Pipeline mapping system

A system comprises a plurality of magnetic field sensors 16 arranged in an array of fixed spacing l for detecting and locating a buried pipeline 4 or other elongate structure that is not visible for inspection. The sensors are arranged to be spaced apart in a direction transverse to a longitudinal axis of a remote elongate structure to be located. Each sensor senses a magnetic field of the structure induced by the Earth's magnetic field in at least two orthogonal directions. From the known spacing of the sensors and the magnetic field readings, an angular spacing of the magnetic field sensor about the longitudinal axis is determined, from which the location and depth of the structure is identified. This system can help account for ground movement affecting the pipeline or inaccurate recording of its location at the time of laying and avoids potential pipeline damage during excavation.