FVS can take accurate measurements from imagery
Photogrammetry is the scientific process of obtaining reliable information about physical objects and the environment through the processes of recording, measuring and interpreting photographic images.
This very skilled process means we can accurately compare a person or object in CCTV footage against an object of known height.
FVS use the ‘known-scale’ photogrammetry technique, which has been developed from scaling and measuring methods taught by the Joint School of Photographic Interpretation (JSPI). This is a very accurate and tried and tested method, used by photogrammetrists in the Armed Forces and other leading UK forensic imagery companies.
This is how it works — using a known scale, in this case a full-size scale bar, the subject’s height is compared with that of the scale bar in a reconstruction exercise.
FVS typically use a scale bar (pictured below) to obtain measurements, however any object of known dimensions can be used. We have previously used the known length of a London Taxi captured in footage to determine the length of another vehicle that was travelling behind it.
FVS can also accurately plot distances and angles of incedence. In this case (below), FVS were asked to plot the angle and spread pattern of a shotgun fired at a St Neots' nightclub in a 'drive-by' shooting, by plotting the location of holes in the doorframe against the offenders' vehicle as it drove past.
We were also asked to compare the offenders' vehicle with one recovered by police. This involved a reconstruction exercise where the vehicle was driven past the camera to enable a like-for-like comparison.
How accurate is Photogrammetry?
Height can vary depending on where a person is in their stride and stance, so to counteract this we select images where a person is standing on a straight leg and displaying their full height.
It is also accepted that a person standing bolt upright is unnatural and does not represent their height when standing in relaxed position. All photogrammetrists (and many scientific disciplines) quote a tolerance or ‘margin of error’ in their work. FVS offer a tolerance of +/- 1.5%, which typically provides a range of +/- 1” (one inch).
In practice, this means that a measurement of 5’ 6” +/- 1” means that if a suspect is between 5’5” - 5’7” they would be a possible contender on height and therefore cannot be eliminated. If, for example, the subject's height is obscured by a tall hat we will measure to the eye-line.
Below is a video produced by FVS briefly outlining the procedure.