Transcript String Method

Introduction to Bloodstain Pattern
Analysis (BPA):
(Continued)
Calculating the Angle of Impact
Mechanism of how blood forms elongated stains is critical for
understanding how to determine the angle at which the blood impacted
with the surface.
When a blood droplet hits a surface it forms a right triangle, whose
angle can be calculated using trigonometric functions.
The step-by-step procedure
Approximate the outline of the stain as shown in the slide on
the
right below - white oval. Do not include the stain’s tail
(E) in the
approximation.
 Measure the length (D) and the width (d) from the stain.
 Calculate the fraction by dividing the width by the length
(d/D).
 Calculate the impact angle by taking the arcsine (sin-1) of the
fraction (d/D). A fraction of 0.5 represents a 30o angle.
Length
(L)
Width (W)
Calculating Angle of Impact
Point of Contact
With Surface
Angle of Impact
Arc sin W/L
•Measure length ‘L’
•Measure width ‘W’
• Calculate ratio ‘W/L’
•Calculate arc sin W/L
W
L
Direction of
Travel of Droplet
Procedure
2D Area of Impact
Determining the Origin of
an Impact
Point of convergence or Back
Projection in 2-Dimensions
Determining 2D Point(s) of Convergence
Represents the 2D location of
an impact,
 Blunt force trauma when
someone has been beaten
with an object or a fist.
 For impact stains, approach is
helpful in determining a possible
2D Point of
minimum number of blows
 Spatter cannot occur until
Convergence
blood present to be
splashed).
 Multiple points of
convergence suggest
Procedure
multiple blows.
Mechanism for selecting the
•Determine direction of travel
proper bloodstains from within a •Back project through Axis of Stain
bloodstain impact spatter pattern • Determine 2D convergence point
to use in determining the 3D area •Repeat for 20 stains
of impact.
Multiple points of
convergence suggest
multiple actions
2D Points of Convergence
3D - Area of Origin
Area of Origin in 3-Dimensions
 The spatial location of an impact that caused impact spatter
pattern
 Third dimension of the 2D determination.
 If impact location determined in 2D, converting that into a
3D image is simple
 Accomplished using 3D imaging techniques, such as
3D scene imaging systems, computer aided design (CAD)
programs or software specifically designed for the
application.
 Also approximated @ scene using string method
o Manually or by using lasers.
 Alternatively, the spatial area (point) can be calculated
using a combined manual/trigonometric method..
The Tangent Method – Step-by-Step
 Use 2D back project method discussed above and back project stains to point of convergence
 Step 1 in the diagram below and illustrated in previous slide.
 Measure the width (d) and the length (D) of two droplet stains.
 Calculate the impact angle by taking the arcsine of the width divided by the length of the
bloodstain.
 Measure the distance from the point where the blood droplet hit the surface to the point of
convergence.
 The height of the area of impact (Z)?is calculated using the tangent function as shown in the
slide below.
3D Point of Impact
Z (height)
2D Pt. of Impact
2. Determine Impact Angle
Using sin-1(w/l) function
1. Back
Project
1. Back Project
Tangent Method
z= tan (impact angle) x Length
String Method
3D - Spatial Determination of Impact Area
The String Method
 Manual procedure for approximating the area of origin of an impact @ the scene
 Select several stains (10-20) from bloodstain pattern
 Most easily from a previous 2D back projection determination.
 These stains have specific characteristics:
Characteristics of Droplets for String Method
 Droplets must be traveling linearly - means they still posses most of the energy
imparted by impact. The easiest way of determining which stains are appropriate is to
identify them by performing 2D analysis using back projection.
 Stains losing energy begin to arc and fall - no use for determining the spatial origin
of an impact.
 Stains should be as elongated as possible, given the complexities of the pattern
analyzed. Measuring these stains is less error prone than more circular stains.
The String Method:
Step-by-Step
Choose as many as 20 elongated stains
traveling linearly from impact point
before striking the surface.
Number each stain sequentially and
tape number to the wall (surface) next to
stain. Record the information in a log.
Calculate the impact angle for each
stain (arcsine width/length).
Record angle in log and onto tape next
to the stain on the impact surface.
Tape end of a long piece of string
where the blood droplet hit the surface.
String Method – Continued
 Ensure tape holding the string is secure.
 Use zero edge protractor to track string through
the center of the stain’s axis along the protractor at
the calculated impact angle.
 Pull the string taut and attach it to the surface,
e.g., floor or other object.
 Re-check the measurements and the stain
trajectory (the string) to ensure the angle of the
droplet impact is correct.
 Repeat the process for as many as 20 stains in the
pattern.
