Date of Award
Master of Science
In 2009, a National Academy of Sciences report called for investigation into the scientific basis behind tool mark comparisons (National Academy of Sciences, 2009). Answering this call, Chumbley et al. (2010) attempted to prove or disprove the hypothesis that tool marks are unique to a single tool. They developed a statistical algorithm that could, in most cases, discern matching and non-matching tool marks made at different angles by sequentially numbered screwdriver tips. Moreover, in the cases where the algorithm misinterpreted a pair of marks, an experienced forensics examiner could discern the correct outcome. While this research served to confirm the basic assumptions behind tool mark analysis, it also suggested that statistical analysis software could help to reduce the examiner's workload.
This led to a new tool mark analysis approach, introduced in this thesis, that relies on 3D scans of screwdriver tip and marked plate surfaces at the micrometer scale from an optical microscope. These scans are carefully cleaned to remove noise from the data acquisition process and assigned a coordinate system that mathematically defines angles and twists in a natural way. The marking process is then simulated by using a 3D graphics software package to impart rotations to the tip and take the projection of the tip's geometry in the direction of tool travel. The edge of this projection, retrieved from the 3D graphics software, becomes a virtual tool mark. Using this method, virtual marks are made at increments of 5° and compared to a scan of the evidence mark. The previously developed statistical package from Chumbley et al. (2010) performs the comparison, comparing the similarity of the geometry of both marks to the similarity that would occur due to random chance. The resulting statistical measure of the likelihood of the match informs the examiner of the angle of the best matching virtual mark, allowing the examiner to focus his/her mark analysis on a smaller range of angles.
Preliminary results are quite promising. In a study with both sides of 6 screwdriver tips and 34 corresponding marks, the method distinguished known matches from known non-matches with zero false positive matches and only two matches mistaken for non-matches. For matches, it could predict the correct marking angle within ±5-10° . Moreover, on a standard desktop computer, the virtual marking software is capable of cleaning 3D tip and plate scans in minutes and producing a virtual mark and comparing it to a real mark in seconds.
These results support several of the professional conclusions of the tool mark analysis community, including the idea that marks produced by the same tool only match if they are made at similar angles. The method also displays the potential to automate part of the comparison process, freeing the examiner to focus on other tasks, which is important in busy, backlogged crime labs. Finally, the method offers the unique chance to directly link an evidence mark to the tool that produced it while reducing potential damage to the evidence.
Laura Denise Ekstrand
Ekstrand, Laura Denise, "Virtual tool mark generation for efficient striation analysis in forensic science" (2012). Graduate Theses and Dissertations. 12849.