1. A method of locating a three-dimensional model of an object comprising the steps of: providing a first three-dimensional search model of an object, the three-dimensions being three spatial dimensions; determining values corresponding to the first three-dimensional model that are substantially rotationally and translationally invariant in the three spatial dimensions; and, using a processor, searching for three-dimensional models, the models comprising data relating to each of three spatial dimensions, in a database associated with similar substantially rotationally and translationally invariant values within predetermined limits to those of the first model.