Because objects can move in three dimensions, we use 3-D Cartesian axes (X, Y and Z) to map out the paths that droplets of blood take from a source of injury. Police often want to reconstruct the exact position where the injury occurred. A blood spatter analysis can give us two pieces of information, first, the directions in which the droplets were travelling, and second, the angles at which they struck a horizontal surface such as the floor (impact angle).
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The first step is to fit an ellipse to the oval shape of the stain. The direction of the path of the blood droplet at the instant of impact is given by the long axis of the ellipse. Next, the impact angle is worked out from the relationship: sin (impact angle) = W / L where W
and L are the width and length of the ellipse.
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Fortunately, when the parabolic arc is viewed from above, it appears as a straight line. These lines are called STRINGS, because lengths of string were once laid out from each stain found at a crime scene. When viewed from above all the strings pass through the same point. This convergence point (CP) of the intersecting strings is the mapped location of the blood source.
The pattern analysis was performed by computer software designed by F. Carter, Carleton University, Ottawa.
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This graph shows the string paths from several blood stains, labelled 1 to 12. The red cross marks the convergence point (CP). The laws of physics require that the strings all pass over the source point. However when it comes to estimating the height of the blood source, the same laws require information that we do not have, such as the sizes and speeds of the droplets. Therefore the best we can do is estimate an upper limit for the height of the blood source using the side or end views of the strings. |
Graph and blood stain image courtesy of Fred Carter
