Section 5.5: Impression Evidence
Tire Impressions
A comparison of the accident/crime scene impressions can result in an identification, inclusion or elimination of a tire. Impression(s) can be found in a variety of substrates including soil and snow, as well as on cement and asphalt. The evidentiary value of a comparison usually depends upon the quality of the impression and the manner in which it is documented and collected.
The decision to cast is affected by the conditions of the substrate that the impression is in or on. Impressions in fine, humus soil, wet sand, and even snow are often excellent candidates for casting. Coarse substrates may not always the best substrate for retaining detail of the tire impression(s). It is recommended that all impressions are photographed and cast, to recover the maximum amount of impression detail.
Photography is a valuable way of collecting impression evidence for later comparison. As with all evidence, overall photographs should be taken using a standard-format lens showing the impressions in relation to the other features of the scene. It is critical that distortions are minimized by adhering to the following: Impression photography requires the use of a tripod and detachable flash. Documentation must include a photograph with a measurement scale. The scale should not be placed over or across the impression. The scale must be level with the bottom of the impression and be approximately the same size as the impression for proper documentation. The scale should contain case identification information. Case number Orientation Documentation should indicate the direction of travel, if this can be determined.
The camera should be mounted on a tripod directly over the impression, with the film plane parallel to the impression. The impression should be shaded from direct sunlight. It is recommended that the detachable flash or other light source be at an angle of 45 degree or less depending on the depth of the impression. A variety of flash angles are recommended. These oblique light photographs should be taken with the direction of the flash coming from at least 3 different directions around the impression. Tire impressions are photographed in an overlapping series that should continue the length of the impression in which detail is present. Each frame should overlap by approximately 20%, and no more than two feet should appear in each frame. A scale or tape measure placed the length of the track will help reconstruct the length of the entire impression from the separate photographs. Single-lens reflex or similar cameras with changeable lenses should be used for capturing impression evidence.
Tire & Vehicle Measurement
Tire Vehicle Measurements The following measurements should be recorded: The track width of a vehicle is the distance between the center of the tire mounted on one side of the vehicle to the center of the tire on the opposite side. The front and rear track widths may be different. The wheelbase of a vehicle is the distance between the center of the front axle and the center of the rear axle.
Measure the track width and wheelbase recorded in the impressions, if possible: The track width may be measured from the inside of one tire track to the outside of the adjacent tire track, if they can be determined to be a pair made by a single vehicle. If the positions of the front and rear tires can be determined where a vehicle stopped, these positions can be measured to determine an approximate wheelbase measurement.
After photography, casting may be performed to document the impression in three-dimensional form. The decision to cast is affected by the substrate conditions and other environmental factors. Impressions should be photographed before casting. Do not remove soil adhering to the cast or attempt to clean the cast after recovery as this may damage cast detail. Place each casting in a protective, breathable container after drying.
Chemicals and/or powders may be used to enhance or optimize impression(s). It should be determined prior to chemical application if a sampling of the blood is required, as the chemicals used to optimize the impression(s) may interfere with DNA analysis.
Forensic light sources may optimize visualization and photography of the impression(s). Examination of the impression(s) using a forensic light source may be conducted prior to applying any chemicals or powders. Blood does not fluoresce, but views as black in infrared (IR) range and may offer contrast between the impression and the substrate of the item it is on.
Footwear Impression Evidence
A shoe or boot may be associated or disassociated to a crime scene by footwear impressions. A comparison of the crime scene impressions can result in an identification, inclusion or elimination of a footwear outsole. Impressions can be found in soil, snow, on counters, tile floors, doors, wood and vinyl furniture, paper items, as well as other surfaces. The decision to cast an impression is affected by the conditions of the substrate the impression is in or on. Impressions in fine, humus soil and even snow are often excellent candidates for casting. Coarse substrates may not always be the best substrate for retaining detail of the tire impression(s). It is recommended that all impressions are photographed and cast, to recover the maximum amount of impression detail. Documentation (also see Photography) Photography is a valuable way of collecting impression evidence for later comparison. The evidentiary value of a comparison usually depends upon the quality of the impression and the manner in which it is documented and collected. Overall photographs should be taken using a standard-format lens showing the impressions in relation to the other features of the scene. It is critical that distortions are minimized by adhering to the following: Impression photography requires the use of a tripod and detachable flash. Documentation must include a photograph with a measurement scale.
