Area of Origin: Stringing and Trigonometric Methods

What Is Area of Origin?

When blood strikes a surface, it leaves stains whose shape, size, and direction reveal where the blood came from. Investigators distinguish two related concepts. The area of convergence is the two-dimensional zone on a floor or horizontal plane where the paths of individual stains appear to meet when you project them backward along their long axes. The area of origin adds the third dimension — the estimated height above that convergence zone from which the blood was shed. Together, these two findings help reconstruct the position of a victim, a wound, or a transfer event during a crime.

Determining area of origin is a core skill tested by the IAI Certified Crime Scene Investigator (CCSI) examination and is central to courtroom-ready documentation under any state evidence technician framework.

How Blood Stains Tell Direction

An elliptical bloodstain has a long axis and a short axis. The ratio of the short axis width to the long axis length gives you the sine of the impact angle. A stain that is nearly circular struck the surface at close to 90 degrees (nearly straight down). A stain that is very elongated struck at a low, glancing angle. The tail or satellite spatter, when present, points in the direction of travel. Always photograph each stain with a scale before touching or disturbing the surface.

Stringing: The Two-Dimensional Step

Stringing is a physical technique for finding the area of convergence.

1. Select stains. Choose at least six well-formed elliptical stains spread across the scene. Avoid stains that are smeared, overlapping, or whose long axis is unclear.

2. Measure and record. Using two fixed reference points (a corner, a doorframe), record the position of each stain's center in your field sketch. Note which end is the tail end.

3. Affix string. Anchor a length of colored string at the tail end of each stain and run it along the long axis of the stain, extending backward (opposite to the direction of travel) across the floor toward where the strings converge. Tape flag each string at the stain so you can identify it later.

4. Read the convergence. Where the strings cluster together is the area of convergence. It will rarely be a perfect single point; map the bounded zone where most strings cross and record its dimensions and position relative to your reference points.

5. Photograph from above. Use a stepladder or overhead vantage to photograph the full string layout. Include a scale and north arrow in the image.

If your strings do not converge, re-examine your stain selection. A stain whose long axis is misread will throw a string far off.

Trigonometry: Adding the Third Dimension

Once you have the area of convergence on the floor, you can estimate how high above that point the blood originated. This requires two pieces of information for each stain:

• Impact angle (α): calculated from the stain dimensions using `sin(α) = width ÷ length`.
• Horizontal distance (d): measured from the stain's position on the floor (or wall, converted to a horizontal equivalent) to the center of the convergence zone.

The height of origin is then:

> H = d × tan(α)

Where H is the estimated height, d is the horizontal distance, and α is the impact angle. Calculate H for each of your six or more stains and average the results. Outliers may indicate a stain that was not part of the primary event — note them but do not discard them without written justification.

Wall Stains and Projection

When stains appear on a vertical wall, the geometry changes slightly. You must determine the horizontal distance from the stain's location on the wall to the convergence point on the floor, and the vertical height of the stain on the wall. The trigonometric relationship still applies, but the "d" value is now the straight-line horizontal distance from the base of the wall below the stain to the convergence point. Measure carefully and document all intermediate measurements.

Documentation Standards

All measurements must be recorded in your field sketch at the time of collection — not reconstructed later. Use metric or US customary units consistently throughout a single case. Your written interpretation paragraph must:

• State the mapped convergence zone dimensions and location.
• State each impact angle used and the resulting height calculation.
• State your averaged height of origin.
• Acknowledge the margin of variation among individual stain calculations.

A finding that cannot be traced back to recorded field measurements will not survive cross-examination.

Safety in the Lab

Simulated spatter scenes use non-biological materials, but standard PPE habits must be practiced every time you process a mock scene, because the habits carry over to real scenes. Wear nitrile gloves and eye protection throughout. If any real biological material were present, a fluid-resistant gown and face shield would be required per OSHA Bloodborne Pathogens standard. Keep the floor clear of loose string that could cause a trip hazard, especially when moving with a camera or climbing a stepladder. A second person should steady any ladder used for overhead photography.