Abstract: A method of aligning rail parameter measurements comprises receiving measurement data indicating variations in a parameter of the rail along the length of the rail derived from measurement runs performed at different points in time, each item of measurement data comprising a value for the parameter and an estimate of the location at which the parameter was measured. A signal is formed corresponding to each measurement run. Pairwise relative offsets are found between spatially overlapping segments in the signals corresponding to consecutive windows within the signals. For each signal and window the average relative offset to other signals is computed. Errors in the location estimates are determined based on the computed average relative offsets. The determined errors are used to refine the location estimates in the signal data.