1. A method for providing a digital model of at least a part of an object referenced to a global coordinate system, comprising, for the or each said part: (a) providing an initial digital model representative of the respective part using an optical device at a respective relative location with respect to said object, wherein said initial digital model is referenced to a local coordinate system associated with said optical device, and wherein said optical device comprises a plurality of visually exposed optical markers, said markers being located at known locations with respect to said optical device referenced to a marker coordinate system; (b) determining said location of the optical device in step (a) with respect to an external camera arrangement, based on relating the relative locations of said markers with respect to one another, as they appear from a viewpoint of said external camera arrangement, to the relative locations of respective said markers with respect to one another as referenced to said marker coordinate system, and wherein said viewpoint is at a known location referenced to said global coordinate system; and (c) generating a transformed digital model representative of the respective part and referenced to said global coordinate system, based on modifying said initial digital model according to said determined location.