What measurements do you need to calculate the impact angle?

To analyze a bloodstain pattern, an expert relies on three main interrelated elements: the size, shape and distribution of bloodstains. Since we've already discussed stain size, let's dive right into shape. Blood drops that fall straight down, with little but gravity and air resistance affecting them, make round stains. Blood moving at an angle and sped along by some force, however, tends to make elongated marks, especially when it strikes a nonporous surface.

As a rule, following the long axis of the stain from the blunter end to the sharper, more disturbed edge reveals the direction the blood traveled. If a number of stains radiate outward, analysts can draw lines backward along these axes to an area of convergence. But this gives them an area in only two dimensions. Investigators must also determine the blood's impact angle [source: Dutelle].

The lower the angle at which blood strikes a surface, the thinner and more elongated the stain. The converse holds true as well. For example, if a blood droplet has a 10-degree drop, it creates a highly elongated stain, whereas a droplet with a 90-degree (vertical) drop leaves a round stain [source: Dutelle]. Measuring a stain's width and length, analysts use the following mathematical formula to calculate the impact angle:

angle of impact = arcsin (stain width/stain length)

The greater the difference between the width and length, the sharper the angle of impact [source: Dutelle]. For example, imagine a bloodstain 0.08 inches wide by 0.16 inches long (2 by 4 millimeters). The width divided by the length equals 0.5. The ArcSin of 0.5 is 30, meaning the blood hit the surface at a 30-degree angle. In a bloodstain measuring 0.04 by 0.16 inches (1 by 4 millimeters), the impact angle comes out to about 14.5 degrees.

If the bloodstain has a tail, as can occur in droplets striking a surface at certain angles and speeds, it should be left out of this calculation [sources: Dutelle, Eckert and James].

Once analysts know all three angles, they can move on to the most visually striking part of the process, and therefore the one most featured in shows like "Dexter" — the technique of stringing. Stringing involves running strings from the rear edges of the bloodstains upward at the appropriate angles to find where they roughly meet — the area of origin. This technique provides only an approximation, however, and is mainly used to establish whether a victim was seated, standing or lying down when the event occurred. The presence or absence of blood on other surfaces, combined with common sense, also aids in this analysis [source: Dutelle].

Increasingly, analysts are taking advantage of computer programs that allow them to store spatter data, calculate values such as impact angle, and display information in helpful 3D renderings. Initially, this software required manual data entry, which was tedious but perhaps not as tedious as stringing.

Developers are working on software that can automatically reconstruct a single coordinate frame from several images, limiting user input. Others are working on using laser scanning and even machine learning to analyze the blood spatter.

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

Becoming a Bloodstain Pattern Analyst

The field of bloodstain pattern analysis requires knowledge of math, physics, biology and chemistry. Students in criminology and criminal justice learn about bloodstain pattern analysis in forensic science classes or classes specifically on blood spatter. A common path for professionals involves earning a degree or certificate in criminal justice or forensic science, augmented by courses, workshops and seminars taken periodically throughout their careers [source: RRCC]. Many law enforcement officers train in bloodstain pattern (or blood spatter) analysis through the International Association of Bloodstain Pattern Analysts (IABPA). The IABPA developed criteria for basic and advanced courses on the subject [source: IABPA]. Other organizations, such as the International Association for Identification (IAI), offer workshops and seminars in addition to advanced courses that lead to certification in blood spatter analysis.

Lesson: Angle of Impact Lab

Objective: For students to learn and use trig functions in the real world.  Students should be able to solve for angles in a right triangle.

Standards: Apply content to real-world scenarios.

Set-up: 10 minutes before class.

Procedure:

• As students enter the classroom, students will begin work on the daily question.

• After two students go up to the board to work out the daily question, go over the correct answer with them.

• Spend 5 minutes going over any missed or confusing questions the students had on the assignment.

• Before the start of the angle of impact lab share with them a quick way of determining the blood splatter pattern. It should look like the image to the right.

• Have students spend 20-30 minutes working on the angle of impact lab.  Worksheet is attached.  Students should be in groups of 2 or 3.

• 
  When students are finished with the angle of impact lab, students are to complete the final part of the lab with a poster.  Students should spend the remainder of the classroom working on the poster and putting their finishing touches on the assignment.

• 2-3 minutes before the bell rings students should fill out their exit slip, for an informal assessment.

The length of the blood stain is the hypotenuse while the width is the opposite side of the angle of impact. The angle of impact can be calculated from the measurement of the opposite side which is the width and the length is hypotenuse of the blood stain. The angle of impact can be calculated using the formula angle of impact = arcsin (opposite side/hypotenuse) the investigator has to do the following to obtain the data: Measure the length and width of the splatter. Divide the width of the splatter by its length. Determine the arcsin of that number, typically using a calculator with an arcsin function. The point of convergence of all the blood spatters can be located by drawing an extended line and locate the point of intersection of all the lines. A string can be pinned on each blood spot and raise it following its angle of impact. A three-dimensional model can be created and the point of convergence can be shown