1. An image guided surgery system comprising: an imaging device having an x-ray source and an x-ray receiver for generating a plurality of two-dimensional images of a body part; a localizing device for determining the three-dimensional locations and orientations of the imaging device and a surgical tool; an imaging model with imaging model parameters for modeling the projection and mapping of points between the x-ray source and the x-ray receiver of the imaging device to the images wherein the imaging model parameters are dependent on the orientation of the imaging device; first means for determining first imaging model parameters while the imaging device occupies an orientation; storage means for storing a set of the first imaging model parameters and corresponding imaging device orientation for a plurality of imaging device orientations; and second means for determining second imaging model parameters corresponding to a given orientation of the imaging device and calculated from the set of first stored imaging model parameters and corresponding imaging device orientations; wherein the imaging model and the second imaging model parameters for the given orientation permit accurate superposition of a representation of the surgical tool on the images of the body part.