Footwear and Tire Impression Examinations
Contributed by Cheryl Lozen
TA footwear or tire track impression left at a crime scene can be utilized by an investigator in an effort to establish if there is a connection between the impression and some suspected known footwear or tire. Other aspects of the crime may also possibly be determined such as the number of persons or vehicles involved, mode of entry, direction of travel, make and model of footwear or tire, etc.
Impressions can be three-dimensional when left in snow or soft soil, or they can be two-dimensional when a dirty, bloody, or wet origin impression is left on a surface. Questioned impressions from crime scenes can be photographed, lifted, or cast with a variety of materials to preserve for further examination. Many methods of enhancement can be performed to help visualize and collect the scene impressions as well.
Footwear and tire impressions are examined in a similar fashion to each other. Examinations are performed utilizing various visualization and enhancement methods including lighting techniques, photography, magnification, chemical enhancement, powdering, lifting, and processing with computer software.
The primary task of a footwear/tire track examiner is to determine whether or not a particular footwear or tire could have made the scene impression. If there is sufficient detail to do so, a comparison of physical size, tread designs and/or general wear patterns can lead to an elimination or an association. The questioned impression and known footwear or tire must correspond in the class characteristics of design, physical size, and general wear (if observed) in order to be associated. .
The highest degree of association expressed by a footwear and tire impression examiner is an identification. This requires that the questioned impression and the known footwear or tire share agreement of class characteristics along with randomly acquired characteristics (such as scratches, cuts and stone-holds) of sufficient quality and quantity. It will be the opinion of the examiner that the particular known footwear or tire was the source of, and made the questioned impression.
If there is agreement of class characteristics but insufficient detail to compare randomly acquired characteristics (or they are absent) the resulting opinion of the examiner will be that the known footwear or tire is a possible source of the questioned impression and therefore could have produced the impression. Other footwear or tires with the same class characteristics observed in the impression would also be included in the population of possible sources.
If there are unexplainable differences in class characteristics and/or randomly acquired characteristics, the resulting opinion of the examiner will be that the known footwear or tire did not produce the questioned impression.
An unknown footwear or tire impression can also be searched in a laboratory or commercially available database to determine the possible make and model of shoe or tire that could have made that impression. There are no databases available at this time however, that contain all makes and models of footwear and tires.
Calculations of tire track width, turning diameter and wheel base measurements at the crime scene can be utilized to generate a list of possible makes and models of vehicles that have similar measurements to the crime scene tire track impressions.
Comparison of barefoot morphological and sock-clad foot impressions from known standards to crime scene impressions is also performed by some examiners. This may also include comparison of barefoot morphological impressions on a shoe insole to known standards in order to identify the possible owner of a shoe from discarded or lost footwear. Barefoot morphology refers to the shape of the foot, and does not include the examination of friction ridge skin impressions.
References
www.theiai.org
1. Bodziak, W. J., Footwear Impression Evidence, 2nd ed., CRC Press: Boca Raton, FL, 2000.
2. Bodziak, W. J., Tire Tread and Tire Track Evidence, CRC Press: Boca Raton, FL, 2008.
3. Abbott, J. R., Footwear Evidence, Charles C. Thomas Publisher: Springfield, IL, 1964.
4. Cassidy, Michael J., Footwear Identification, Canadian Government Publishing Center, 1980.