Classroom Discussion Questions

• What characteristics did you look for to identify the shoe prints? – Could you identify the size of the individual based on

the shoe size? Why or why not?• What characteristics did you use to identify the

tire tracks?• What were some of the unique tool marks you

noticed?• What could be difficult about matching dentition

marks to bite marks on a victim?

Reminders

Check grades – a LOT of people didn’t turn their trajectory/projectile motion problems back.

Some didn’t turn their quizzes back in – I HAVE NO GRADES FOR YOU THEN.

EC Opportunity by Monday: Schaeffer model paragraph on how castings & impressions are used in forensics --

Forensic Science: Fundamentals & Investigations, Chapter 15 2

Forensic Science: Fundamentals & Investigations, Chapter 15 3

Chapter 15 Cast and Impressions By the end of this chapter you will be able to:

distinguish between patient, latent, and plastic impressions

describe how to make foot, shoe, and tire impressions

use track width and wheel base information to identify vehicles

prepare dental impressions and match them with bite marks

All Rights Reserved South-Western / Cengage Learning © 2009

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Introduction Introduction

People, vehicles, and objects leave evidence of their presence at an accident or crime scene.

Patent impressions are two-dimensional. Latent impressions are hidden to the eye. Plastic impressions are three-dimensional.

Individual or Class Evidence

Depending on how it is made, impression evidence may be either class or individual evidence– When would it be class evidence?– When would it be individual evidence?

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Shoe Impressions Shoe Impressions

The size of a shoeprint can tell the size of foot of the person.

The depth of a foot or shoe impression can tell something of the person’s weight.

The type of shoe can tell something of the person’s job or personality.

Databases contain the names of specific manufactures and tread patterns used to identify different types of shoes.

Gait and Tracks

Several prints together can help us determine a person’s gait, or walking habits. – What are some things we could determine

from a person’s gait?A limp or injuryRunning vs walking

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Problems can arise when manufactures use the same generic sole patterns– What else can a footprint tell us that

can help identify a person?

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Shoe Impressions Shoe Impressions

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Shoe Wear Patterns

Some factors that personalize a person’s footwear include: – Body weight. – The way a person walks. – The surface on which the person usually

walks. – Debris that became embedded in the tread or

unique holes and cuts in the tread.

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Collection of Shoe Impression Evidence

Why would the following steps be important?

1. Take photos as soon as possible. 2. Take multiple photos of the impression from

at least two different orientations. 3. Place an identifying label and a ruler in

position with the impression for the photo. 4. Use oblique lighting when possible.

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Lifting Latent Impressions

Different methods to make latent prints visible include: Luminol will make bloody footprints visible. Dusting the latent print reveals an impression. Electrostatic lifting and gel lifting (impression

shown above) techniques also can capture hidden impressions.

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Electrostatic Dusting and Lifting Electrostatic dusting can reveal dust left with

each step and create an impression. Electrostatic charges can lift impressions

from paper, carpeting, wood surfaces, linoleum, asphalt, and concrete.

Other methods to recover latent impressions include gel lifters.

These methods can provide information about the person who left the shoe print.

Casting Impressions

A 3D impression is called a cast and can be made to preserve physical evidence– Shoeprint in mud or snow

– Plaster of Paris is often used to create castsA light film of hair spray is applied to prevent the

impression from collapsing under the weight of the plaster

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Foot Length and Shoe Size

A shoe print is not a direct record of the person but it provides information about the person who left it– A person’s height is generally related to

his or her shoe size but it is impossible to predict someone’s exact height from foot size

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Tire Treads and Impressions Tire Treads and Impressions

To channel water away and provide traction, a tire’s tread is divided into ridges and grooves.

Tires can leave patent, latent, or plastic tread patterns.

Tire evidence usually indicates the type of vehicle that left the mark, may be used to link a suspect or victim to a crime scene, and can reveal the events that took place.

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Recording Tread Impressions Ridges and grooves of discovered tire

impressions are counted across the entire width.

Unique characteristics such as wear or pebbles embedded in the grooves are noted.

A print of a suspect’s tire impressions (through one revolution) is taken.

Comparison with impressions from the crime scene, then, can be made.

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Identifying a Vehicle

Identifying tread patterns may not be enough to link a suspect with a crime scene. Other things can help with this: – Front and rear track widths (center of each tire to

the center of the opposite tire). – Wheelbase measurement (center of the front

axle and the center of the rear axle). – Turning diameter.

Databases can be checked to find the vehicle with these specifications.

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Accident Reconstruction

Drivers may not recall the exact series of events before, during, and after an accident.

People, vehicles, and objects, however, can leave evidence of their actions at the scene of an accident.

Debris patterns and tire marks, for example, can be clues to speed, direction, and vehicle identification.

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Accident Reconstruction

There are three basic types of tire marks:

Skid marks—can be clues to the distance when brakes were applied and the vehicle’s speed.

When someone brakes suddenly and lock the wheels

Yaw marks—can show a sideways skid. When a car travels in a curved path faster than it can handle

and skids sideways

Tire scrub—can determine the area of impact.

Made by a damaged or overloaded tire during or right after impact

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Dental Impressions Dental Impressions

Occasionally a perpetrator will leave behind a bite mark.

Differences in the size of teeth and jaws, position, fillings, crowns, caps, breakage, and crowding make bite marks individual evidence.

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Dental Patterns in Forensics

The investigation consists of recognizing, docu-menting, collecting, and analyzing evidence.

What are the two basic ways dental patterns can be used in forensic investigations?

Up to 76 points of comparison can be used when comparing a suspect’s dental patterns with bite marks left at a crime scene.

What is the common method used to reveal whether there is a match?

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. . . . . . . . . . . . . . . . . Summary. . . . . . . . . . . . . . . . . Summary

Impression evidence—whether patent, latent, or plastic—will be considered class evidence unless it has individualizing features.

Debris patterns and tire marks can be clues to speed, direction, and vehicle identification.

Differences in dental patterns can connect a person to a crime scene.

Documentation (including photos early on) is extremely important in an investigation.