1. Apparatus for defining a 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 the 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 digital image signals, one said set of digital 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 digital image signals therefrom, said computing means being operable to: establish a workspace coordinate framework in three dimensions from a first selected one of said pairs of images made when said fiducial means is positioned within said workspace, determine workspace coordinates in said workspace coordinate framework of any selected point which can be identified from both images of said first selected image pair, receive a set of scan coordinates for each of three or more selected landmarks present in a scan made in a scan coordinate framework, and correlate said scan coordinates with the workspace coordinates of said landmarks as derived from one of said image pairs, said landmarks being selected from the group of anatomic features, surgical implants, and radiologic implants, compute conversion functions for converting the scan coordinates of a selected feature in said scan to corresponding workspace coordinates in said workspace framework, and for converting observed workspace coordinates of a selected feature observable in both of said images of a selected one of said image pairs to corresponding scan coordinates in said scan coordinate framework, and perform said conversion functions for user-selected features found in said scan or in a subsequent selected one of said image pairs.