A method of monitoring the shape of a structure , wherein the method comprises: using an optical fiber distributed acoustic sensor deployed in a known spatial relationship with respect to the structure to detect, at a plurality of acoustic sensor points along the fiber , acoustic signals emitted by a plurality of acoustic sources ; wherein the method is characterised by : the structure to be monitored being moveable; the known spatial relationship being such that the optical fiber is co-located and fixed to the structure such that it is known which part of the optical fiber is adjacent and connected to which part of the structure ; the plurality of acoustic sources being deployed at known positions (X1, X2, X3, X4) in an area in which the structure to be monitored is located; and the method further comprising the steps of: calculating relative positions of a plurality of the acoustic sensor points in dependence on the detected acoustic signals from the acoustic sources ; from the calculated positions of the sensor points along the fibre , determining a shape, or shape and position, of the structure in dependence on the known spatial relationship between the fiber and the structure . using an optical fiber distributed acoustic sensor deployed in a known spatial relationship with respect to the structure to detect, at a plurality of acoustic sensor points along the fiber , acoustic signals emitted by a plurality of acoustic sources ; wherein the method is characterised by : the structure to be monitored being moveable; the known spatial relationship being such that the optical fiber is co-located and fixed to the structure such that it is known which part of the optical fiber is adjacent and connected to which part of the structure ; the plurality of acoustic sources being deployed at known positions (X1, X2, X3, X4) in an area in which the structure to be monitored is located; and the method further comprising the steps of: calculating relative positions of a plurality of the acoustic sensor points in dependence on the detected acoustic signals from the acoustic sources ; from the calculated positions of the sensor points along the fibre , determining a shape, or shape and position, of the structure in dependence on the known spatial relationship between the fiber and the structure . the structure to be monitored being moveable; the known spatial relationship being such that the optical fiber is co-located and fixed to the structure such that it is known which part of the optical fiber is adjacent and connected to which part of the structure ; the plurality of acoustic sources being deployed at known positions (X1, X2, X3, X4) in an area in which the structure to be monitored is located; and the method further comprising the steps of: calculating relative positions of a plurality of the acoustic sensor points in dependence on the detected acoustic signals from the acoustic sources ; from the calculated positions of the sensor points along the fibre , determining a shape, or shape and position, of the structure in dependence on the known spatial relationship between the fiber and the structure . calculating relative positions of a plurality of the acoustic sensor points in dependence on the detected acoustic signals from the acoustic sources ; from the calculated positions of the sensor points along the fibre , determining a shape, or shape and position, of the structure in dependence on the known spatial relationship between the fiber and the structure . <EOS>