The scale should not be placed over or across the impression. The scale must be level with the bottom of the impression and be approximately the same size as the impression for proper documentation. The scale should contain case identification information: Case number Orientation The camera should be mounted on a tripod directly over the pattern, with the film plane parallel to the impression. The impression should be shaded from direct sunlight for flash photographs. It is recommended that the detachable flash or other light source should be at an angle of 45 degrees or less depending on the depth of the impression. A variety of flash angles are recommended. These oblique light photographs should be taken with the direction of the flash coming from at least 3 different directions around the impression. The entire impression should be captured in one frame unless overlapping photographs are needed to capture sufficient resolution.
After photography, casting may be performed to document the impression in three-dimensional form. The decision to cast is affected by the substrate conditions and other environmental factors. Impressions should be photographed before casting. Do not remove any soil adhering to the cast or attempt to clean the cast after recovery as this may damage cast detail. Place each casting in a protective, breathable container after drying.
Two-dimensional Footwear Impressions
Two-dimensional impressions are usually a deposit or removal of material to or from a surface. These may be found on paper items, doors, counters, tile floors, and other hard surfaces/substrates. There are generally two ways footwear impressions are made: by the removal of dust or other material from a surface by adhering to an outsole leaving a void (negative) impression the deposition of a material or contaminate such as blood, dirt, and oil present on a footwear outsole, transferred to a surface, leaving an impression If possible, submit the entire item that has the impression on it. If that is not practical, the impressions may be lifted using various techniques such as: Electrostatic dust lifter Gel print lifter Tape or clear adhesive material (if no other material is available)
Chemicals and or powders may be used to enhance or optimize impression(s). It should be determined prior to chemical application if a sampling of the blood is required, as the chemicals used to optimize the impression(s) may interfere with DNA analysis. Forensic light sources may optimize visualization and photography of the impression(s). Examination of the impression(s) using the forensic light source may be conducted prior to applying any chemicals or powders. It should be noted that blood does not fluoresce, but views as black in IR range and may offer contrast between the impression and the substrate of the item it is on.
When making a cast, be prepared to act quickly and methodically. Time is often a critical factor in successfully making a cast. When casting tire impressions, cast as much of the tire impression as possible. Use dental stone for casting in dirt, soil and sand as well as snow. Sulfur may be used for casting in snow. Impression coating materials such as SnowPrint Wax, stabilizers (e.g., hairspray) and highlighters (e.g., paint) may be used depending upon conditions present. Casts should be marked with the impression identifier, general evidence collection information (date, case number, etc.) and with a directional indicator. Casting material should be allowed to thoroughly harden before removal from surface.
Follow the manufacturer’s instructions for preparing the casting material. A sturdy plastic bag is used for mixing dental stone for footwear impressions, a bucket is usually necessary for mixing casting material for tire tracks. The initial pour of the casting material should occur off the impression to avoid damaging detail; the casting material should be allowed to flow into the impression.
Casting Impressions with Dental Stone
If necessary, prepare impressions for casting. When casting a fragile impression, it may be necessary to apply a fixative. Care should be exercised when applying fixatives to minimize any possibility of damage to the impression. When casting in dense soils, it may be necessary to apply a release agent. Care should be exercised when applying release agents to minimize any possibility of damage to the impression. Add appropriate amount of water to pre-measured amount of dental stone. The average footwear impression requires approximately two (2) pounds of dental stone and approximately ten (10) ounces of water.
The amount of water required may vary depending on the casting product. The resulting mixture should have the viscosity of heavy cream. The viscosity of the mixture may need to be adjusted based upon the nature of the impression. Mix continuously for a minimum of 3–5 minutes so that the powder can thoroughly absorb the water. Pour casting material carefully outside the perimeter of the impression and direct the flow into the impression.
Ensure the impression is completely filled and/or covered evenly. In the event that the casting material does not flow completely into the impression, the top surface of the casting material can be agitated to help it flow. Casts should be of sufficient thickness to avoid breakage. If necessary, additional casting material may be poured over the top of the original cast to complete the cast and/or add thickness. For fragile and shallow impressions: pour casting material from outside the perimeter so that it rapidly flows over the impression. A thinner mixture of casting material is necessary for this technique. Avoid pouring directly onto the uncovered impression.
Larger quantities of dental stone can be mixed in a bucket to cast large segments of tire impressions. Impressions under water may be cast using dental stone and specialized techniques. Casting impressions with dental stone and sulfur in snow: If necessary, prepare impressions for casting. It is noted that snow varies considerably in texture and type. Application of highlighting materials (such as Snow Print Wax™ or aerosol paints) may be advantageous during photography.
