An apparatus and methods for determining the position of an elongate structure. A number of magnetic field sensors arranged at fixed spacing, each sensor being arranged to sense a magnetic field of a remote structure induced by the Earth's magnetic field in at least two orthogonal directions. In use, the magnetic field sensors are arranged remotely of an elongate structure having a longitudinal axis, such that the magnetic field sensors are spaced in a lateral direction relative to said longitudinal axis. An angular spacing for the magnetic field sensors about the longitudinal axis is determined according to the magnetic field readings in the two orthogonal directions and a distance between one or more of said magnetic field sensors and said elongate structure is determined based on said angular spacing determination.