1. A method of pruning a reference library of signatures and corresponding known structure parameter vectors, each signature comprising a reference signal vector, the method comprising the steps of: for a given reference signal vector, interpolating a parameter vector assuming that the given reference signal vector and its corresponding parameter vector were deleted from the reference library; and deleting the given reference signal vector and the corresponding parameter vector if an interpolation error between the corresponding parameter vector and the interpolated parameter vector is less than a threshold value.