Welcome to this learning object: stain measurement and calculating angle of
impact. The process of measuring a blood stain
and determining its angle of impact uses trigonometry x' sine function. Because a
mathematical relationship exists between the width and length of an elliptically
shaped blood stain it is possible to calculate the angle of the impact for
the original drop of blood. if a stain is well formed we can accurately measure
the width and length of the stain by simply dividing the stain along its
major and minor access. Because the opposite halves would generally be equal
to each other this calculation establishes the angle of impact blood
drops that are projected onto an object form ellipsis --is when they impact the
object at an angle that is less than 90 degrees. Directionality is easy to
determine here, it is from right to left. if the angle of impact is 90 degrees
then the blood drop is in the shape of a circle not an ellipse. directionality is
not always easily determined with near circular stains. in the case of the
stains shown directionality can be determined by the visible scalloping
which is from right to left.
under ideal circumstances of velocity impact angle and target surface, a drop
of blood on impact would create a stain that was perfectly uniform oval shape.
ideal circumstances rarely if ever exist particularly an actual casework. the
distortion created in response to the landscape of the target surface will
prevent this stain from being a perfect oval. if the volume of the drop of blood
is great enough the blood that has not been stabilized by deposition on the
surface will continue along a path of travel creating even more deformation
from the ideal elliptical stain. all of this distortion is the result of causes
other than the angle of impact for that reason any deformation at the terminal
edge of a stain must be negated when the length of the stain is measured for
impact angle determination. to eliminate the distortion from the terminal edge
and form an ellipse we must measure the distance from the centre of the widest
point to the leading edge of the stain. this value is then doubled to arrive at
the overall length of the undistorted stain
to calculate the angle of impact the first step is to fit an ellipse to the
oval shape of the stain the ellipse must be divided along the major or minor axis
to do this we draw a line for the length through the middle of the stain from the
leading edge to the terminal edge creating the major axis line we
determine the widest part of the stain by drawing a line to show the width of
the stain creating a minor axis to form the ellipse we must measure the distance
from the center of the widest point the minor axis of the stain back toward the
leading edge of the stain this value is then doubled to arrive at the overall
length of the undistorted stain the two halves wouldn't be approximately equal
to each other
the device used for measuring stains depends on the analyst some analysts
prefer a loop with a built-in scale while others use calipers or metric
rulers in this example we will use a metric ruler the major axis has been
completed and we have determined that one side of the minor axis is 1.5
millimeters we arrived at this by measuring the distance from the centre
of the widest point the minor axis to the leading edge point of the stain now
we double the 1.5 millimeters to make three millimeters which eliminates the
distortion and creates an ellipse we now fit an ellipse to the oval shape of the
steam the width of the ellipse of the stain is three millimeters keep in mind
that you must never include any portion of the satellites caliper spine that may
be present in the stain including that in the measurement would alter the
overall length to width ratio and change the calculated impact angle you need to
envision a perfect ellipse superimposed on the stain now it's time to determine
the width of the stain
using a metric ruler we determined that the width of the stain at its widest
point is one millimeter and the length of the stain is three millimeters with
this information we can determine the angle of impact using the formula that
states the sine of an angle equals the width divided by the length we can
calculate the angle by using either a scientific calculator or a trigonometric
table
next we will use a scientific calculator to determine the angle of impact the
formula as previously mentioned is the sine of the angle equals the width
divided by the length the calculator must have the sine and inverse functions
after you turn on the calculator enter the width of one millimeter and then
divide that by three millimeters the length this will e will 0.33 millimeters
which is the sine of the angle push the inverse button followed by the sine
button which will then give you the angle of a impact nineteen point five
degrees it is that simple but you must use a scientific calculator with the
functions listed above to make it this easy
now we will use the trigonometric table to determine the angle of impact use the
formula previously mentioned the sine of the angle equals the width divided by
the length on your calculator enter the width of one millimeter and then divide
that by three millimeters the length this will equal 0.33 millimeters which
is the sine of the angle now go to the trigonometric table and find 0.33 which
is between the 19 and 20 degree angle of impact this is how to measure stand and
calculate the angle of impact using a trigonometric table
we have demonstrated how to measure and calculate the angle of impact of an
acute angle sting acute angle stands are elongated and elliptical in shape now we
will demonstrate how to measure and calculate the angle of a more circular
shaped state if the angle of impact is ninety degrees then the blood drop is in
the shape of a circle not an ellipse as stated earlier directionality is not
always easily determined with near circular stains
under ideal circumstances of velocity impact angle and target surface a drop
of blood on impact would create a stain that was perfectly uniform oval shape
the distortion created in response to the landscape of the target surface will
prevent the stain from being a perfect oval usually to eliminate the distortion
from the terminal edge and to form an ellipse we measure the distance from the
center of the widest point to the leading edge of the stain in this near
circular stain we can see there is very little distortion which results in
little if any terminal edge being present
to calculate the angle of impact the first step is to fit an ellipse to the
oval shape of the stain forming an ellipse on this near circular stain is
next to impossible because there is very little distortion the ellipse must be
divided along the major or minor axis to do this we draw a line for the length
through the middle of the stain from the leading edge to the terminal edge
creating a major axis we determine the widest part of the stain by drawing a
line to show the width of the stain creating a minor axis to form the
ellipse we must measure the distance from the center of the widest point the
minor axis of the stain back toward the leading edge of the stain. This value is
then doubled to arrive at the overall length of the undistorted stain the two
halves would be approximately equal to each other.
to measure a blood stain some analysts prefer a loop with a built-in scale
while others choose to use calipers or metric rulers in this example we will
use a metric ruler the major axis has been completed and we have determined
that one side of the minor axis is 0.75 millimeters we arrived at this by
measuring the distance from the centre of the widest point the minor axis to
the leading edge point of the stain. now we double the 0.75
millimeters to make 1.5 millimeters which eliminates the distortion and
creates an ellipse we have now determined the length of the nearest
circular shaped stain the width of the ellipse of the stain is 1.5 millimeters
remember you must never include any portion of the satellite scallop or
spine that may be present in the stain. in the stain shown here there is
scalloping which should not be included including that in the measurement would
alter the overall length to width ratio and change the calculated impact angle
now it's time to determine the width of the stain
the device we used here to measure the minor axis width is a metric ruler the
stain width is 1.4 millimeters and the stain length is 1.5 millimeters with
this information we can now determine the angle of impact to do so we must use
this formula the sine of the angle equals the width divided by the length
we can calculate the angle of impact by either using a scientific calculator or
trigonometric chart next we will use a scientific calculator
to determine the angle of impact the formula as previously mentioned is the
sine of the angle equals the width divided by the length. the calculator
must have the sine and inverse functions turn on the calculator enter the width
of 1.4 millimeters and then divide that number by 1.5 millimeters the length
this will equal 0.9 three millimeters which is the sine of the angle push the
inverse button followed by the sine button which will then give you the
angle of impact of sixty eight point nine degrees it is that simple but you
must use a scientific calculator with these functions to make it this easy
finally let's use the trigonometric table instead of a calculator to
determine the angle of impact. use the formula previously mentioned which is
the sine of the angle equals the width divided by the length on your calculator
enter the width of 1.4 millimeters and then divide that number
by 1.5 millimeters the length this will equal 0.9 three millimeters which is the
sine of the angle now go to the trigonometric table and find 0.93 which
is a sixty nine degree angle of impact. You have now seen how to measure stains
and calculate the angle of impact of different shape stains. Congratulations,
you have completed this activity: stain measurement and calculating angle of
impact.