String Method
 If pattern is from a single event AND if
the appropriate stains were selected, AND if
measurements determined correctly AND if
the string was run at the correct angle
 Strings should converge in an area in space that
approximates the area of the impact.
 If the area of convergence seems too large, the
process might not have been performed correctly.
o Or indicates multiple blows
http://en.wikipedia.org/wiki/File:BPA_AOC.png
Troubleshooting The String Method
Reason
Poor stain selection
Incorrect stain measurement
Incorrect angle using the zero edge
protractor
Multiple impacts occurring in the same
general spatial location
Correction Method
Re-examine the impact pattern and the stains
selected.
Repeat measurements to ensure
measurement accuracy
If angles were calculated correctly (above),
check the protractor positioning. If placed
correctly, re-check string positioning.
Use 2D back projections to determine if
multiple impacts occurred. If so, use 2D
method to choose stains from each impact
and make overlapping 3D area of impact
determinations.
Cast off
Blood Leaving a Moving Object
An Example of Centripetal Force
Centripetal Force
 Only one force is at work, centripetal
force.
 Directed toward the center of the path of
the moving object.
 When the adhesive forces holding the
blood onto the object are greater than the
centripetal force, the blood will fly off the
object in a tangentially straight line.
 Straight line … impact site and hence its
angle … direct link to location of object at
the exact time blood left it.
Movement of Object
Left-to-Right
Castoff
 When blood leaves a blood-covered object it can
 Drip passively or be propelled.
 If propelled blood leaves the blood-covered object when the centripetal
force acting on it overcomes the adhesive forces holding the blood on the
object.
 True whether blood cast from object occurs while the bloody object is
being swung or if it comes to an abrupt halt (cessation castoff).
Example
 Blood on a bloody knife
 Knife is swung in an arc,
 Blood will be propelled (castoff) the knife in a path that is tangentially
straight from that point and travel until it hits impacts a surface.
 If blood hits the surface while still traveling in the straight line, the shape it takes
will represent the angle of the impact.
 Trigonometric functions can be used to calculate the impact angle
 Backtrack to the spatial origin of the castoff when blood left the object.
Castoff
 Created from bloody knife blade and
swinging the blade in a downward motion in
front of the target.
 Volume of drops and impact angles change as
the knife continues its downward motion.
Alternate Terminology
Bevel & Gardner
Spatter Family - Linear - Castoff
James, Kish & Sutton
Spatter - Projection
Mechanism - Cast-off
Wonder
Spatter Groups - Cast-offs Swing
Old IABPA
Cast-off Pattern
Cessation Castoff
Direction of
Travel
Bloody knife hitting floor
Breadth of Castoff Pattern
More than one droplet usually leaves bloody object as it
moves through its path
 Preponderance form a pattern characteristic of the
surface of the bloody facing the impact surface.
 Example
 Bloody hand is held sideways to the impact surface,
say a wall, the castoff pattern formed represent that
surface of the hand facing the impact.
 Pattern will be broad line of individual droplets.
 As hand moves through its arc, it will change position
causing individual fingers to face wall.
 Castoff pattern broadens and might show castoff from
individual fingers.
 Breadth of pattern reflects the blood-covered surface
area of the surface facing the impact site.
Thus, a knife blade will often give a single line of castoff
staining, while a baseball bat can give a broader castoff
pattern.
If the knife’s flat side of blade faces impact area, the breadth
of the castoff pattern will reflect the width of the knife blade,
but not its actual dimensions.
Cast Off Sequence
 First blow causes bleeding
 Subsequent blows contaminate weapon with
blood
 Blood is cast-off tangentially to arc of upswing or
backswing
 Pattern & intensity depends on:
 type of weapon
 amount of blood adhering to weapon
 length of arc
Bubble Stains
 Bubble stains form when air is present in the droplet. An
example or expectorated blood is shown below.
 The arrows point to the air bubbles in the stain, which are
expected in blood expectorated from the lungs for from the
mouth.