These materials may or may not be necessary for the casting process. To increase the contrast of the detail, a thin application of highlighting spray may be directed at the impression from an oblique angle. The application of highlighting sprays to the snow impression may increase melting; therefore, the impression may need to be shielded from the sun until it can be photographed and cast. A thick application of SnowPrint Wax™ may be applied if needed before using the dental stone casting material. Casting with dental stone:
Add a heaping tablespoon of Potassium Sulfate to the pre-weighed bag of dental stone. Add snow to the water source and place the bags of dental stone in the snow to pre-cool the ingredients. Add the appropriate amount of water to the pre-measured dental stone. A thicker mixture should be used for snow. Pour the casting material from outside the perimeter and direct the flow into the impression. The surface of the casting material can be agitated to help it flow.
Collect and package debris that may have fallen from the cast when it was removed. Store the packaged debris with the cast. Do not clean the cast. Package the debris in a bindle or other container that will securely store it. Label the container with your name, identification number, and the date and time. Place the cast into the labeled container. When necessary, use shock absorbing and protective materials to cushion the cast.
A cast is delicate and can be easily damaged. Package the cast so that it is stable and secure and the face of the impression is up and protected from disturbance during transport. If a cast is too large to box, wrap it in clean paper and shock-resistant material (such as bubble-wrap). The goal is to seal the impression and protect it from damage during transport. Never discard a cast, regardless of condition when removed from the impression.
Be sure to save and submit all casts to the laboratory. Even a broken cast may be useful during the examination process. Close the container and seal the entire opening with evidence tape. Write your initials and identification number, and the date and time across the evidence tape seal. Make sure that the container is labeled with a description of the item cast, your initials and identification number, the date and time, location and, when possible, evidence number.
Lifting Footwear Impressions in Dust
Photograph the impression before collecting or removing it. An electrostatic dust lifter is appropriate to use only when the impression is left in dry dust. Whenever possible to do so without damaging the impression, collect the object containing the impression. Always use clean gloves when handling evidence. Always collect soil/rock samples from the immediate surrounding area when the impression is on the ground.
Properly photograph the impression before lifting. Label a container and an identification label for the developed impression with your initials and identification number, the date and time, evidence number, location, and evidence description. Make sure that the information on the container matches the identification labels. Each piece of evidence must have a unique number. This number should correspond to the placard next to the evidence. The evidence description includes:
- Type of print collected
- Location of the impression
- Orientation of the impression to north, or to a feature of the object with the impression on it, or to a nearby object
Label the container just before collecting an object, and seal the container immediately after collection. Lifting the Impression When lifting impressions, examiners should use the least destructive method first. If in doubt, treat all impressions as dry-origin and apply the methods listed below. If unsuccessful, attempt collection as indicated for wet-origin impressions. All procedures shall be performed when applicable and noted when appropriate. The order and use of these individual techniques is determined by considerations such as substrate, components of the impression, and environmental conditions.
Electrostatic Dust Lifters
Electrostatic lifting is useful for the detection and lifting of dry-origin dust and residue impressions that are the result of tracking from dry, dirty surfaces onto relatively cleaner surfaces. Electrostatic lifting is normally the first technique used, as unsuccessful attempts will not prevent subsequent lifting and enhancement techniques. There are a number of electrostatic lifters available. Consult the manual provided by the manufacturer for specific operating instructions.
All of these devices utilize a film which has a black side and an aluminum-coated side. The black side of the film is placed against the impression, and a high-voltage charge is applied to the film, resulting in the transfer of the dry dust or residue impression. To visualize the lifted impressions, the lifts should be examined in a darkened room with a high-intensity light source held at an oblique angle to the surface of the lift. The lifting film should never be reused.
Smaller lifts can be stored in individual clean file folders. These folders should never be reused. Larger lifts can be stored by carefully rolling with the aluminum side out. After rolling, the edge can be secured with a small piece of tape. Electrostatic lifts are fragile and impressions can be destroyed by any wiping action across the surface of the lift. Consideration should be given to photographing lifts prior to packaging. Electrostatic lifts retain a charge and should never be packaged in cardboard, cardboard boxes, or plastic bags.
Adhesive and Gelatin Lifters
Footwear-size adhesive and gelatin lifters are used for the lifting of dust and residue impressions, wet-origin impressions, as well as impressions developed with fingerprint powder. Gelatin lifters are available in white, black and clear. White lifters provide greater contrast with impressions enhanced with dark-colored powders. Black lifters provide greater contrast with light-colored powders and residue impressions. Clear lifters normally do not provide good contrast. Gelatin lifts of residue impressions should be photographed as soon as possible after collection.
