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Impression Evidence II. 3D Impressions. 3-D Impressions Impressed Evidence. Typically found in exterior surfaces Shoe deforms surface Sand/soil/snow – other soft surfacesCharacteristics Vary widely Shallow or deep Quality varies Can have great detail or none. - PowerPoint PPT Presentation
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Impression Evidence II
3D Impressions
3-D Impressions Impressed Evidence
Typically found in exterior surfaces Shoe deforms surface
Sand/soil/snow – other soft surfacesCharacteristics
Vary widely Shallow or deep Quality varies
Can have great detail or none
3-D ImpressionsValue of 3D Impressions
Impressions with sufficient detail can be associated with a specific item of footwear Impressions in clay-based soil
Can retain great detail Impressions in sand & small rocks
Little detail Dry packed snow
Greater detail than wet, melting snow
Collecting 3D Impression Evidence
SequenceDocumentation Sketching
Give lab knowledge where occurred @ sceneLab can recreate as closely as possible to original
o Differences in how recreated in lab v scene can affect making proper match
Field notes Photography
Necessary for court presentationCastingLifting
Most impression evidence has 3D characteristics … surface topography.
For forensic purposes, 3D impressions have depth in addition to length and width and commonly found outdoors in a soft or malleable receiving surface such as soil, sand or snow.
The quality (detail) varies with, The receiving surface’s malleability, texture and composition. The detail present in the source origin. The mode by which the impression was transferred to the
surface. The affects of weather: temperature, rain and snow.
Collecting 3D Impression Evidence
Sequence
The scene investigator has no control over how the impression was generated or its clarity.
Responsibility: capture detail as completely and as clearly as possible. Two activities: 1. Photography and 2. Casting.
Casting defined:
“the filling of a three-dimensional footwear impression with a material that will acquire and retain the characteristics that were left in that impression by the footwear.”
Each archiving technique complements the otherPhotography and casting are not an either or decision. Both are critical and
both must be done in order to properly archive the impressions.
Collecting 3D Impression Evidence
Sequence
Lifting & Preserving 3D Footwear Impression Evidence
What to CastIndented (Impressed) Dry
ImpressionsWet, 2D Prints on Concrete
Impressions in Snow
Impressions Covered by Water
Photography -v- CastingPhotography Casting
Shows impression as found at the scene Direct mold of the original impression
Condition and detail Reproduces microscopic characteristics
Can give best reproduction from coarse surfaces
Gives reproduction of sides of outsoles not reproduced in photographs
A backup for casting Backup for photography
Casting Footwear Impressions
Characteristics of Forensic Quality Casting Material
Produce very fine detailFlow easily into impressionCleaned without loss of detailEasily obtained with consistent quality &
propertiesEasily mixed – not require special equipmentSet in reasonable timeUnlimited shelf life
Quality of CastsReceiving surface malleability,
texture & compositionSandClayLoamy soil
Detail present in the impression Mode by which impression transferred to the surface
Mechanics of making the impressionEffects of weather:
Temperature Rain/snow Wind
Casting Gypsum manufacturing processes
CaSO4-2H2O 110-130 Deg C
GypsumCalcium Sulfate Dihydrate
(CaSO4)2-H2O 130-200 Deg C
PlasterCalcium Sulfate
Beta-hemihydrate
CaSO4 200-1000 Deg C
StoneCalcium Sulfate
Alpha-hemihydrate
Plaster of ParisRequires more water
Irregular Crystals & PorousConsistency > 50
AutoclavePressure
Steam
Open Oven
Dental StoneRequires less waterUniform CrystalsDense Crystals
Consistency < 50
Choosing Dental Stone
Regular plaster of Paris is 5,000 psi compression strength and will chip fairly
easily.
Hydrocal is around 8,000 psi and is more durable.
Merlin's Magic is around 14,000 psi and is even more durable yet.
Excalibur and Die-Keen are 18,000 psi and dry so hard it's almost like a ceramic
material.
Several kinds of dental stone … always check the compression strength measured in psi
(pounds per square inch).
Casting Footwear ImpressionsGypsum Consistency vs Compressive Strength (psi)
ConsistencyWater-to-Powder (W:P)
ratioLbs per 100lb plaster
Plaster per pounds of water
Dry compressive Strength(psi)
50 2lbs-0oz 3750
38 2lbs-10oz 7000
33.3 3lbs-0oz 9750
30 3lbs-3oz 11,000
21 4lbs-12oz 15,000
W:P – The quantity of water (by wt.) per quantity of powder (by wt.)A “30 Consistency” means – 30 parts of water/100 parts powderHigher the consistency – longer setting times – lower strength - softer
Mixing Merlin's Magic Merlin's Magic: Special type of casting material.
