A measurement system processes data representing an images of an optical target, e.g. on a contact probe, to determine position of each of a number of points on a vehicle or other object. The system uses two reference frames and processes an image of the two frames, to define a three-dimensional (3D) coordinate system, for example, in relation to a designated first frame. The image processing determines the position of the other frame in that coordinate system. For any measurement in which the first reference frame is visible in an image with the probe, processing directly determines position of the point in the 3D coordinate system. For any measurement in which that reference frame is not sufficiently visible, but the second reference frame is sufficiently visible, the image processing determines position relative to the second reference frame and transforms that position into a measurement in the defined three-dimensional coordinate system.