1. A method for determining by an x-ray projection a position of a target radiopaque marker disposed on a catheter in a subject's body, comprising: assigning a virtual landmark in the body as a reference point; obtaining a multiplicity of images comprising a detectable marker disposed on the catheter; determining an auxiliary location relative to the reference point using at least one additional radiopaque marker disposed on the catheter; and compensating for periodic movements of the body on the multiplicity of images using the virtual landmark; obtaining two optional locations of the catheter tip based on the auxiliary location; selecting a target radiopaque marker location from the two optional locations in which the catheter has the same projection on the multiplicity of images; determining the position of the target radiopaque marker with respect to the reference point; wherein the method is performed by a general purpose computer.