The specificity and reproducibility of general Schallamach pattern on heel part of rubber outsole

The Schallamach pattern is an acquired characteristic that is sometimes observed in crime scenes. However, the detailed pattern is not always visible, although its general pattern is still recognizable. To determine the discrimination power (DP) of the general Schallamach pattern (GSP), 55 outsoles worn by 33 volunteers were chosen for the experiment. Their prints were lifted after different intervals for six months using magnetic powder and self-adhesive lifters. After the prints were segmented into grids using Photoshop CS6, the presence/absence, angle, and density of the GSP were observed and measured in each grid. There were 18747 pairs of same-source samples among the 1447 dust impressions. An algorithm (weight combination of the presence/absence, angle, and density) was determined when a weight combination of the parameters considered recognized most of the same-source sample pairs. The results showed that the angles of the GSP on the left and right shoes were concentrated in the range [-5.5°, -20.43°] and [8.8°, 24.97°], respectively, and the DP was only 37.5%. With a simply optimized algorithm, the DP reached 54.82%, and the DP of sample pairs lifted within one day reached 79.04%. The study demonstrates that one can distinguish the Schallamach pattern on the heel part of the outsole without information regarding the presence/absence, angle, and density of patterns.