Impact Force
Droplet Size
Impact
Spatter
Drip Pattern: Secondary Spatter
 Free-falling drops dripping into wet blood
 Large irregular central stain
 Small round & oval satellite stains
Arterial Spurt
Blood exiting body under arterial pressure
Large stains with downward flow on vertical
surfaces
wave-form of pulsed flow may be apparent
Heartbeat
Object moves through a
wet bloodstain
Feathered edge suggests
direction
Alternate
Terminology
Wipe Patterns
Bevel & Gardner
Non-spatter Family - Irregular
Margin - Smear - Wipe
James, Kish & Sutton
Altered - Sequenced - Wipe
Wonder
Spatters Not a Criteria Transfers - Moving - Wipe
Old IABPA
Wipe Pattern
 Wet, bloodied object
contacts a secondary surface
 Transfer from:
Swipe Patterns
 hand, fingers
 shoes, weapon
 hair
 Transfer to:
 walls, ceilings
 clothing, bedding
 Can produce mirror-image
of bloodied object
Alternate Terminology
Bevel & Gardner
Non-spatter Family Irregular Margin - Smear Swipe
James, Kish & Sutton
Passive - Contact/Transfer Swipe
Wonder
Spatters Not a Criteria Transfers - Moving - Swipe
Old IABPA
Swipe Pattern
Flow Patterns
Blood flows horizontally & vertically
seeking the path of least resistance
 Altered by contours, obstacles
Often ends in pool at the lowest or
most blocked point
Forward and Back Spatter
Forward Spatter
Backspatter
(AKA Blow Back)
Bloodstains on Clothing
Whose blood on the clothing?
How was blood deposited?
 Passive staining
Transfers
Fabric Impressions
Flow patterns
Saturation stains
Dripping blood
 Dynamic staining
Impact spatters
Arterial spurts/sprays
Expirated stains
Castoff
Bloodstains on Clothing
 How was garment collected?
 Photos of person wearing garment
 Examine stains BEFORE DNA analysis
 Non-destructive examination
History of garment
 How handled
Emergency room floor
o After suspect’s injuries treated
» Additional bloodstains added
» Existing ones altered
Sources of Error
 Differential expansion of blood droplet upon
impact
 Where width of stain is NOT equivalent to
droplet diameter!
 Fabric dependent
 Measurement error – significant figures
 Droplet volume and velocity unknown
 Surface properties
 Texture
 Porosity
 Resiliency
 Droplets not originating from a single source
Spattered Blood
 Random distribution of bloodstains that vary in
size
 Produced by variety of mechanisms
 Size range varies considerably by any one mechanism
 Quantity varies depending on quantity of blood
available
 Force applied
 Gunshot
 Beating
 stabbing
Identify Spatter Patterns
Archiving Bloodstain
Patterns
Archiving Bloodstain Patterns
All archiving principles apply. However scene investigators are not BPA analysts … need to
understand and recognize them and then follow guidelines to properly preserve them.
 Acquire knowledge: Study bloodstain patterns and the underlying scientific
principles relevant to BPA and Take an approved BPA workshop,
 Gain on-scene experience,
 Experiment
 First: Examine bloodstain pattern carefully to ascertain its overall
characteristics. Answer the following questions.
How much area does the pattern encompass?
Is the pattern a composite of multiple patterns?
How might this pattern have occurred?
Begin archiving. Capturing these properly requires several photographs: …
 Establishing
 Midrange of each staining area.
 Close up photographs provide detail,
o Misting, etc.
o Close-ups of the relevant details present in various areas of the overall pattern.
o Iinitial photographs taken without scales & then repeated with scales.
 Preserve the overall size of the pattern as well as sub-patterns within the whole.
Scene Example
 Photograph before scales in place … at a scene as midrange shot of the bed
and wall behind the bed.
 The stains on the bed, those on the wall behind the bed and those on the floor
next to the bed were important.
 Not shown in the photograph are the castoff stains: on the ceiling above
the bed, on the wall to the left of the bed and wall opposite the bed.
 Multiple superimposed
patterns to capture for meaningful
archive.
 Multiple impact spatters
 Large swipe/wipe pattern on
the wall behind the bed leading
from the just above the bed and
flowing downward toward the
floor
 Smaller swipe patterns on the
wall to the right of the main
impact spatter.
Step
Reason
Determine the total area the pattern covers by examining adjoining walls,
floors and ceiling.
Measure perimeter of each area of pattern staining. Record measurements
in appropriate log.
Bloodstain patterns should be photographed first in the raw, without scales,
so that subsequent analysis is not hindered by their presence.
Photograph the area(s) without scales.
Ensures that subsequent archiving will not miss important
areas of the pattern
Archives entire pattern without scales which can cover
droplets from adjoining or overlapping patterns.
Cordon off each pattern using adhesive measuring tape ruled in inches.
Include the entire pattern - width and length. Photograph the pattern with
the scales in place using forensically appropriate photos: establishing, mid
range and close-ups, if appropriate. If not possible to capture its entirety,
grid the area and photograph grid each separately
Scales ensure that the size (with and length) of a pattern is
captured.
Video the pattern using a digital camera having video capability (or a
dedicated video camera). Capture each area of the pattern. Show
relationship of each to the whole. Ensure to capture areas adjacent to the
pattern - walls, floor and ceiling.
