1. Casts and Impressions
Chapter 15

2. Types of impressions
Patent impressions – visible, 2-dimensional impressions produced when an object moves through soil, dust, paint, blood, or other fine particles and leaves a trace
Latent impressions – hidden to the eye but can be visualized through the use of special dusting & electrostatic techniques or chemical developers. Oils, fine soil, & other minute debris can be carried onto a floor & transferred. Even clean shoes or feet can transfer materials onto newly waxed/polished floors

3. Types of impressions
Plastic impressions – 3-dimensional imprints left in soft materials, such as snow, mud, soil, or soap. Easily lost

4. Individual or class evidence?
Individual Evidence
Tread patterns in shoes or tires can identify brands and size unless
Tread patterns in shoes or tires with specific marks or scuffs on them.
Dental impressions are individual evidence

5. Shoe impressions- shoe wear patterns
how a person walks wears specific patters into the soles of their shoes. Some factors that personalize shoes are:
Whether the person walks on their heels or toes
Body weight
Direction a person’s toes point – straight, inward, or outward
Shape of the foot
A person’s activities
Surface that a person usually walks on
Unique holes, cuts, or debris that can be embedded in the shoe

6. Gait and tracks Gait – walking habits
A limp or injury creates an asymmetrical gait – 1 foot is angled or makes a deeper impression than the other
Someone carry a heavy weight

7. Gait and tracks
Tracks – indicate if a person is running or walking by the length of stride and pressure and shape of the impression
Can also tell:
# crime scene
Movements of crime scene
Entrance & exit to crime scene

8. Collection of shoe impression evidence
Photograph impressions from multiple angles, use identifying labels & rulers, & good lighting
Lift latent prints
Luminol for bloody prints
Dusting with fingerprint dust
Electrostatic lifting – used on paper, flooring, carpeting, wood, linoleum, asphalt, and concrete
Gel lifting – used on oily or moist impressions
Casting plastic impression – using plaster or dental stone

9. Tire treads & impressions – anatomy of a tire
Purpose - to channel water away & to provide traction as it makes contact
with the pavement
Width & angle of grooves are engineered for different surfaces.
- touring tires – small grooves to channel high speeds
- off-road tires – wide grooves for traction in mud

10. Recording tire tread impressions
Ridges & grooves are counted across the entire width of the tire
Unique characteristics such as wear or pebbles embedded in the grooves are noted
A tire impression involves 1 full rotation of the tire

11. Identifying a vehicle
Tire impressions can identify the make and model of a a crime scene.
Some identifying characteristics
Front and rear track width – center of 1 tire to the center of the opposite tire

12. Identifying a vehicle Some identifying characteristics
Wheelbase – distance from the center of the front axle to the center of the rear axle

13. Identifying a vehicle Some identifying characteristics
Turning diameter – measure of how tight a circle can be driven or minimal space required to make a U turn.
The larger the wheelbase, the larger the turning diameter

14. Establishing Car Movements from Tire tracks
A vehicle’s direction of travel can be determined from…
Vegetation disturbed as vehicle entered or left the road
Debris patterns cast off by a moving car
Splash patterns
Substance transfer (Ex. Oil leaking from vehicle to pavement)
Tire marks left

15. Accident reconstruction – 3 types of tire marks
Skid marks – formed when someone brakes suddenly & locks the wheels
Shows distance the breaks were applied
Used in calculating speed of the vehicle

16. Accident reconstruction – 3 types of tire marks
Yaw marks – produced when a vehicle travels in a curved path faster than it can handle & it skids sideways
Tires and road surface melt from extreme temperatures
Audible squeal & often smoke occurs

17. Accident reconstruction – 3 types of tire marks
Tire scrub – produced by damaged or overloaded tire(s) during or immediately after impact
Usually curved, irregular in width
May have striations, look like stripes
Determine area of impact

18. Dental impressions – individual evidence

19. Dental impressions – tooth structure

20. Dental Patterns – factors affecting dental impressions
# of teeth / missing teeth
Color of teeth
Size of teeth
Alignment and distance between teeth
Unique fillings, crowns, or caps
Overall condition of teeth (broken / chipped teeth, teeth that have been ground down, etc.

21. Bite mark comparisons May have up to 76 points of comparison
Dental chipping
Surface indentions
Distances between teeth
Alignment of teeth
Angle of mouth arch
Absence of teeth
Bite marks should be photographed with a ruler included asap!
Bite marks should be swabbed for DNA (in the saliva)