A method of processing data is privided. The method comprises the steps of: comparing first and second samples of a space, the first and second samples being related by a set of transformations, to provide a set of pairs of signals, each of said pairs of signals corresponding to substantially the same feature in the space in the first and second samples; for each of the pairs of signals, evaluating residuals corresponding to the differences between the values of the space in each sample; evaluating a first estimate of the set of transformations from the residuals; estimating a tensor field representing orientation of flow in the space, the tensor field covering at least one point in the space; and recalculating, from the first estimate and the tensor field, the residuals for each of the pairs of signals to improve the estimate of the set of transformations.