frequency with which the detector and processing circuitry scan the sources is compatible wiht the mining machine. The sources can be retro-reflectors, in which case a primary optical source is mounted adjacent to the detector.