I’ve mentioned before on my blog, blood has many facets. DNA typing uses STR markers to individualize a person based on their DNA, blood typing can be used to more swiftly remove a person from being under suspicion, given that the tested DNA is a type opposing a tested person (e.g. finding blood, testing it and finding it to be O+ and the blood of the main suspect’s is AB+).
The physical natural patterning of blood droplets are just as important. They can be analogous to matrices for shoe prints or fingerprints (etc.)(like a fingerprint in clay), they are also an indicator of where people were positioned, if you can read it accurately. There are certain shapes of droplets, measurements, calculations and physics formulas that can be used to project a map. Which lead to understandings like indications of direction, location of wound due to volume of blood and height or it’s violence in patterning.
Using a drops length and width, you can calculate the angle of impact. You use a trigonometric function called arcsin, because of the application of sin on right angles, and the spattering taking the form of a two dimensional projection on a 3D space. The formula for angle of impact, is [angle of impact = arcsin (length/width)], which turns the width and length into ratios within the formula. So, a truncated version of the formula and an example would be an ellipse pattern drop 5 inches long and 3 inches wide.
[36.87° = 0.6 (arcsin)] Bloodstains cannot be wider than they are longer due to the basic physics, unless its a spill, which is more than just a drop of blood.
It requires requisite knowledge of geometry, physics, physiology, a good sense of logic and some requisite licensing (40h of schooling, 3 years of training, 240h in associated fields of study (related to bloodstain pattern analysis ((BPA)BSA).
Terms to know:
Passive bloodstains- any droplets or blood left by falling, flowing or pooling.
Transfer bloodstains- any swipes of blood, pattern transferring, or contact bloodstains.
Impact/Projected bloodstains- spatters (back spatter), splashes, cast-offs (from hand weapons being flung back overhead, etc), gushes or spurts from arteries.
Fly spots- occur because of insect activity (usually feeding on the blood)
Void spot- discontinuance of blood due to an object blocking the path of the blood
Skeletonized stain- when blood is allowed to partially dry, but is then wiped (O’s)
(TBC)