Epipolar photogrammetry: a novel method for forensic image comparison and measurement

We present a new method of photogrammetry, requiring no direct measurements in the observed scene, no availability of the original camera taking the snapshot and no knowledge of its position. The practical setup involves minimal human intervention, with only the datum of another snapshot (which may not be simultaneous to the first one, i.e. can be temporary anterior or posterior) of the environment from a calibrated camera. This largely automatic configuration permits to eliminate usual points measurements data acquisition step (e.g. in forensic photogrammetry), and to get accurate lengths, angles, and areas measurements in the original scene. The crucial observation is that the two snapshots, even if not simultaneous and from different cameras, provide a stereo system. Therefore the correspondence of 8 points in the views gives the epipolar geometry of the stereo setup, and as one of the camera is calibrated, the calibration can be "transferred" to the other camera through the epipolar matrix. This transfer yields a calibration of the original camera (internal parameters and position in the scene) even if it is not available any more, its settings have changed, its orientation is different or it was moved. A related to this "automatic" photogrammetry application is ability to compare relative size of objects captured by different non-co-located cameras at different times, but at the same general scene location.