Example Digital Forensic Work

One of our clients, Accident Consultants, approached us for assistance with some of the technical aspects of a criminal case where they were acting as an expert witness for the defence.

The case involved an Asda van that was travelling on a motorway. It passed through a pair of average speed cameras, that were monitoring the speeds of vehicles passing through roadworks. The speed limit was 50 mph.

As each vehicle passes by an average speed camera, the numberplate is captured by two cameras, and ANPR technology reads the vehicle's number plate. The precise time that this occurs is also recorded. The time is provided both by a GPS signal and also verified against an internal clock which is manually set. This hence gives two independent data points from two sources.

Once the vehicle passes by the second average speed camera, the difference between the two timestamps can be calculated, giving the precise time that the vehicle took to travel the known distance between the two average speed cameras. The average speed can the be calculated from the distance and the time.

In this case, according to the RedFusion Speed Violation Report, the vehicle's average speed was recorded at over 65 mph. Well in excess of the 50 mph speed limit.

Our client, a leading UK forensic expert, had already established:

Cogent Digital was able to ascertain that:

Example Forensic Image Analysis

Below we can see the images provided in the RedFusion speed violation report. Note that identifying features have been redacted for use on this website:

RedFusion Speed Camera 1 Image RedFusion Speed Camera 2 Image

As commercial vehicle fleets often have very similar number plates, it was important to ensure that the two numberplates were the indeed the same. Specifically, optical character recognition software, and people, often have difficulty determining similar letters on numberplates, such as E and F; N and M; C, D, O, and U; and so on, especially when there is dirt on the numberplate, and reflections and shadows come into play. To do this, the number plate is extracted from each image:

Numberplates from both cameras

The images are then contrast enhanced, to separate the letters from the background. If done correctly, this also helps to remove the effects of shadows and reflections.

Contrast enhanced images

The first image is slightly smaller, and taken at a different angle than the second image. So the size and perspective of the image is adjusted to match the second image.

Size and perspective adjusted image

Finally, a difference map of the two images is calculated. There are a number of types of difference maps that can be used, but in this one (actually an addition map) where the images are identical it shows as grey. Any differences will be tend towards white or black, depending on how the images differ. In this case, we can see that the only differences are the outline of the characters of the numberplate. This just means that the size of the characters in each image was slightly different. As we're not seeing any complete sections of characters that are different, then this proves that the numberplates are indeed the same.

Difference map image

Furthermore, we can enhance the images so that we can better look at the vehicle and driver. Here we can take a number of measurements. The height of the driver is the same. His arm is in a similar position. The distance from his seatbelt to the name tag on his jacket is the same. His head is the same shape. The distances from his eyes to his ears and nose are the same. Also there are a number of stickers and other marks on the vehicle which are identical. There are no changes to the vehicle (nothing disappears or appears between the two photographs).

Driver and vehicle image