Casts and Impressions Chapter 15 Types of impressions

Casts and Impressions Chapter 15

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

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

Individual or class evidence? 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

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

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

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: • # people @ crime scene • Movements of individuals @ crime scene • Entrance & exit to crime scene

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

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 water @ high speeds - off-road tires – wide grooves for traction in mud

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

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

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

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

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

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

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

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

Dental impressions – individual evidence

Dental impressions – tooth structure

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.

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)