Pairwise approaches to registration can produce reasonable correspondences, but suffer from the problem that the results obtained depend on the choice of reference. Refinements of the basic approach are possible, where the reference is initialised and updated so as to be representative of the group of images as a whole. It is important to note, however, that even in this case the correspondence for a given image is determined solely by the information in the image and the reference. More recently, there has been considerable interest in groupwise methods which aim to make more systematic use of the information in the complete set of images when establishing correspondence. The remaining two methods we tested fall into this category.