1. A correlation apparatus comprising, storage means containing respective storage location for storing data relating to a first succession of representations of a scene, a second succession of representations of the scene corresponding to the first succession of representations but having a scale less than that of the first succession of representations, and a reference representation of at least a portion of the scene having a scale greater than at least the initial representation of the second succession of representations, first comparing means for comparing the reference representation with the first succession of representations to form respective first comparison signals indicative of the degree of similarity between compared representations, second comparing means for comparing the reference representation with the second succession of representations to form further respective comparison signals indicative of the degree of similarity between compared representations, further means for comparing the first comparison and further comparison signals, and control means for replacing the reference representation with a selected one of the first succession of representations in dependence on the comparison of the further comparison means.