An impression of a person's footwear in sand, mud, soil or even snow can be used to associate an individual with a crime scene by matching wear patterns, cut marks and other alterations. The strength of the association can depend strongly on the quality of the evidence collected.
Crime Scene Tape
In order to prevent the contamination of footprints during an initial crime scene investigation, the area around footprint evidence should be secured with crime scene barrier tape. In bad weather, further protection of evidence with a small tent or other shelter may be necessary to prevent deterioration or loss of evidence.
According to the California Department of Justice, photography is the most valuable way of collecting impression evidence for later comparison. Standard lens cameras must be used to photograph evidence, and on a tripod, as instant or autofocus cameras are not suitable for making comparisons. The photograph is most useful if it contains a tape measure or ruler at the same depth as the impression to show scale, as well as identifying information such as the case number, evidence number, North arrow and date.
After photographs have been taken, a three-dimensional cast of the footprint can be taken. A liquid casting medium such as Dental Stone, Traxtone or Diecast, often made from reconstituted powder, is poured into and around the footprint. After the amount of time specified by the manufacturer, the cast is hard enough to be removed from the footprint and can be taken to the lab for further analysis.
Some environmental conditions and casting mediums require a frame to be placed around the footprint in order to contain the casting medium after it has been poured. Reusable aluminium casting frames can be purchased that are adjustable in size in order to fit the size of the evidence.
Hardening Spray or Snow Wax
Footprint samples in loose or sandy soils can sometimes benefit from an application of clear acrylic hardening spray, which can help the impression resist the weight of the casting materials without any loss of detail. An aerosol wax is capable of preserving details of impressions left in snow. As well, snow wax can provide insulation against liquid casting materials that are not only comparatively warm but can create an exothermic (heat-producing) reaction as they solidify.
Footprints used as evidence need a reference sample against which they can be compared in order to positively associate an individual with a crime scene. This means that impressions of the suspect's footwear must be taken at the laboratory as soon as possible after an incident. As well, imprints of the footwear of any police officials or other individuals at the crime scene should also be taken in order to rule out possible contamination of evidence.
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