1. Apparatus for defining the location of a medical instrument relative to features of a medical workspace including a patient's body region, comprising: workspace imaging means positionable for producing a plurality of pairs of images of a medical workspace, each of said image pairs comprising two images made along one of each of a different one of two sightlines, said sightlines intersecting at an angle; digitizing means operably disposed for digitizing each of said images of said image pairs to produce sets of image signals, one said set of image signals corresponding to each of said images; fiducial means removably positionable in said workspace for providing a series of fiducial points held in fixed spatial relation to one another; and computing means connected to said digitizing means to receive said image signals therefrom, and including memory means for storing pattern recognition data and instrument structure data correlated to each of a plurality of different medical instruments, said computing means being operable to: establish a workspace coordinate framework in three dimensions from one of said pairs of images made when said fiducial structure is positioned within said workspace, determine workspace coordinates in said workspace framework of any selected point which can be identified from both images of said pair, recognize one of said medical instruments when it is visible in both images of one of said pairs, and compute workspace coordinates of an operative portion of said recognized instrument.