One of the jobs of a blood spatter analyst is to determine the location, or point of origin, of a source of the blood. In order to do that, the analyst must look at the different shapes of the blood droplets and figure out the direction and angle at which they struck the surface.

Direction of Impact

If a blood droplet hits a surface at a 90 degree angle then the shape will be a perfect circle. At any other angle the blood droplet will take on the shape of an ellipse and will typically have a tail extending from it.

                Blood Droplet                                                Tail

The tail is formed by a smaller droplet of blood breaking off the main droplet early on in its flight and landing slightly ahead of the larger one.

If there are many blood droplets in a spatter pattern, the direction of each droplet can be determined and traced back in the direction of its source with a string. All the strings will intersect at a region called the Point/Area of Convergence. This location is not necessarily a single point which is why it is most offten referred to as an “area” of convergence.

The Area of Convergence just shows the distance on the surface from where the trauma impact occurred, but it’s not the true location of the incident. The Point/Are of Convergence needs to be “raised” off the surface into three dimensional space to show the Point/Area of Origin.  To determine that area, the angles of impact of the blood droplets must be calculated.

Angle of Impact

It is a fairly straightforward process to determine the angle of impact of a blood drople it does, however,  require a ruler and a little trigonometry. The two measurements that are needed are the droplet’s width and length, as recorded without the tail. In the image below the yellow ellipse demarks the shape of the droplet minus its tail.

 

 

 

 Once the measurements have been taken, a simple trigonometric formula (that can be done on any scientific calculator) can be used to determine the angle of impact.

Angle of Impact = sin-1 (Width/Length)

 

It is important to take the angle measurements of each droplet, then “raise” the string attached to that droplet by the calculated angle from the surface. Once that is done with each droplet, the Area of Origin will be determined. Alternatively, you can calculate the Area of Origin if you measure the distance of the droplet from the Point/Area of Convergence, then use the following trigonometric formula for each droplet.

Height from Surface = Distance from Area of Convergence x tan(Angle of Impact)