Blood spatter
analysis
An adult human contains about 5 litres
of blood.
Loss of approx 30% blood volume (1.5
litres) through either internal or external
bleeding usually results in a person
losing consciousness.
The loss of 40% (2 litres) blood volume
can be fatal.
What can forensics do?
Forensic scientists can reconstruct events at
crime scenes from the blood spatter patterns
present.
They can determine from a blood stain…
• Direction of travel
• Angel of impact
• Nature of the force
• Relative positions of individuals at a scene
• The sequence of events that may have occurred
3 velocities of blood spatter
• High velocity blood spatter may have come
from a gunshot wound inflicted by a bullet
moving at 900 feet/second.
• Medium velocity may have resulted from a
spurting artery or a blunt instrument striking
the already bloody head or limb of a victim.
• Low velocity blood may have dripped from
a wound or blood-soaked item.
High velocity blood spatter
• High velocity blood spatter is produced by an
external force greater than 100 feet/sec.
• The stains, sometimes referred to as a mist,
tend to be less than 1mm.
• Usually created by gunshots or explosives,
high velocity patterns also may result from
industrial machinery or even expired air, such
as coughing or sneezing.
• High velocity droplets travel the shortest
distance because of the resistance of the air
against their small mass.
Medium velocity blood spatter
• An external force of greater than 5 feet/sec but
less than 25 feet/sec causes medium blood
spatter.
• The stains generally measure 1 to 3 mm.
• Most medium velocity stains found at crime and
accident scenes form patterns created by blood
flying from a body to a surface as a result of
blunt or sharp trauma or the body colliding
with rounded or edged surfaces.
• It may result from a punch, stabbing, or a series
of blows or, in the case of an accident, the body
striking surfaces inside or outside a vehicle.
Low velocity blood spatter
• Low velocity blood spatter is created by an
external force less than 5 feet/sec (normal
gravity)
• The stains are generally 3 mm and larger.
• It usually results from blood dripping from a
person walking or running or from a bloody
weapon.
• Dripping blood often falls at a 90-degree angle
and forms a 360-degree (circular) stain when it
hits a flat surface.
The longer the
blood stain the
lower the angle
of impact
Spatter
diameter
increases with
height +
spikes appear
at edge of
stain
Direction
of blood
The stringing
method
Used to determine point
of convergence of several
blood spatter stains (i.e.
location of the victim)
After measuring the width &
length of the blood spatter a
simple calculation can be
done to determine the angle
of impact.
Area of convergence
(2D origin of spatter)
Angle of impact
Example
What to record:
- Length & width of each blood drop
- Distance to point of origin from the back of each blood drop
Point of Origin of Spatter
(3D)
2 methods to calculate point of origin (3D)
Both methods need these initial steps at the crime
scene:1. Identify stains that have a common area of convergence
2. Draw lines along the central axis of each stain to determine
location of convergence
3. Measure the distance from the back edge of the stain to the area
of convergence
4. Calculate the angle of impact for each stain individually
5. Ensure at least 3 stains are used (the more the better)
Method 1 - graphing
1. X-axis represents the location
of the blood stain (eg. floor)
2. Z-axis represents the height
from the stain
3. Both axes must be drawn
using the same scale
4. Mark on the x-axis the distance
to the point of convergence for
each individual stain
5. Use a protractor to draw a line
at the angle of impact from the
mark on the x-axis until it
touches the z-axis
6. The area on the z-axis that the
lines converge will be the likely
point of origin
Distance
to PoC
Angle of
impact
Stain 1
8cm
68o
Stain 2
16cm
52o
Stain 3
23cm
41o
Method 2 - Trig
Example:
If D = 30cm
&
H = tan35 x 30
H = 21cm
i = 35o
Now its your turn!
1. Draw lines of convergence
2. Work out the angle of impacts for all 3
blood stains
3. Work out the point of origin for the 3
blood stains using:a) Graphing
b) Trig
c) Triangulation (strings)