Adhesive lifters are available in white and clear. They include footprint-sized sheets and various widths of rolled tapes. White backgrounds are recommended for clear adhesive. Clear adhesive on a clear background is not recommended for residue impressions, these lifts are normally used for impressions developed with dark-colored powders. Residue or powdered impressions may also be lifted with tape if other lifting material is not available.
Sections of tape may be overlapped and lifted as a single lift to recover the entire impression intact. The tape-lifted impression should be placed on a contrasting or white background. Rolled tapes are available in five-inch widths and are preferred to narrower tapes. Dental Stone can be used to lift impressions such as mud and tire residues from surfaces such as concrete and tile.
A thick layer of dental stone can be poured over the impression area and lifted when dry. Note that a border of cardboard or other material should be placed around the impression to aid the lifting of the dental stone after drying. Mikrosil™ or other polyvinylsiloxane (PVS) casting materials can be used to lift impressions enhanced with powder, from any surface. These products lift the complete powdered impression and are particularly useful on textured surfaces. Packaging Insert the lifted impression into the labeled envelope. Do not bend or fold the lifted impression to fit it into the envelope. Close the envelope in which the print is stored and will be transported, and seal the entire opening with evidence tape. Write your initials and identification number, and the date and time across the evidence tape seal.
Vehicle Examination Procedures
Process a vehicle with the same caution and detailed approach as you would any crime scene. Obtain warrants as you would other crime scenes if required. Establish well-defined boundaries around the vehicle using boundary markers (such as rope, tape, cones, etc.) to preserve evidence. Photograph the interior and exterior of the vehicle; including the vehicle identification number (VIN). Consider exterior photography from all four corners of the vehicle, towards the center of the vehicle. Collect DNA, fingerprint or trace evidence before moving the vehicle to avoid damaging or losing it when the vehicle is moved.
Examine exterior surfaces for latent prints including the side mirrors, fenders (when a wheel is missing), and the six-inch-wide areas surrounding the sides, hood, trunk, and roof support post. Examine interior surfaces for latent prints including door handles, rear-view mirror, seat belt buckles, windows and window handles/buttons, stick shift knob, and glove box door. Tow the vehicle to a well-covered, dry, secure area, such as a police compound, when a detailed search for evidence is required. Take steps to ensure that any evidence that can fall from the vehicle during towing can be retained; e.g., placing a tarp below the car or stabilizing bullet holes in glass with tape.
Maintain and continue to protect the boundary around the vehicle until it is towed. Documentation Record critical information about the vehicle before it is moved. Information to record in your notes includes: Odometer reading Gas level Apparent damage State of windows, head and tail lights, turn signal lights and mirrors Ambient temperature and radiator/hood temperature Possible reason for vehicle being at the location Whenever appropriate, wrap all stray vehicle parts related to the vehicle in clean paper. Lay the object on the paper and fold the paper around it.
Label the container for the object with your initials and identification number, the date and time, evidence number, location, and evidence description. Each piece of evidence must have a unique number. This number should correspond to the placard next to the evidence. The evidence description includes:
- Type of item Location of the item found
- The proximity of the item to the vehicle
Label the container just before collecting an object, and seal the container immediately after collection. Packaging Place the wrapped object into the labeled container. Some objects, such as tires, may be transported without placing into another container. If the object will not fit into a container, seal the paper wrapping the object with evidence tape. Close the container and seal the entire opening with evidence tape. Write your initials and identification number, and the date and time across the evidence tape seal.
Seal the openings of the vehicle with evidence tape. It is important to seal the vehicle by taping exterior parts of the vehicle that can be opened with evidence tape. Avoid applying tape to areas that might be or are known to contain evidence that could be damaged when touched. When the weather is inclement, cover the necessary areas with a clean, weather-resistant material, such as plastic.
The vehicle should be accompanied to a secured storage facility. When the vehicle arrives at its destination, sign the evidence log to verify that the vehicle was checked into the storage facility, then check that all seals on the vehicle are intact. When seals are broken, note when the break occurred and reapply evidence tape to secure the vehicle.
References and Further Reading
Speir, J. A. (2017). A Quantitative Assessment of Shoeprint Accidental Patterns with Implications Regarding Similarity, Frequency and Chance Association of Features. Available: https://www.ncjrs.gov/pdffiles1/nij/grants/251522.pdf
Modification History File Created: 05/02/2019 Last Modified: 04/30/2021
This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.
