1. A method for registering coordinate systems, comprising the steps of: positioning a calibration jig having markers that are opaque to an imaging modality in an imaged area, wherein the markers comprise at least two collinear marker quadruples having respective marker cross ratios; producing with the imaging modality an image of the markers; detecting the markers on the image; on the image identifying collinear image quadruples of the detected markers; computing respective image cross ratios of the collinear image quadruples; defining candidate lines by associating at least one collinear marker quadruple and at least one collinear image quadruple that have matching marker cross ratios and image cross ratios according to predefined criteria; performing respective registrations of the calibration jig with the image using pairs of the candidate lines; determining residuals of the registrations; and selecting one of the registrations wherein the pair of the candidate lines thereof is associated with a qualifying residual that is smaller than a predetermined value.