Made to pour into molds easily with very few air bubbles.
Mix differently than for regular plaster. Below are instructions to mix up enough plaster to fill one regular size mold.
First you need to make a measuring cup that can be reused.
http://www.hirstarts.com/casting/dental.html
Need two disposable plastic cups - nested. Pour 2 ounces (60 ml) of water into the top cup and place a black mark on the outside of the bottom cup where the water line is. Place an additional 2.5 ounces of water into the cup (for a total of 4.5 ounces or 135 ml). Place another black mark on the outer cup at the water line. Remove the inner cup and you have a reusable measuring cup.
Here is how you mix the plaster
Procedure for Merlin’s Magic
Insert a new cup into your measuring cup. Pour in water until it reaches the first line. Carefully shake in the powder until the mixture reaches the second line. The powder must be absorbed into the water before you can determine if the second line is reached. Remove the inner cup, mix up the plaster and pour it into your mold.
Mixing by weight,
Use table @ right. Need scale to measure the weight of the powder.
"ounces" shown here are a liquid measurement (not weight).
Mixing instructions on the package of Merlin's Magic will be different. Their instructions are used
for dental castings, which use a vibrator to shake thicker plaster into their dental molds, which can give mix that is too thick to pour into a mold.
Number ofmolds to fill
Ounces (or ml)of water
Amount of powder(by weight)
1 2 ounces (60ml) 186 grams
2 4 ounces (120ml) 372 grams
3 6 ounces (180ml) 558 grams
4 8 ounces (240ml) 744 grams
Casting Using Dental Stone
Photographing the Impression
Proper positioning of scales
Positioning of Retainer
Placing the casting frame in place
Mixing water (3/4 cup/lb stone)and dental stone
Pouring mixture onto impression
Curing the castAllow to sit 30-40 minutes before lifting
Do not remove adhering soil
Final cast Allow to cure up 24-48 hours Carefully remove soil Save Soil for comparisons
Casting Underwater Impressions
Drain or remove excess water – Good – BUT - Not Necessary Pipette or syringe Carefully Absorb with paper towel Pour casting material as usual
If water can’t be removed? Frame the impression Sift dry casting powder gently into water above
impression Allow to fall to bottom Sift until 1” of powder covers the areaPowder builds up & saturated with powderUse additional dental stone slurry to fill the framed area
Will settle into the water later & into impression – cover entire impression
Allow to set for at least 1 hour
Casting in SnowCasting Procedure
Spray Impression with Snow Print Wax or Dust with Snow Print Powder 2-3 layers
Allow to dry for 2-5 minutes between applications
Slowly Add slightly cooled dental stone
Cooled dental stone minimizes melting the snow
Non-Casting Procedure Auto primer spray paint before
Snow Print Better contrast for
Photographs Photos critical
Can’t directly cast with primer paint
Many snow prints not cast
Spray the impression lightly with the Snow Print Wax at an angle in order to highlight the raised areas of the impression. Do not cover the entire area of the impression. This is the best time to
photograph the sprayed impression. Do not hold the spray too close to the impression because the blast from the
aerosol can damage the impression’s detail. Spray the impression with 2 or 3 more applications, ensuring that the entire
impression is covered in wax. A properly sprayed impression will not reveal much detail when viewed from above.
The red (or other colored sprays) attracts heat: Shield the impression from the sun.
Allow the wax mold to dry for approximately 5-10 minutes. Prepare a dental stone slurry and allow it to sit longer than usual so that when the
slurry begins to harden, the heat generated will not melt the impression. When the cast is hardening, scratch initials and date into the cast. Allow the impression to sit for approximately 60 minutes before lifting.
After lifting, immediately photograph the cast. Snow print casts are fragile, and the wax molding is easily destroyed. Keep the cast
away from sunlight. If melting occurs, cast detail may
Snow Casting Procedure
Snow Print Wax
Casting with plaster
3D surface scanning
3D model of the sole
GOM ATOS II system (Gesellschaft fu¨r Optische Messtechnik mbH, Braunschweig, Germany).
3D Scanning
http://cs.iupui.edu/~tuceryan/pdf-repository/Buck2007.pdf
Packaging Casts
Allow cast to dry for 24 hours or longer Do not attempt to remove soil or clean until cured
Will destroy impressionSoil used for comparison purposes
Loosely & individually wrapped in paper or paper bag Do not wrap in tight plastic
Place wrapped cast in cardboard box & tape seal
Place in shock absorbent or porous packaging material
Identification:
Before cast hardens, place CSI or investigator’s initials, date and case & number on back side of cast.
