Forensic Video Analysis
Forensic Video tests the acquisition, preservation, interpretation and presentation of video evidence, including the technical limits that constrain conclusions. It rewards candidates who protect the original and document every transformation.
Acquisition and preservation
- Work from a copy; hash the native export (MD5 or SHA-256) at acquisition and re-verify later.
- Record the recorder make, model and firmware, player version, export method, and any clock offset between the system time and real time.
- Use a write blocker for storage media; never analyse directly on the original device.
Format fundamentals
- Frame versus field: interlaced 1080i carries 50 or 60 fields per second, so a single frame may hold two moments in time.
- Frame rates are often 25, 29.97 or lower, and may be variable; do not assume 30 fps.
- Pixel aspect ratio matters: standard-definition D1 video is 4:3 with non-square pixels, and careless resizing distorts measurement.
- Temporal compression (GOP) means frames depend on others; export losslessly or transcode with full documentation.
Analysis and comparison
- Photogrammetry derives subject height from a known reference using vanishing points or homography; always report uncertainty.
- Enhancement steps (brightness, contrast, stabilisation, super-resolution) must be logged with their limits.
Worked example. A suspect stands beside a door frame measured at 2.03 m. After correcting pixel aspect ratio, four independent measurements place the subject's head between 1.78 m and 1.84 m; the report states a range, not a single height. The screen capture is rejected in favour of the hashed native export.
Traps that cost marks
- Filming a screen or using phone-recorded playback instead of a native export.
- Deinterlacing that discards the field containing the identifying detail.
- Stretching aspect ratio to “fill the screen”.
- Assuming the on-screen timestamp is accurate without documenting offset.
- Promising zoom-and-enhance clarity that the source resolution cannot support.