1. A system for measuring the position of a hand-held tool relative to a body in at least five degrees of freedom, the system comprising: (a) a rigid or semi-flexible tool having a distal end for insertion into the body, and a proximal portion for manual manipulation outside the body; (b) a marker arrangement comprising a patch configured to be applied to an external surface of the body at a point of insertion of the tool into the body, said patch carrying a plurality of fiducial points; (c) a camera for generating images; (d) a mechanical linkage mounting said camera on said proximal portion of said tool such that: (i) said camera moves together with said proximal portion of said tool, and (ii) said camera is directed with a field of view including at least part of said patch when said patch is deployed on an external surface of the body and said distal portion of said tool in inserted into the body; and (e) a processing system in data communication with said camera and configured to process images from said camera to determine a position of at least part of said tool relative to said fiducial points.