Amount Desired: Standard -Evidence - Up to 2 ft.
Preservation: Use mesh reinforcing and let dry 24 hours before putting in package.
Wrapping & Packing: Surround with packing material in box too prevent
shifting or breakage. Avoid sealing in plastic bags.
Miscellaneous: Dental stone is the preferred casting material. Take
photos of impressions before casting. Mark package FRAGILE. Do not clean.
Do not use twigs for reinforcement.
Illinois State Police Packaging Procedure for Cast Impressions
http://www.isp.state.il.us/docs/6-420.pdf
Impression Evidence On-Scene Overview
[1] Adapted from Hilderbrand, Dwane S. Footwear, The Missed Evidence, Skaggs Publishing 1999, pages 61-62.
Impression[1]
And SurfaceCategory
of Impression
Archiving Method
Enhancement Method
Preservation Method
Wet soil or Mud (fine, even consistency)
3D Photograph 1. Dust with black/fluorescent or bicolor magnetic powder
Cast using dental stone
Concrete 2D Photograph quickly
1. Quickly dust with black/fluorescent or bicolor magnetic powder
1. Re-photograph2. Cast using dental stone
In dry soil (Hard, packed or loose. Varying in consistency from coarse to powder-fine)
3D Photograph 1. Stabilize with hair spray or lacquer2. Spray with automotive primer paint of appropriate contrasting color
1. Re-photograph2. Cast using dental stone
Sand(variable texture – retains small amount of impression detail)
3D Photograph 1. Stabilize with spray paint (black) or “snow print wax” or “snow print powder”2. Spray with automotive primer paint.
1. Re-photograph2. Cast using dental stone
Impression[1]
And SurfaceCategory of Impression
Archiving Method
Enhancement Method
Preservation Method
Snow(Temperature affects textures, cohesiveness and impression detail)
3D Photograph 1. Stabilize with automotive primer paint (gray) and/or “snow print wax or powder”2. Spray with automotive primer paint.
1. Re-photograph2. Cast using dental stone or Jade Stone
Dry Residue - Dust on Hard Surfaces(Produce highly detailed impressions on hard surfaces)
2D Photograph 1. Lift using electrostatic or gelatin lifter) 2. Chemical enhancement.
Re-photograph
Wet Residue – Dust or Mud on Hard Surfaces(Highly detailed impressions)
2D Photograph 1. Cannot be lifted using electrostatic lifter. Can be lifted using gelatin lifter2. Dust with magnetic powder (dual color or black)3. Treat for the presence of iron or other metals4. Cyanoacrylate fume followed by powder dusting or fluorescent staining (surface dependant)
1. Re-photograph2. Lift with gel lifter
Dust – Fabrics(Carpet, upholstery & bedding can produce detailed impressions within the fabric nap or on the surface)
2D Photograph 1. If dust – lift as above. 1. Re-photograph2. Cast using Jade Stone … or3. Lift with gel lifter
Blood(Can produce high-detail impressions)
2D Photograph 1. Light surfaces – Stain using Acid Violet 17.2. Dark surfaces – stains using Basic Yellow 7 or DFO.
1. Re-photograph2. Lift using gelatin lifter.
Exemplar Impressions
Exemplar Footwear ImpressionsMany people leave impressions @ scene
Categorize designs of shoesVictimsWitnessesMedical personnelOther innocent peopleOfficersSuspects
Known impressions made Specialty impression materials available
Biofoam
http://site.utah.gov/dps/impressions-fw-evidencecollection_000.htm
Gellifter
Inkless Lifter
http://site.utah.gov/dps/impressions-fw-evidencecollection_000.htm
Forensic Footwear DatabasesTreadMark™
The number of shoe prints at a crime scene can be so large that the process of impression recovery becomes very time-consuming.
Commercial product using four parameters—pattern, size, damage, and wear—to identify individual outsole impressions.
Compared with shoe print data from two sources: suspects in custody and crime scenes.
A match could yield the name, date of birth, criminal record number, places of interest, and similar offenses for possible suspects.
How does TreadMark™ work?
Impressions from crime scene obtained using photograph, gel lift, dust lift, and adhesive lift.
Input directly into the analytical system by high-resolution digital imaging. Same procedure used with impression of a suspect’s shoe print:
Operator measures, analyzes, and compares crime-scene and suspect images.
Both image sources can be searched within themselves and against each other, allowing such images to be transmitted to other users.
www.csiequipment.com/systems.aspx Exit Notice.
