Technical note: The Next Step - a semi-automatic coding and comparison system for forensic footwear impressions.

Footwear impression evidence is a key tool in criminal investigations, connecting suspects to the crime scene; in addition, it may provide valuable forensic intelligence linking different crime scenes in the absence of a suspect. This paper presents the development of a computer database and semi-automatic system for shoeprint comparison. The database is based on structured manual coding of elements by CSI (Crime Scene Investigators). The computer algorithm then compares the pattern information of the query footwear impression to the entire database, returning a list of possible matches ranked by pattern similarity and crime scene geographic proximity. Initial results using a database of 284 footwear impressions from real crime scenes revealed that a "match" (i.e., the same shoe model) was found for 30% of the impressions; in most cases, the "match" ranked within the top five places of the "hit" list generated by the algorithm. Our results confirm that this semi-automatic footwear comparison system is simple, cost-effective and efficient, providing great potential for linking crime scenes.

Location distribution of randomly acquired characteristics on a shoe sole.

Footwear comparison is used to link between a suspect's shoe and a shoeprint found at a crime scene. Forensic examiners compare the two items, and the conclusion reached is based on class characteristics and randomly acquired characteristics (RACs), such as scratches or holes. An important question concerns the distribution of the location of RACs on shoe soles, which can serve as a benchmark for comparison. This study examines the probability of observing RACs in different areas of a shoe sole using a database of approximately 13,000 RACs observed on 386 outsoles. The analysis is somewhat complicated as the shoes are differentiated by shape and contact surface, and the RACs' locations are subject to measurement errors. A method that takes into account these challenges is presented. All impressions are normalized to a standardized axis to allow for inter-comparison of RACs on outsoles of different sizes and contact areas, and RACs are localized to one of 14 subareas of the shoe sole. Expected frequencies in each region are assumed to be Poisson distributed with rate parameters that depend on the subarea and the contact surface. Three different estimation approaches are studied: a naive crude approach, a shoe-specific random effects model, and an estimate that is based on conditional maximum likelihood. It is shown that the rate is not uniform across the shoe sole and that RACs are approximately twice as likely to appear at certain locations, corresponding to the foot's morphology. The results can guide investigators in determining a shoeprint's evidential value.

Recovery via SfM photogrammetry of latent footprint impressions in carpet.

Impression evidence retained in carpet is usually recovered, if at all, in two dimensions via a vertical photograph. Here, we show that recovery is also possible via SfM photogrammetry and this gives good quality results that allow digital measurements both in the x-y plane and by depth (z-axis). This study focuses on recovery from polypropylene carpets which are widespread due to their resistance to wear and low cost. We show how traces can be recovered using both SfM photogrammetry and conventional photography with illumination provided via a crime scene light source. Experiments show that traces are retained for considerable time periods if left undisturbed, in excess of four weeks, but are quickly lost in under 8 hours by subsequent footfall. A simple simulation shows how the movement of an individual can be determined from carpet traces and the value of 3D recovery is illustrated via a set of experiments conducted with barefoot traces. We draw attention to the fact that 3D models allow a more statistical-based approach to be taken to match bare footprints at crime scenes. SfM photogrammetry is shown to provide a useful compliment to existing techniques and therefore worthy of further experimentation and potentially operational use.

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.

Digitally processing an image of a shoe impression in blood.

Shoeprints are valuable crime scene exhibits because, given a reasonable-quality impression and a suspect shoe, the forensic investigator can correlate the impression with the shoe and pin down a suspect. In similarity to bloody fingerprints, a common practice with bloody shoeprints is that the crime scene investigator photographs the impressions at a 90° angle with a scale, develops them with amido Black, and then photographs again. In most cases, the post-development prints will feature better and more details that are usually sufficient to perform a comparison between the impression found at the crime scene and suspect's shoes. This study examined shoeprints in blood that had been collected in an apartment in northern Israel where two bodies were found. The floor tiles in the apartment had featured a colored design which in the post-development photographs of the shoeprints blended in with the blood on the floor. As a result, the shoeprint impression was partial and small details were masked. In the laboratory, we processed the pre-amido Black photograph in several steps designed to increase contrast. The result of this digital processing was a full shoeprint sufficiently clear to display randomly acquired characteristics of the sole and subsequently establish identification between the impression found at the crime scene and suspect's shoes. When chemical amplification is not sufficient, it is worth exploring other methods before proceeding with the comparison, as it is sometimes still possible to extract information from the same data using alternative methods in order to achieve a conclusive result.

Assessing the quality of footwear marks recovered from simulated graves.

Footwear marks are one of the most frequently encountered evidence types recovered from a crime scene and can provide valuable scene intelligence regarding potential suspects. It has been acknowledged that impressions of footwear and tools can be recovered from graves, but previous studies have only focused on tool mark recovery. This has led to a lack of published information regarding footwear mark recovery from graves. It is therefore important to demonstrate whether the recovery of footwear marks is feasible and, if so, under what conditions this can be achieved. To address recovery, this study, placed 60 three dimensional (3D) impressions of footwear marks within 60 simulated graves. This was done to assess time (1, 2, 4 months) and at known depths (20, 30, 40 cm). The footwear marks within the graves were covered with clothing or left uncovered. The shoe's design patterns were grouped and counted in a photographic comparison between the 3D footwear impressions, placed within the test-pits, and any recovered impressions. A grading system was adapted by the authors to score the quality of footwear impressions observed during recovery. The results demonstrate that the preservation and recovery of footwear impressions from graves is feasible. The simulated graves covered with clothing showed better preservation of footwear impressions, but there was no clear evidence that time or depth had an effect. The authors note that careful consideration and vigilant excavation skills are needed when excavating graves which may bear potential footwear marks, as their recovery will lead to an increased amount of intelligence that can link suspects to homicide scenes.