Photograph each area of the pattern having special characteristics.

Radial impact spatter might have value in determining the spatial area
of the impact. If analysis not be done at scene, capture individual
droplets (numbered sequentially and taped to the surface) so that
subsequent length/width measurements for stain pattern analysis
software.

Impact pattern with imbedded mist pattern - capture the latter,
measuring overall size and that of representative droplets.

Complex pattern with multiple, overlapping characteristics, (multiple
impact patterns and/or multiple castoff patterns or is an impact
pattern containing cessation cast off). Capture sub-patterns separately
showing relation to overall pattern.
Gives the BPA analyst the overall perspective of the pattern.
Gridding is a least preferred method, but is useful to
capture detail in extremely large patterns.
Alerts the BPA analyst to pattern complexity or that
information within the pattern might have additional
interpretative data.
Bloodstain Artifacts – Selected Examples
 No Scene is virginal … must be considered a scene that is not as it
was when the crime took place.
 Blood evidence is not exempt from evidence dynamics.
 Examples of things that happen to blood evidence range from
cleanup activity, officials responding to the scene, emergency
medical personnel trying to save lives are common to weather
complications.
 Artifacts, a common concern
 Can occur from people who work on the scene
 Insects who dine on blood or walk through the scene.
Insect Activity
 It is well known by experienced BPA analysts that insects can leave artifactual
marks in blood at the scene. Forensic Entomologist Jason Byrd wrote
“Insects can also affect the interpretation of blood spatter pattern analysis. Roaches simply
walking through pooled and splattered blood will produce tracking that may not be readily
recognizable to the untrained observer. Specks of blood in unique and unusual areas (such
as on ceilings) may mislead crime scene technicians unless they are aware of the
appearance of blood contaminated roach tracks.
Similarly, flies and fleas may also track through pooled and spattered blood. However, flies
will also feed on the blood and then pass the partially digested blood in its feces, which are
known as "flyspecks". Flies will also regurgitate and possibly drop a blood droplet on a
remote surface, which may serve to confuse bloodstain analysis.
Fleas feeding on the living pass a large amount of undigested blood (used as the larval food
source) on many household surfaces. If a crime occurs in a heavily infected apartment, fecal
drops already present would serve to confuse analysts as those droplets would test positive
for human blood. Therefore it is important to recognize and properly document the natural
artifacts that may occur from the presence, feeding, and defecation of roaches, flies, and
fleas.”
Flies
 Contamination and artifacts from insects
dining on dried or wet blood at the scene should
be anticipated by the BPA analyst,
 Investigators should understand that such
activity can be common.
 Artifactual staining has confused BPA
analyists but that from flies should not have.
 Shapes and sizes of the individual stains
are characteristic of the patterns that form,
and these have been described
Type
Characteristics
Size
Fecal
Symmetrical and asymmetrical round
spots having 3 levels of pigmentation:
Creamy, brownish and dark.
0.5-4mm
Vomit
regurgitation
Craters from sucking activity are
surrounded by raised edges having a dark
perimeter. The surface is irregular and
reflective.
1-2 mm
Trailing
(swiping due
to defecation)
Distinguished by two segments, a body and 4.8-9.2mm
a tail appearing sperm-like, tear shaped or
snake-like.
Cockroaches
Cockroaches feast on blood. Like flies, they leave telltale marks of their activity
that can be misinterpreted by inexperienced scene scientists/investigators.
On-Scene Investigative Activity
 The following illustrate how crime scene unit activity can affect
bloodstains. Note the wall next to the deceased.
 The photographs illustrate the importance of on-scene archiving before
actively working the scene or removing evidence.
The bloodstain artifacts on wall in the left photograph were created by the
crime scene unit removing the evidence from the corner .
The perspective in each photograph is slightly different, which is why some
of the detail in the left photograph is missing in the right photograph.
Photograph: original before
the crime scene unit arrived
at scene.
.
Before Processing Began
Photograph taken after the unit
completed processing but
before the body removed
After Processing
Fabrics – Differential Absorption
Interpreting bloodstain patterns from blood deposited on garments can pose problems
because modern fabrics are often blends of different types of fibers, each of which has
different absorptive properties.
Packaging/Preserving Bloodstain Evidence
 Photography @ the scene
 Clothing of homicide victims/witnesses
Dry all wet evidence if possible
 If not possible – do not fold stains on top of stains
 Procedure
 Place clean paper under & over garment
 Fold garment over the paper so that no stains come
into contact with other stains
 Place folded garment into paper bag & seal