The picture above examines how an analyst determines the area of origin of a spatter event. The gray lines on the floor determine the 2 dimensional area of convergence, while the blue lines determine the area of origin. Note that the intersection of the blue lines demonstrate a maximum height; therefore, the event happened at this point or at a point below it. This is because blood does not travel in a straight line (as is done in stringing a spatter pattern) but in what's called a parabolic arc. We get this arc due to the Earth's gravitational pull and air resistance acting on the blood drop in flight. To find the area of origin the analyst must first select a few well formed stains which seem to visually cross paths. Next, would be to determine the impact angles of theses selected bloodstains, then compute the tangent (TAN) of these angles. Next, the analyst will measure the distance from each stain to its first line of convergence (next gray line it meets). Now, armed with the Tangent of each given impact angle the analyst will multiply each by the distance for each stain and arrive at the approximate height above the surface from which this event happened. With the use of a protractor and strings you could run them back (blue lines) on their predetermined angles and attach to a vertical structure such as a camera tripod. Today, many analysts use computer software to display their findings; however, the use of strings is still used by some analysts today and is frequently taught in the basic 40 hour course.
Here is a link discussing Trigonometry & Right Triangles!
A form you can use when documenting bloodstains!