SoleMate Commercial database contains information—manufacturer, date of market
release, an image or offset print of the sole, and pictorial images of the uppers—for more than 12,000 sports, work, and casual shoes.
Sold on DVD, updated and distributed to subscribers every 3 months.
Limitation is that different manufacturers often use the same sole unit. Therefore, it may be difficult to determine the exact make and model of a shoe. The software links such records, however, so that all footwear that might match a crime-scene print can be considered.
How does SoleMate work?
The pattern of an unidentified shoe print is assigned a set of codes to isolate basic features, such as circles, diamonds, zigzags, curves, and blocks. Options, with variations, are presented pictorially, allows investigator to code features that best match the shoe print.
These codes form the database search, with results presented in descending order of pattern correlation.
Foster & Freeman USA Inc., at 888-445-5048.
TreadMate
Maintained by the same United Kingdom company that markets SoleMate, this database contains information:
5,000 vehicle tires and tire tread patterns, Manufacturer, Date of market release, Pictorial image, Pattern features.
Because manufacturers sometimes use the same tread, it may be difficult to find the exact make and model match of a tire. In these cases, records are linked so that all tires that might match a crime-scene tire mark may be considered.
How does TreadMate work?
The pattern of an unidentified tire mark is assigned a set of codes for pattern features, such as waves, lines, diamonds, zigzags, curves, and blocks, which then form the basis of the database search. Results are presented in descending order of correlation.
Foster & Freeman USA Inc., at 888-445-5048 or [email protected].
Tire Print Evidence
Evidence Often Overlooked
Vehicle-involved scenes run the gamut of scene types: homicides, sexual assaults, burglaries, drive-by shootings, terrorist events, etc. Identifying vehicle should be a critical aspect of any on-
scene investigation.
In typical homicide investigation, investigators must consider the possibility that specific categories of physical evidence related to the crime are present. Hit and run crimes, whether vehicle-vehicle, vehicle-
person, vehicle-other object, involve vehicles that leave the scene.
In these crimes, damage creates physical evidence Evidence could prove the culprit vehicle was at the
scene; o This evidence should be collectable.
Vehicle-Involved Scenes
The following article from the New York Daily News is such an example[1]. In this case, the suspect and his vehicle were found. The NYPD crime scene unit had the responsibility of working the car to prove it was the vehicle that struck the victim.
The abducted child case. No evidence found inside the suspect vehicle proving the child there … Must consider other, indirect, avenues to move investigation forward.
The abductor vehicle had been at the scene. Success includes thorough investigation of the outside where the vehicle
might have been parked Physical evidence collected and scene archived.
One example of physical evidence vehicles leave behind is tire track impressions.
Hypothetical - Kidnapping Involves a vehicle for transport Critical to find physical evidence that the child had been inside the car, Reality: Such evidence may not be present or had been removed.
Case ExampleKidnapping
Tire Track Impression Evidence Evidence Often Overlooked
Footwear Impression: Dealt with locating, enhancing, photographing, etc, footwear impression
evidence, Much of that discussion is applicable to tire track evidence as well.
Similarities with Footwear Impressions Tire track evidence:
Classified as two-dimension (2D) or three-dimension (3D). Commonly in dust or are otherwise contaminated 2D impressions or
impressions in a soft surface. Considered Class or individualizing:
Physical characteristics needed for meaningful criminalistic-quality comparisons
Former provides information about the tread design Latter provides information imbedded into the tread of the tire from daily
usage.
Provides evidence of the individual (shoe impressions) or the vehicle (tire impressions) being at the scene, For tire tracks, suggests the individual vehicle was
used in the crime. Tire track impressions geared to identify vehicle …
not the person, Although the person might have been driving the
vehicle.
Footwear & Tire Track DifferencesTheir Intrinsic Forensic Values
How Important is Scene Evidence?
Identifying specific vehicle requires recovering it and making direct comparisons with physical evidence from scene Scene data allows investigators and laboratory
analysts to narrow the search among universe of vehicles.
Until suspect vehicle is located and impounded, all scene data must be archived, collected and preserved. Determining which tire tracks to photograph,
enhance and or cast is critical
Critical Vehicle InformationCrime Scene Procedures
Direction of travel Relationship of impressions
@ scene to arrangement of tires on suspect vehicle
Position of front of vehicle Which impressions made
by front & rear tires Which impressions to
photo/cast Locations where vehicle
track measurements will be recorded
Other relevant evidence Footwear impressions Fluid spills
How vehicle was maneuvered?
