1. A method of finding a transformation which relates data points in a first data set to data points in a second data set, comprising calculating for each of a plurality of candidate transformations a likelihood measure representing the likelihood that the candidate transformation correctly represents the relationship between the two data sets, the likelihood measure being based on local parametric joint and marginal probability density functions (pdf) calculated for the pairs of data points in the two data sets which are regarded as corresponding under the candidate transformation, the parameters of the functions being set by maximising a local likelihood criterion for the pairs of data points.