Forensic footwear evidence can be used in legal proceedings to help prove that a shoe was at a crime scene. Footwear evidence is often the most abundant form of evidence at a crime scene and in some cases can prove to be as specific as a fingerprint. Initially investigators will look to identify the make and model of the shoe or trainer which made an impression. This can be done visually or by comparison with evidence in a database; both methods focus heavily on pattern recognition and brand or logo marks. Information about the footwear can be gained from the analysis of wear patterns which are dependent on angle of footfall and weight distribution. Detailed examination of footwear impressions can help to link a specific piece of footwear to a footwear imprint as each shoe will have unique characteristics.
Types of footwear evidence
Footwear evidence can come in at least three forms, footwear outsole impressions, footwear insole impressions and footwear trace evidence.
Footwear outsole impressions
Footwear outsole impressions are impressions left on an object that was caused by contact with a piece of footwear. These can be left on the ground or raised surface by persons treading over it, left on doors or walls by persons attempting to kick or climb over a wall or even left on other persons after being kicked or stomped on.
There can also be latent impressions not easily visible to the naked eye, on many different surfaces such as floor tiles, concrete or even carpet. Detection may require the use of additional specialized light sources such as portable ultraviolet lighting. Recovery typically includes photography as well as lifting with "gel" or "electrostatic" dust lifters.
Footwear insole imprints
Imprints of a person's foot can be left on the insole (inside) of footwear. Due to the sweat and dirt from the foot the insole impression can be left regardless of the wearers foot being in a sock or other covering. The size and arrangement of foot and toe imprints can be used to determine whether a person has worn a piece of footwear. The analysis and comparisons of foot imprints is part of the discipline of forensic podiatry. The insole will show a virtual image of the bare foot print of the wearers foot. This can be compared to the actual barefoot print of the shoe owner to gain a match. A 3d optical surface scan can then be used to build up a model of the foot itself. Useful in forensic evidence casting and identification. H Farmer BSc Hons Fs 2018
Footwear trace evidence
Footwear trace evidence is trace evidence that is recovered from footwear. Types of trace evidence that could be recovered include skin, glass fragments, body hair, fibers from clothing or carpets, soil particles, dust and bodily fluids. The study of this trace evidence could be used to link a piece of footwear to a location or owner. DNA can be one of the contributing factors in forensic footwear evidence.
Detection of footwear evidence
Footwear impressions can be detected with a variety of methods including: Visually using natural or artificial lighting. Using artificial light sources to provide oblique, coaxial, and polarized light for detection of visible and latent impressions. Using electrostatic lifting devices to lift dusty impressions. Using fingerprint powder to develop latent impressions. Using physical or chemical enhancement methods to develop or enhance faint impressions.
Recovery of footwear impression evidence
Footwear evidence occurs most often as either footwear impressions left in a soft surface, such as mud, or as dust deposits, which are difficult for the human eye to detect. At violent crimescenes footmarks can be left as a result of a person standing in blood and subsequently trailing it as they move around the scene.
Lifting
Footwear impressions can be lifted from surfaces with tools such as adhesive lifters, gelatin lifters or electrostatic lifting devices.
Casting
Evidence left via impressions can generally be recovered utilizing a plaster cast. Initially the impression is isolated by framing the area with a solid boundary. Following this a plaster mix can be gently poured inside the frame; it is generally considered not best practice to pour directly onto the impression. In some cases where the surface is not ideal for casting prior techniques can be utilised to gain a better cast of the impression. Sand can often be fixed in place by applying an aerosol resin or glue although hair spray is often used. Wet mud impressions can be dried using a combination of pipetting water from the surface and applying hot air, often in the form of a hair dryer.