History and relevance of footwear impressions
The discipline of forensic comparative science comprises the examination of multiple forms of evidence, most notably, latent prints, track impressions (footwear and tire), firearms and questioned documents. Track impressions, however, have been credited to be the oldest form of pattern evidence that employ comparative examination. Track impressions are defined as a single impression or a series of impressions left along a path by a person, animal or thing, after it has moved through that path The utility of these impressions have roots dating back to 9600 BC, when Paleolithic man hunted for food and would gauge the irection of travel or the type of animal, they sought, based off the hoof or footpad tracks in soil. Since then, the observance and understanding of track impressions expanded and it started to develop and become more prominent and relevant in various facets of human life. The first criminal case that shone light upon the relevance of footwear impression evidence came about through the Richardson case in Scotland, in the year 1786. Boot prints found around the body of the murdered victim were retrieved and compared with every individual attending the victim’s funeral, the following day. The crime scene prints led to a match with the outsole design of the boots belonging to a man named Richardson. Since then, the usefulness of footwear impressions in a forensic investigation has gained considerable impetus, through extensive research and studies conducted in the field. Currently, it is one among the many valuable forms of evidence, to be located and retrieved from a crime scene, in order to possibly include or exclude the association of a person’s footwear with a scene.
Basic theory and utility of footwear impression evidence
Footwear impressions form an integral component of crime scene reconstruction by providing valuable information such as the number of persons involved in the event and the interaction between them, the direction of travel, point of entry and exit, the speed of movement and, at times could even aid in determining the physical traits of the individuals involved The frequency of observing footwear impressions, on any crime scene are almost equal or may be even greater than fingerprints. The observance and timely retrieval of these impressions from the scene, however, largely depends on the skill and training of the forensic analysts processing the crime scene. Impressions are created when footwear is pressed or stamped against a surface such as floor or furniture, which results in the generation of a static charge between the footwear outsole and the surface in contact. This static charge could result in deformation of the surface or transfer of material from the shoe to the surface. When material is transferred from the outsole of the shoe to the surface the shoe is in contact with, the impression is deemed as a positive impression. However, if the shoe creates an impression on a dirtied surface by removing material from it, the impression is termed as a negative impression. While footwear impressions may be present at any crime scene, the ability to locate them, largely depends on the degree of clarity of the impression. Based on their appearance, footwear impressions are classified into three types – Visible impressions, Plastic impressions and Latent impressions. Visible impressions are created due to transfer of material from the outsole of the shoe to the surface in contact. These impressions are visible to the naked eye, without additional enhancement or lighting. Examples of visible impressions are bloody shoe prints on a floor or carpet. Plastic prints are three dimensional impressions created on softer surfaces like snow or mud. Latent impressions are those that are not easily visible to the naked eye and require additional processing and enhancement using chemical reagents, powders and alternate light sources. Examples of latent impressions are impressions on a hardwood floor or a window sill. Latent and visible footwear impressions are categorized as two-dimensional impressions, while plastic footwear impressions are considered as three-dimensional impressions. This classification is defined by the characteristics of the surface, on which he impression is deposited. Two-dimensional patterns are those that lie on the surface and are essentially superficial, while three dimensional impressions possess some degree of depth within the surface in contact.