angle of impact of a blood droplet

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Stringing, Sine and Spatter Shapes

The technique of "stringing" -- attaching strings to each spatter to see where they converge -- is just one way to determine the area of convergence, or source of the blood. Most TV shows, like "Dexter," focus only on the analyst looking at the strings and making statements about the crime, not on the process involved. In the stringing method, which is still used by many analysts, the analyst documents the location of each spatter using the coordinate system. Then, he or she establishes a level line to show how the spatter is oriented in relation to the floor and ceiling.

Using elastic string, the analyst draws lines from each spatter through the level line. Then, he or she uses a protractor on the level line in the area where the strings converge to determine the angle of flight for each spatter. If the spatters are mostly on a wall, the analyst can measure the distance from the area of convergence to the wall to find out where the victim was located.

Some analysts use trigonometric calculations to find the area of convergence, which basically means that they use triangles to figure it out. The measurements of the blood stain become the sides of a right triangle. The length of the stain is the triangle's hypotenuse, and its width is the side opposite the angle the analyst is trying to find.

First, the analyst has to locate each spatter and measure its length and width using a scale, a ruler or calipers. Then, he or she computes the angle using this formula:

angle of impact = arcsin (opposite side/hypotenuse)

Here's what an analyst has to do to for this to work:

  1. Measure the length and width of the splatter.
  2. Divide the width of the splatter by its length.
  3. Determine the arcsin of that number, typically using a calculator with an arcsin function.

A drop of blood that fell perfectly vertically, or at a 90-degree angle, will be round. As the angle of impact increases, the drop of blood gets longer and develops a "tail." This tail points in the direction that the drop traveled, but its length isn't part of the measurements.

blood droplets elongate

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The greater the difference between the width and length, the sharper the angle of impact. For example, imagine a bloodstain 2 millimeters wide and 4 millimeters long. The width divided by the length is 0.5. The arcsin of 0.5 is 30 -- so the blood hit the surface at a 30-degree angle. In a bloodstain that is 1 millimeter wide and 4 millimeters long, the ratio between them is 0.25. In this case, the blood hit the surface at about a 14-degree angle.

A third method involves taking the length and width of each blood spatter as well as other measurements of the area and putting them into a computer program, such as No More Strings. These programs create three-dimensional models and animations of the crime scene and show the area of convergence. When used to present evidence, computer programs can be more compelling and convincing than jargon-filled expert testimony or two-dimensional photographs.

area of convergence

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So far, we've discussed how bloodstain pattern analysis can work when trained law-enforcement officers implement it correctly. We'll look at the history of bloodstain pattern analysis, as well as an infamous case that contains botched bloodstain pattern analysis practices, next.