Empirical Evaluation of the Reliability of Photogrammetry Software in the Recovery of Three-Dimensional Footwear Impressions.

This paper examines the reliability of Structure from Motion (SfM) photogrammetry as a tool in the capture of forensic footwear marks. This is applicable to photogrammetry freeware DigTrace but is equally relevant to other SfM solutions. SfM simply requires a digital camera, a scale bar, and a selection of oblique photographs of the trace in question taken at the scene. The output is a digital three-dimensional point cloud of the surface and any plastic trace thereon. The first section of this paper examines the reliability of photogrammetry to capture the same data when repeatedly used on one impression, while the second part assesses the impact of varying cameras. Using cloud to cloud comparisons that measure the distance between two-point clouds, we assess the variability between models. The results highlight how little variability is evident and therefore speak to the accuracy and consistency of such techniques in the capture of three-dimensional traces. Using this method, 3D footwear impressions can, in many substrates, be collected with a repeatability of 97% with any variation between models less than ~0.5 mm.

Dataset of Digitized RACs and Their Rarity Score Analysis for Strengthening Shoeprint Evidence.

In recent years, there is a growing demand to fortify the scientific basis of forensic methodology. During 2016, the President's Council of Advisors on Science and Technology (PCAST) published a report that states there are no appropriate empirical studies that support the foundational validity of footwear analysis to associate shoeprints with particular shoes based on specific identifying marks, which is a basic scientific demand from the field. Furthermore, meaningful databases that can support such studies do not exist. Without such databases, statistical presentation of the comparison results cannot be fulfilled either. In this study, a database of over 13,000 randomly acquired characteristics (RACs) such as scratches, nicks, tears, and holes, as they appear on shoe sole test impressions, from nearly 400 shoe soles was collected semi-automatically. The location, orientation, and the contour of each RAC were determined for all the RACs on each test impression. The statistical algorithm Statistic Evaluation of Shoeprint Accidentals (SESA) was developed to calculate a score for finding another feature similar to a particular scanned and digitized RAC in the same shape, location, and orientation as the examined one. A correlation was found between the results of SESA and the results of real casework, strengthening our belief in the ability of SESA to assist the expert in reaching a conclusion while performing casework. The score received at the end of the process serves the expert as a guiding number, allowing more objective and accurate results and conclusions.

A new method for the recovery and evidential comparison of footwear impressions using 3D structured light scanning.

Footwear impressions are one of the most common forms of evidence to be found at a crime scene, and can potentially offer the investigator a wealth of intelligence. Our aim is to highlight a new and improved technique for the recovery of footwear impressions, using three-dimensional structured light scanning. Results from this preliminary study demonstrate that this new approach is non-destructive, safe to use and is fast, reliable and accurate. Further, since this is a digital method, there is also the option of digital comparison between items of footwear and footwear impressions, and an increased ability to share recovered footwear impressions between forensic staff thus speeding up the investigation.

Inherent variation in multiple shoe-sole test impressions.

Shoeprints left at crime scenes are seldom perfect. Many prints are distorted or contaminated by various materials. Noisy background often contributes to vagueness on the shoeprints as well. Test impressions made from the suspect's shoes in the laboratory are considered a genuine replication of the shoe-sole. This naïve attitude is far from being correct. Consecutive test impressions made in the laboratory under strict similar conditions revealed differences among the exemplars of the same sole. Some of them are minor, but some are major, and can mislead the less experienced practitioners during the comparison process. This article focuses on the inherent within source variability between controlled shoeprints made from the same shoe, as it appears on the RACs. To describe and analyze this variability, repeated test impressions were prepared, and datasets were created. Several RACs were marked on each test impression, using an expert assisting software tool (developed in the authors' lab). The variance in repeated test impressions is demonstrated and possible sources are discussed. This variance should be considered when trying to establish the degree of matching between individual characteristics.

Collection of Wet-Origin Footwear Impressions on Various Surfaces Using an Electrostatic Dust Print Lifter.

Electrostatic dust print lift method is known to be able to recover only dry-origin footwear impression. However, the wet-origin footwear impression could also be recovered using this method. As the amount of dust accumulated before deposition of the wet-origin footwear impression increased, the intensity of the footwear impression lifted with this method became stronger. If the footwear impression is not affected by moisture after it is made, the 28-h old wet-origin footwear impression could be recovered using this method. The intensity of the lifted footwear impression did not decrease significantly even when the number of sequential steps increased as long as the shoe sole is wet. However, when the moisture on the shoe sole depleted, the intensity of the footwear impression decreased sharply. This method has the advantage of being able to enhance the footwear impression without being affected by the footwear impressions deposited in the past.

A comparison of enhancement techniques for footwear impressions on dark and patterned fabrics.

The use of chemical enhancement techniques on porous substrates, such as fabrics, poses several challenges predominantly due to the occurrence of background staining and diffusion as well as visualization difficulties. A range of readily available chemical and lighting techniques were utilized to enhance footwear impressions made in blood, soil, and urine on dark and patterned fabrics. Footwear impressions were all prepared at a set force using a specifically built footwear rig. In most cases, results demonstrated that fluorescent chemical techniques were required for visualization as nonfluorescent techniques provided little or no contrast with the background. Occasionally, this contrast was improved by oblique lighting. Successful results were obtained for the enhancement of footwear impressions in blood; however, the enhancement of footwear impressions in urine and soil on dark and patterned fabrics was much more limited. The results demonstrate that visualization and fluorescent enhancement on porous substrates such as fabrics is possible.