Vehicle characteristics Stance Track measurements Wheelbase Tread wear
indicators Wear bars
No. vehicles & no. occupants
Were objects loaded or unloaded?
On-Scene Considerations
Unknown: Whether something seemingly unimportant and ignored
will be important as evidence AND suddenly plays a prominent role in the investigation. Consider everything at vehicle-involved scenes as
potentially probative. Misperception that tire track impressions have little
forensic or investigative value. General scene investigative principles apply equally to
vehicle-involved scenes; Management, archiving, searching, etc, are an integral
aspect of the investigation.
Success At the SceneOne Shot at It
Crime Scene Procedures
Official Busines
s
Scene&
Command Center
Official Busines
sSegregatedWitnesses
& PotentialSuspects
Media
Discrete Security Areas at the Scene
Public Public
Secure Travel Routes for Emergency Equipment / Personnel
Secure the area Tire prints protected
Obtain information Case information Vehicle information Scene information
Establish safe path to view evidence
Archive
Collect/package/preserve evidence
ManagementShould employ guidelines
previously discussedWhile these principles are
inviolate Unique characteristics
for vehicle-involved scenes. Specifically,
identifying, archiving, characterizing and preserving the physical evidence associated with identifying a suspect vehicle.
Investigative Questions Archiving Sketching and critical on-scene
measurements Vehicle information Suspect information Reconstruction
Archiving
Photographing, sketching, video, 3D-Imaging tire-track and other vehicle-involved scene evidence
Much like for footwear impression evidence.
For tire track impressions – Document at least 24” of impression
For comparison purposes
Archiving
Photography of Tire Track Impressions.
Photos must Include markers that Identify the tire position on vehicle
– Establishing or Overview Photographs• Tell the story and relationship of how the tire tracks relate to the overall
scene. Photos with and without evidence markers in place.
– Midrange• Tell the story of how the tire track evidence relate to each other and to
nearby objects. Photos with evidence markers are important to establish evidence-to-evidence relationships.
– Close-ups• These include at least 24” of all 4 impressions, appropriately identify
which tire, e.g., right front, etc, being photographed. Photos with and without scales must be included.
Considering Lighting and Glare3-D Impression
Blocking Sunlight Using Ambient Light
Photographing 3-D Impressions Photograph before casting Reproduces class characteristics
Accidental characteristics are often lostSpray paint may enhance sufficiently
Outdoor lighting may make it necessary to block direct sunlight Sometimes sunlight may be superior to oblique
lighting Use polarizing filters to eliminate glare
Look for sidewall information in impression
Photography – Effect of Lighting
• Existing light blocked out and oblique light provided with off camera Flash
Archiving 3D ImpressionsCasting
http://projects.nfstc.org/ipes/presentations/Bodziak_Footwear-Non.pdf
Archiving 3D ImpressionsCasting
Tire Impressions in the SnowReverse Sidewall Impression
Marking Tire Impression EvidenceVehicles Still at the Scene
Photographer’s name, Date, Time Use scales in plane of impression Mark tire position on vehicle & on scene
Use spray fluorescent paint for all wheels Rt front, etc
Arrow pointing to front of vehicle Establishes inside & outside edge of impression
Impression number 1st or 6th impression recorded @ the scene
N/S directional
SketchesTire Track Impressions
Plan sketches: Overview of impressions In-Depth detail not important
Measurements Photography
Sufficiently detailed to permit a determination of specific vehicle characteristics
Dependent on the amount and detail of the tire track impressions present.
Appropriate measurements must be made. The measurements are the critical data
needed to compare the on-scene tracks with a suspect vehicle.
Archiving – SketchesInclude Vehicle-Critical Measurements
Anatomy of the Tire
http://www.jacks-tire.com/Tire101.aspx
Tread Design
PitchEliminating Road Noise
Pitch
SmallLarge Medium Medium Small
Vehicle Information
How Was the Vehicle Maneuvered?
Closely examine tire tracks
Using known factors
Common sense
Direction of Travel
Direction of Travel Spinning tires Striations by sidewall in furrow
Shows which way tire rolling Location where vehicle stopped,
backed up to change direction Overlapping front & rear tire
tracks Look to see if coming or going
from scene Grass or small plants
Direction they were flattened Directional tire tread patterns Damp soil or snow
Tires lifted soil or snow slightly in direction of travel
Deposition of transferred material in direction of travel
Mud, dirty water or fluids splashed or thrown in direction of travel
Make the Appropriate Tire Track Measurements
Wheelbase Front track width Rear track width Turning diameter Tread design width (arc width) Tread depth (skid depth) Tire circumference
Front Tires Turned
Front TireTrack Width
Rear TireTrack Width
ApproximateWheelbase
WheelbaseDefined as the distance between the leading edge of
the front and rear tires.
