subject: Crime Scene Forensics - Getting Tire Impressions From the Crime Scene [print this page] Crime Scene Forensics - Getting Tire Impressions From the Crime Scene
These tire impressions could suggest what vehicle was used to transport a dead body to or from a scene of a murder.
How do investigators go about getting tire impressions for study in the crime lab?
In this article, I will discuss what two-dimensional and three-dimensional tire impressions are and how CSI's go about obtaining them for forensic analysis.
If you drive your vehicle over a firm surface such as mud or paper, you will leave what is known as two-dimensional tire tracks in the wake. Suppose you drive across your lawn after a mild shower. You will leave behind what is known as three-dimensional tire tracks. It is these same tracks that your vehicle will leave on any malleable substance such as mud or snow. CSI's have several methods for documenting these different types of tire tracks.
Obtaining two-dimensional tire impressions
Crime scene investigators normally take pictures of two-dimensional tire tracks from a 90-degree angle relative to the ground. By doing so, this helps to minimize any distortions. Next, they put a ruler next to the tracks to display the size of the tires and their tread patterns in the pictures.
Some two-dimensional tracks are latent; i.e. not apparent to the naked eye. In that case, CSI's many times use laser or some sort of light source to get the tire tracks to become more visible so they can be documented and photographed.
CSI's then obtain a comparison tread pattern from suspected vehicles by applying ink to the tires, rolling them down long pieces of white paper, and comparing those tire tracks with the ones taken from the crime scene. Recording the entire tread pattern on paper is very important. It is important because documenting patterns from manufacturer defects and from unique tread wear are essential to making a perfect match. Recording only a foot or two of the entire circumference of the tire may exclude this crucial information.
Obtaining three-dimensional tire impressions
In contrast with two-dimensional tire impressions, CSI's take pictures of three-dimensional tire tracks from an angled source of light. This helps to expose detail and depth in the photograph that one would normally not see in a two-dimensional tire impression. After taking pictures, CSI's then make what is known as a cast of the tire impression. Castings are advantageous because they display more detail such as tread depth and design contours and expose any uneven tread wear. Photographs are not as likely to disclose such details.
Castings can also be applied with ink and used to make two-dimensional impressions on white paper. These two-dimensional impressions can then be compared with any other two-dimensional impressions obtained from a suspected vehicle.
Having detailed tire impressions can be crucial to helping solve a crime.
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