Recovery of 3D footwear impressions using a range of different techniques.

Three-dimensional (plastic) footwear impressions are frequently found at, or in the vicinity of a crime scene, and may provide a valuable form of evidence or intelligence. This paper compares the traditional methods of casting and/or two-dimensional photography with Structure from Motion (SfM) photogrammetry. We focus both on the recovery of class characteristics (sole pattern) and randomly acquired characteristics caused by damage. We examine how different recovery techniques influence visualization of outsole features and discuss what effect this may have on evidential value. Five shoes and their associated three-dimensional impressions made in both sand and soil were compared using a grid system and tread descriptors commonly used in the UK. We conclude that within the limitations of this study SfM photogrammetry allows superior levels of visualization of both class and randomly acquired characteristics, giving a better definition in detail in some instances. The use of SfM as a complementary approach can therefore lead to a potential increase in evidential value.