WheelbaseFront Wheels Turned
Inside leading edge of track will be rounded Make rt. Angles to center
line of impressionWill run laterally across
the width of impressionProject line along inside
edge – parallel to center lineo Intersection is fixed point
on inside leading edgeo Repeat of all 4 tires
Wheelbase in Mud & Snow
Camber
http://en.wikipedia.org/wiki/Camber_anglehttp://autorepair.about.com/od/glossary/ss/def_camber.htm
Camber Tilt” of the tires as they
rest on the ground.
Mechanical issues with vehicle affect how impressions appear at scene
Differences between normal and abnormal camber (positive or negative) signifies
improper alignment or worn front-end parts of the vehicle.
Positive camber: tires tilted further apart, “out,” at the top.
Negative camber tires tilted closer, “in,” at the top,
Normal Camber
Positive CamberTire tilted “out” @ top
Negative CamberTire tilted “in” @ top
Narrower
Wider
Track Width or Stance
Front and Rear Track Widths
• Track widths– Midpoint of leading edge of
front and rear tire track impressions
Front TireTrack Width
Rear TireTrack Width
Front and Rear Track Widths
Defined as distance between middle of leading edge of the front and back tires.
Mechanical issues can affect how these impressions appear at the scene.
Among others, one is camber, which is the “tilt” of the tires as they rest on the ground.
Toe Difference between
front & rear of front tiresNormally set “in”
only a few millimetersoCompensate for
normal front end tendency to toe “out” at highway speeds.
Improper alignmentWorn front end
components
Track Width Toe In or Out
Vehicle InformationTurning Diameter
Diameter is defined as the diameter of the circle made when the vehicle is driven in a circle.
Determined from measurements taken @ scene.
Turning diameter
Turning Diameter
A AA
B
C
Front Tire
Rear Tire
Calculating Turning Diameter
On-Scene Tire Impression
BD Bisects A – AA @ C: Line E= Distance B to CDistance C to AA = Line F (= distance A to C)
Turning Diameter = (F / E) + E (11)Alternatively: Turning Diameter = (F /8E + E/2) (12)
EF
22
At the scene Don’t know if the turn is full-lock Curb-to-curb for the outer tires
Not wall-to-wall for outside edge of vehicle
Procedure Select segment reflecting
sharpest portion of turn Measure imaginary line
between 2 points of arc Measure outer margin to outer
margin A-AA
Bisect line & draw line to outer margin of track arc –
(A-C) Draw another line between the
bisect point and the inside of the track arc –
(C-B) Calculate diameter
Approximate the tire circumference by finding an accidental, repeated characteristic, such as a gouge or cut, along the imprint. The distance between these repeated marks is the rolling tire circumference.
Investigators must understand that measured value – accidental characteristic-to-accidental characteristic – different from value obtained by wrapping the measuring tape around the tire in its center line, Larger
Procedure
Measure distance between repeated accidental characteristics on impression Considerations
Measured value is less than when the tape is wrapped around the tire o Tires have a curved arc width, impressions do not.
Measured circumference is larger in the center line of the tire than at the outer edges.
Tire circumference
Vehicle InformationTread Design
Tread design width:
Measurement from one edge of the design to the other.
These measurements must be measured at the scene from the impression.
Information important so manufacturers can help investigators identify an unknown impression.
Tread design width (arc width)
2D impressions visible because of contaminants adhering to the tread; dirt and dust the most common. Other contaminants also create impressions
water, grease (oil) or blood. Each leaves visible impression Each can be enhanced, depending on the contaminant.
Consideration of Enhancement Choices Variety of choices and investigators must make the proper decision.
Archiving the impression photographically is step-one Enhancement choices and the variables offered by the scene:
o Surface material, o Chemistry of the impression material (dirt, oil, blood, etc), and
the o Chemistry of the enhancement method.
