Tire and shoe impressions are a result of the object impacting a surface and characteristics or features transferring onto the receiving surface. The unknown impression from the crime scene is compared to the known tire or shoe and areas of uniqueness in the impression are documented with the intent to identify the specific source or origin. In general the five most important aspects between the impression and a source are the correspondence of design type, physical size and shape, general wear, manufacturing characteristics and individual identifying characteristics.
Impression evidence at the scene can be associated with the item that made it by class characteristics or individual characteristics. With sufficient individual characteristics, a positive identification between the source and the impression can be made.
Types of impressions examined by laboratory personnel:
- Three – dimensional impressions in soil, snow, sand, mud, etc.
- Two – dimensional impressions on floors, counters, car hoods, paper, skin, etc.