1. A method, comprising: accessing a plurality of frames of images, the frames having pixels; selecting pairs of the frames; for each of the selected pairs, finding a correspondence between the pixels or regions in the frames in the pair, including estimating a set of difference transformation parameters for each of the selected pairs of frames, a difference transformation parameter in the set for a given pair being the difference between a first set of unknown transformation parameters for a first member of the pair and a second set of unknown transformation parameters for a second member of the pair; associating a confidence with the difference transformation parameters in the set for each of the selected pairs of frames; and assigning a coordinate system to each of the plurality of frames that is consistent with the correspondence for each of the selected pairs of frames, including weighting the difference transformation parameters in the set for each of the selected pairs of frames based on the associated confidence.