Non-dirt Impressions
Tread Wear
Tread Wear Indicators
Tread wear indicators (wear bars) Located in grooves of tread design
Run laterally across treado 1/16” above base of the groove
Useful for comparison purposeso Divide tire into useful segments
Cars & light trucks 6 evenly spaced
o Rim diameter of 305mm/12 inches must have @ least 6
o Less than 305mm must have @ least 3/12 inches
Wear Bar Indicator
Tread Wear
Tire wear indicator
First thousand miles produces the fastest tread wear
As elements become shorter Flex less & squirming is reduced Slows considerably Accidental characteristics last
longer on well-worn tire than on a new one
Factors Affecting Tread WearExcessive under-inflationExcessive over-inflationBalancingImproper toe-in-outImproper camberChunkingFaulty breaksCombined suspension
steering issuesBad repairs
Equipment: Tread depth gauge (Measures in 1/32nds) or
15mm ruler Newer tires: 11/32 to 16/32”
Maximum depths suggest newer tiresWear bars are set @ 2/32nds, Wear patterns appear as solid bar in the
tire impression
Procedure Estimate height of groove in scene
impressionSet depth gauge on impression grooveCarefully lower gauge into impression &
read depth from the scale Alternatively, measure depth from cast in
dental stone
Measuring Tread DepthNon-Skid Depth
When tires near end of life, U.S. Lincoln penny can be used to confirm the tire's tread depth. If Lincoln's entire head is visible, the tire is worn to approximately 2/32" and is
considered legally worn out in most States. Average new tires used on cars typically start with 10/32" to 11/32" of original
tread depth. Dedicated winter / snow tires and light truck tires typically are deeper - how much
deeper depends on tire's tread type ... Highway Rib, Highway All Season, Off Road All Terrain or Off Road Maximum Traction). Means - original tread depth has only 8/32" of useable tread depth. Useable tread depth calculated by subtracting a worn out tire's 2/32" from the
new tire's original depth of 10/32". The final 2/32" of a tire's tread depth isn't part of the equation when it
comes to calculating tread depth percentages because the tire is already legally worn out.
Remaining tread depth used to calculate tread wear percentages. … o Tire that started with 10/32" of original tread depth and has worn off
4/32" (down to 6/32" of remaining tread depth) is 50% worn.o 2/32" legally worn out tread depth
A tire that starts with 10/32" of original tread depth has 12.5% wear for every 1/32" that is worn away, and a tire that starts with 12/32" " of original tread depth, has 10% wear for every 1/32" that is worn away,
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=197
Tread Depth - Some Statistics
Identifying Tread Wear
Exposed tire bars Tire bars recessed in
sipes & grooves become exposed
• Circular wear Wear around
circumference w/sipes & grooves indicates wear
2/32 TreadLegally Worn
4/32 Tread
6/32 Tread
Enhancing Tire Tracks
Involves many of the same principles as for footwear evidence Photography :
A method for archiving AND enhancing. Chemical enhancements.
Chemical treatments take advantage of the inherent chemical signature of the impression o Minerals, such as iron and aluminum, etc,
organics, or bio-materials – to more clearly visualize the impression.
The chemical enhancements are essentially the same those used to enhance footwear evidence.
Enhancing Tire Track Impressions
Chemical EnhancementMetals in Soil
Impressions with Metals Ammonium or
Potassium Thiocyanate Tests for Iron
Reddish brown 8-Hydroxyquinoline
Iron, magnesium + other metals
Fluorescent under UV light
Impression w/oil or fats Iodine fuming or Iodine
crystals Oils & Other organic
compounds Iodine absorbed
Enhancement Benzophenone
o Forms stable blue color
Thin layer of starch powdero Subsequent steam
treatment turns starch blue
Chemical EnhancementWet Tires with Salt Deposits - Winter
• Silver Nitrate– Converts NaCl (salt) to AgCl (Silver
Chloride)• NaCl + AgNO3 AgCl + AgNO3
Ag Metal– UV or sunlight darkens impression
hvRoad Salt2% Silver NitrateIn Methanol
Tire Tracks in BloodChemical Enhancement
Blood reactive chemicals DAB Leucocrystal Violet Acid Violet 17 Acid Yellow 7 – dark surfaces – yellow fluorescence Amido Black Ninhydrin DFO Crowle’s Stain TMB Coomassie blue Nile Red Hungarian Red Ashley’s reagent
Investigative EvidenceIdentifying Unknown Vehicle
Tire Impressions as Investigational Aids
Identifying manufacturer & brand name Done after processing the scene Tread design
Who makes it & where Tire guides – Boca Raton, FL
Shows tread patterns for all types of tires sold in the U.S.
