Technological innovation in the recovery and analysis of 3D forensic footwear evidence: Structure from motion (SfM) photogrammetry.

The recovery of three-dimensional footwear impressions at crime scenes can be a challenge but can also yield important investigative data. Traditional methods involve casting 3D impressions but these methods have limitations: the trace is usually destroyed during capture; the process can be time consuming, with a risk of failure; and the resultant cast is bulky and therefore difficult to share and store. The use of Structure from Motion (SfM) photogrammetry has been used widely to capture fossil footprints in the geological record and while there is a small body of work advocating its use in forensic practice the full potential of this technique has yet to be realised in an operational context. The availability of affordable software is one limiting factor and here we report the availability of a bespoke freeware for SfM recovery and subsequent analysis of for footwear evidence (DigTrace). Our aim here is not to provide a rigorous comparison of SfM methods to other recovery methods, but more to illustrate the potential while also documenting the typical workflows and potential errors associated with an SfM based approach. By doing so we hope to encourage further research, experimentation and ultimately adoption by practitioners.