Impressions and Tool marks - PowerPoint Presentation

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Impressions and Tool marksChapters 15 & 16

PVMHS, 2015

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Student objectives:

Distinguish between patent, 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 Describe the three major types of tool mark impressions Summarize the steps of a tool mark examination and analysis Match tool marks with the instrument that produced them

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Labs & activities:

Casting a shoe impressionExamining the relationship between height, gait, & shoe size

Examining tires

Compare and contrast tire tread

Find the matchExamination of bite marksCase study of Alampi/Bevard hit-and-runStudy of OJ Simpson case

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PART IImpressionsChapter 15

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Types of impressions:

3 basic categories:

Patent – visible 2 dimensional impressions produced as an object moves through soil, dust, paint, blood, or other fine particles.

Latent – hidden to the eye, but can be discovered by dusting or applying chemiluminescence

materials. Oils, dirt, dust, or other fine particulates can be transferred to a floor or wall and leave a latent impression.Plastic – visible 3 dimensional impressions left in soft materials such as snow, mud, soil, or soap. Plastic prints are easily lost, therefore they need to be photographed immediately and then a cast should be taken.

Cast of shoe

impression

Tire impression –

plastic

Latent shoe impression

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Individual vs. Class evidence

Depending on the impression, it can be categorized as either individual or class evidence.Class evidence: a particular tread pattern may identify a specific brand of shoe or tire, but it will not identify an individual.

Individual evidence: if distinguishing characteristics are identified, such as unusual wear on a tire or a “scar” on a shoe, can be used as individual evidence.

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Shoe impressions

There is a lot of information that can be ascertained from a shoe impression:Shoe size

Weight (plastic impression)

Type of shoe gives information about the suspects job, personality, income, gender.

GaitApproximate heightDatabases exist to aid investigators in researching a tread pattern.Ex: SoleMate, SICAR6of 40

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Shoe wear patterns

If two people purchase the same shoes on the same day, over time the tread will change even though it matched at the start. This is because different people have habits of walking as well as different styles of walking.

Some people walk with more pressure on their toes, heels, outside of foot, inside of foot.

Some people may have a high arch or a flat foot

Some people walk with toes straight, others with toes pointing out, or in. Some people have narrow feet, others are wide.Etc.of 40

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Gait & Tracks

If there are several footprints together, it may be possible to determine the suspect’s gait.If a person has a limp or is carrying something heavy, it may cause an asymmetrical gait – i.e. one foot is angled differently from the other

or

one impression is deeper than the other.

Tracks can indicate if a person was running or walking based on the length of stride, shape of impression, and pressure points on the impression.Tracks can also indicate movements and even pinpoint the order that the movements occurred in.of 4012

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Collecting shoe impression evidence

#1!!! PhotographImpressions should not be touched or tampered with in any way until they are photographed

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Photograph all impressions right away, regardless of the type of impression found.

Take additional photos with a ruler in position.#2 – lift or cast the impression2 dimensional impressions can be lifted using luminol or dustingCasts can be made of 3 dimensional impressions and preserved to be used as physical evidence.Casting impressions in snow can be accomplished using a special spray wax and a material called dental stone.

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Foot length & shoe size

It’s impossible to predict an individuals

exact

height from their shoe size.

It’s important to remember that a shoe print provides some information about the person who left it but:The size of shoe varies by brand/typeThe suspect may be wearing shoes that are not appropriately sized.of 40

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Shoe size & height

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Tire tread and impressions

Tire marks can be used to identify a vehicle at the scene of a crime. Tire marks can indicate the speed and direction a vehicle traveled.

Skid marks at an accident scene may indicate who was at fault or help reconstruct what happened.

Forensic scientists examine tire tread and impression for 2 reasons:

Tread pattern and measurements can identify the type of tire and potentially the make and model of the vehicle,The impression can determine how the car was driven (speed, direction, etc.)of 40

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Tire tread and impressions

Vehicles can leave 3 types of impressions:Patent impressions occur after a vehicle has driven through a fluid material like tar, oil, or blood.

Latent marks may be left on asphalt or concrete roads by oils in the tire that are meant to keep the tire soft and pliable.

Plastic impressions may occur in off road surfaces like mud, snow, or sand.

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Anatomy of a tire

A tires surface is divided into ridges and grooves.Ridges are the elevated regions

Grooves are the indentations

The purpose of this design is to channel water away and provide traction with the ground.

Every model of tire is unique.Width and angle of the grooves are uniquely engineered for each model.Touring tires have small grooves to channel air and water on smooth pavement.Off road tires have wide deep grooves to provide traction in slippery conditions.of 40

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Anatomy of a tire

Tread patterns are symmetrical, with the left and right sides as mirror images.Ribs (the ridge of the tire) and grooves are counted across the entire tread width from shoulder to shoulder.

