Detailed Procedure:

Two-dimensional footwear and tire impressions can be made in any type of substance that can be transferred, such as dust, soil, blood, foodstuffs, etc. Most of the time, two-dimensional impressions are fragile and, therefore, should be carefully protected and processed as soon as possible to prevent degradation. All impressions must be protected from water, wind, equipment, and foot and vehicle traffic until they can be recovered. This can be accomplished by covering with boxes, marking areas with cones or crime scene banner tape, or by posting a guard.

It should be noted that other objects, such as fabrics, can also leave two-dimensional impressions. If another type of object leaves a two-dimensional impression, the collection techniques would be the same as with footwear and tire impressions.

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To collect questioned 2-D impressions:

1. Two-dimensional prints can occur in two ways: when debris is transferred from the footwear or tire onto a clean surface it makes a positive impression, or if the footwear or tire takes a substance away (ie: an impression in spilled powder) it makes a negative impression. Regardless, these impressions should be photographed before any collection or attempts at lifting the impression occur. If using a digital SLR camera, the image should be taken in the .raw or .tiff file setting. Begin by taking photographs in ambient light or with normal flash. The impression should also be viewed with oblique lighting to see if the impression becomes more visible. Place a scale, preferably a right-angle scale (or ruler or tape measure) alongside and parallel to the long axis of the impression, not across the middle. Take closeups of any evident alterations to the tread, such as worn areas, cuts, or divots. When taking photographs of the questioned impression, the plane of the image-bearing surface of the camera (film, or LCD panel for a digital camera) should be parallel to the plane of the questioned impression. A tripod and a small level are useful tools when photographing questioned impressions as they help ensure that the image-bearing surface is level. If the questioned impression is not on a level surface (eg: on a hillside) then attempt to make the plane of the impression and the plane of the image-bearing surface parallel. When photographing tire impressions, take photographs at 1-foot intervals.

2. Wear new, unused, clean latex or nitrile gloves.

3. After photography, if possible, collect the item on which the two-dimensional impression has been made. If needed, tape a clean sheet of paper over the print to protect it. Then, place the item face-up in a rigid box for transport. Be sure to secure the item so that it doesn't move around the container and remove the impression.

4. If the item bearing the impression cannot be transported to the laboratory, contact the lab and describe what substance you believe the impression is made of. They can advise you on the best way to collect the impression if it is possible. For example, an impression in dust may be collected by using an electrostatic lifter or an impression in suspected blood or soil may possibly be collected using a gelatin lifter. The laboratory may also advise the use of chemicals that may enhance the impression, making visible latent portions of the impression. A discussion of these enhancement chemicals is out of the scope of this document.

5. Seal the container with evidence tape. Initial and date the tape.

6. Label the container with identifying information, including case number, date, exhibit number, a brief description, and your name.

7. Store the item in a secure location, such as a locked evidence van or your vehicle, until you are able to transport it to the laboratory.

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Laboratory examination of impressions:

• Type

• Manufacturer identification

• Size

• Unusual wear features

• Trace evidence

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Known footwear and tires can be compared to questioned impressions left at a scene. Associations may be made by comparing tread design and tread design orientation. A positive match may be made if the questioned impression has sufficient individualizing or unique features (eg: cuts, nicks, wear pattern) in common with the known footwear or tire.

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Source:

CFITrainer.Net®. "Physical Evidence at the Fire Scene." Website, video, 2013.

https://www.cfitrainer.net/Training_Programs/Physical_Evidence.aspx

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"SWGMAT Web Site." Scientific Working Group for Materials Analysis, Website, July 1, 2013.