This paper describes a method for correcting static errors in the position component of a 6-degree-of-freedom tracker in a projection-based VR system. This method allows users to observe where errors in the environment are significant and correct them interactively. Later touch-up correction is possible as well. This technique is based on superimposing targets in physical space with their virtual images. The only hardware addition to the VR system required is a few precisely placed targets.