A photogrammetric transformation is presented, which maps photograph image points to the location of the same points on a nominally plane real surface. The transformation uses 4 base points on the real surface that have known locations and is implemented by an interactive program entitled TRANS4; which runs on MS/PC DOS microcomputer systems. The program addresses three of the five major tasks in the process of extracting geometric data from photographs of a traffic accident site and makes the method a realistic option for investigators. The two other tasks are also discussed. The character and relative magnitude of several errors associated with the transformation are discussed, along with techniques for managing the errors.