Comprehensive Visual Identification of all types of Tire Tread Pattern Designs.Covers Passenger, Light Truck and Medium Truck vehicles. Includes Off-Road and Agricultural Tread Patterns. Also covers Motorcycle and retread tire tread designs. Provides information on how to read a tire sidewall and on the different Tire
Sizing Systems. Gives detailed information on Speed-Rating and Load Indexes as well as lug
nut torque and tightening sequences. http://tireguides.com/Products/245
On-Scene InformationLimited Slip Differential (LSD)
Posi-tractionOne wheel was spinning
o Movie: My Cousin Vinnieo Can narrow type of vehicle
Odd mixture of worn tires Suggests an older vehicle
Possible alignment problemsStolen items - gasoline
Location of gas tanko Check footprints @ side of car where gas tank located
Estimate from amount taken whether a truck (pickup-SUV)
Mechanical Problems
Mechanical Problems & Vehicle Design Features
Fluid leaks @ scene Oil, antifreeze, brake fluid, trans fluid
Collect for chemical analysis @ lab
Vehicle Location History
Foreign Material on Suspect VehicleNotice unusual material in tread
impression Collect & take to lab
Identifying The Correct Impression
Age of Tire Impression How Long @ Scene
• How long an object (driven over) has been lying on the ground– Complainant may have this information
• Transferred substance– Recently spilled Fluid
• Knocked over objects• Weather conditions prior to offense
– Help date time of deposit of scene impression
Known Tire Impressions
Known Tire Impressions
Need one full circumference of the tire
Complex pitch in modern radials
Might miss important accidental characteristics
Taking Tire ImpressionsMethods & Materials needed
Large area to make impression
Broom Vehicle lift Tire marking crayons Rag to clean dirt
from tires Roll of paper towel Large jar of
Petroleum Jelly (Vaseline)
Gloves Art board 15” x 40” (3
pieces/tire)
Lg & sm felt marking pens
Magnetic jet black print powder
3” wide magna brush Strips of polyester
plastic 12” x 40” to protect impression
Scotch tape to attach plastic to art board
Information From Each Tire
• Photograph Tire– Exterior sidewall– Tread design– Serial number – Design, mold &
drawing numbers• Show up after
rubbing w/crayon
• ID each tire later– Manufacturer– Tire brand name– Tire size– Type of tire
• Summer, etc– Construction
• Radial, bias ply, etc– Other numbers
• Mold, design, etc– Wear– Number of ribs in design
Recording Known Tire ImpressionsRecording known tire impression
Art board is superior to paper Small debris on surface
can’t come through Reduces/eliminates
wrinkles/tearing common with paper
Use gloves to keep board clean
3 pieces of 40” Board sufficient to record 1 full circumference of tire
Tread Wear Indicators
Recording Known Tire ImpressionsPetroleum Jelly/Magna Brush Method – 2D
Place 2 pieces of art board in path of vehicle travel Tape on reverse side to
hold pieces together Mark adjoining pieces
appropriately Push car over recording
surface Vaseline leaves light
brown impression Mark on art board as
each tread wear indicator (marked on sidewall) rolls by
Mark spaces on art board to correspond to numbers on known tire Tire Wear Indicator Marks
Place locations on sidewall
Recording Known Tire ImpressionsPetroleum Jelly/Magna Brush Method – 2D
Cover art board in front & behind impression w/paper towel to protect it
Clean area of tire that was on the floor when Petroleum Jelly first applied Apply Petroleum Jelly to this area and push
car again
Recording Known Tire ImpressionsVaseline/Magna Brush Method – 2D
Near end of second art board Stop vehicle Remove 1st piece of art board Add thin coat of Vaseline to tire Place 3rd piece of art board in path of vehicle
Continue pushing until known impression is transferred to this 3rd section
Recording Known Tire ImpressionsPetroleum/Magna Brush Method – 2D
• Dust impression as soon as possible– Waiting too long causes defined areas of
tread to diffuse into art board surface• Brush with magnetic brush @ rt. Angles
w/swirling action– Avoid dusting clear areas of board– Black magnetic Dusting Powder
Recording Known Tire ImpressionsVaseline/Magna Brush Method – 2D
Protecting the impression Cover with clear acetate to prevent smearing or
damage during transit & comparison processAllows examiner to draw lines during comparison
Recoding Known Tire ImpressionsInking Method I
Apply ink to art board for 1 full circumference Fingerprint Ink works
Push car (tire) over inked art board This is the inking process
Push car over clean strip of white art board to record the impression
Recording Known Tire ImpressionsInking Method II: Transparent Method
Ink the tire as in inking method
Drive tire over transparent acetate or other transparent media (Mylar) Tape to art board - suitable solid backing
Advantages Can be rolled up for storing Unrolled @ later time for comparison Can be compared directly to impression when
searching for
Comparing Scene Cast with Acetate Exemplary Overlay
The area defined by blue tape isthe segment of the right reartire which corresponds with thecrime scene cast.
Comparing Scene Cast Actual Exemplar Tire