If the tire has a central ridge, it will have an uneven numbers of ribs.

Wear patterns and other imperfections should be noted.

To match the tire pattern of a suspects car to an impression taken at the crime scene, at least 3 meters of tire mark must be obtained as a sample from the suspect vehicle.Done by inking the tire and driving it onto paper.of 4020

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Identifying a vehicle

Because the same type of tire may be used on many vehicles, just identifying tread pattern often isn’t enough.Track width and wheelbase will also need to be measured.

Track width is measured in mm from the center of each tire to the center of the opposite tire.

Wheelbase is measured in mm from the center of the front axle to the center of the rear axle.

Wheelbase

Track width

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Identifying a vehicle

Turning diameter is the measure of how tight a circle a vehicle can make.A longer wheelbase increases the turning diameter.

There is a large database that contains the track width, wheelbase, and turning diameter for all makes and models of cars.

This information can be accessed to identify the vehicle that left an impression.

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Accident reconstructionEstablishing car movements from tire marks

A vehicle’s direction of travel can be established by studying:

Vegetation disturbed by the vehicle.

Patterns of debris cast off by a moving vehicle.

Splash patterns created as a vehicle moves from wet to dry pavement.Substance transfer, such as oil leakage, from vehicle to pavement.Tire marksPhotographs and measurements are recorded to reconstruct the events of an accident. The goal of accident reconstruction is to determine what happened.

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Hit-and-run

In hit-and-run situations, the vehicle is gone but tire marks, splash patterns, pattern of debris, and substance transfer can all be used to determine:

Speed and direction of movement

Whether brakes were applied or not

Whether the vehicle accelerated to “get away” from the sceneCase study: Alampi/Bevard caseof 40

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Tire marks

There are three basic types of tire marks.Skid marks

Yaw marks

Tire scrubs

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Skid marks

Formed when someone brakes suddenlyProvides evidence of the point at which the brakes were applied.

Calculation of velocity can be made from skid marks.

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Yaw marks

Produced when a vehicle skids sideways due to traveling too fast as it turns.Tires and the road surface melt from extreme temperatures (friction).

Audible squeal and smoke.

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Tire scrubs

Produced by damaged or overloaded tires during or immediately following an accident.Usually appear curved, irregular in

width.

May have striations that look like

stripes.Can be used to determine area of impact.

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Dental impressions

Bite marks are considered individual evidence.Factors that contribute to individuality:

Number

Size

AlignmentDental workof 40

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Dental impressions

The individuality of teeth can be used in forensics two ways:To identify remains

To implicate a suspect in a crime

Up to 76 points of comparison may be used when comparing bite marks, including:

ChippingSurface indentations/groovesTooth dimensionsDistances between teethAlignment of teethAngle of mouth archMissing teethDental work

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Processing a bite mark

Gloves should be worn when touching a bite mark.Bite marks should be photographed immediately

A ruler should be included for reference

Bite marks should be swabbed for saliva.

Bite marks descriptions should be recorded in a report.A forensic odontologist should be consultedBite mark photographs should be scanned photographic distortion corrected.Dental molds should be taken from any suspects.of 40

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Part IITool MarksChapter 16

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Tools and crime scenes

Some of the most common type of physical evidence left at a crime scene are tools.Tools generally have unique characteristics, even though they are manufactured in bulk.

There are 3 categories of tool marks:

Indentation marks

Abrasion marksCutting marksPhotographs should always be taken of tool marks when processing a crime scene.of 4035

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Indentation marks

Indentation marks occur when a tool is pressed into a softer surface.The impression can be measure to determine size and shape of the tool.

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Abrasion marks

Made when surfaces slide across one another.Pliers, axes, knives, and gun barrels are just some of the tools that make this type of mark.

Some indentation marks and abrasion marks are made at the same time, such as by a pry bar forcing a door open.

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Cutting marks

Cutting marks are produced along the edge as a surface is cut.Saws & wire cutters leave cutting marks.

By examining the cut surface of a bone, it’s possible to determine the type of saw blade used.

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Tool mark examination

Always document with a photograph, a ruler should be included in photo for reference.Tool mark evidence can be removed from the scene to preserve it.

Investigator may actually cut a section out of a door for this purpose.

A cast can be made using a rubber-based casting material if the evidence can’t be removed.

Always dust for fingerprints first.Always wear gloves.Evidence collected should be wrapped in paper and labeled or placed in a box so that it does not become damaged.of 40

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Any questions?

THE END