1. A method of deriving a feature tracking transformation to map at least one feature from a first data set onto a corresponding feature from a second data set, comprising the following steps: (a) extracting a first set of points relating to said at least one feature in said first data set; (b) associating said first set of points with a second set of points in said second data set; (c) calculating said feature tracking transformation to at least approximately map said first set of points from said first data set to said second set of points in said second data set, wherein said step of calculating said feature tracking transformation comprises the steps of: (i) for each point in one of said two sets of points, estimating a mapping of said point to a selected point in said other set of points to produce individual point mapping estimates; (ii) combining said individual point mapping estimates to produce an estimated transformation to map said one set of points to said other set of points; (iii) applying said estimated transformation to said one set of points to generate an approximate image position; (iv) repeating said steps (i) to (iii) iteratively to produce an estimated transformation for mapping said other set of points to a set of points from said approximate image position; and (v) deriving said feature tracking transformation for mapping said first set of points to said second set of points from a combination of said estimated transformations.