Summary of "Blood Spatter Analysis - Determining area of origin from impact stains."

Blood spatter analysis — Determining area of origin from impact stains

Note: Area-of-origin analysis applies to impact spatter only — blood forced out by an impact, producing a fan of droplets. The result is an approximate 3-D region (area of origin), not a single pinpoint.

Main ideas / concepts

Two goals:
1. Find the 2-D point (point/area of convergence) on the target surface where the lines of flight of individual stains intersect.
2. Determine the height (distance above that 2-D convergence point) of the source by projecting those lines upward using each stain’s impact angle.
Use accurate measurements and multiple well-formed (elongated/ovoid) stains from across the pattern. Small stains are easy to measure incorrectly — use magnification and precision tools.
The final result is an approximate 3-D region (an area/volume), not a precise single point.

Equipment and materials

Bloodstain pattern sample (impact spatter)
Digital calipers (for length and width of small stains)
Magnifier / loupe (10×) if needed
Straight edge (ruler)
Protractor (with center/zero line)
Thin string (dental floss can substitute)
Tape (to fix pattern and strings)
Calculator or scientific phone calculator (for inverse sine and tangent)
Tripod/backstop/wall (to anchor strings and project angles)
Paper/pen to record measurements

Step-by-step methodology

Select stains
- Choose a variety (approx. 8–12 recommended) of well-formed, elongated (oval) stains spread across the pattern.
- Avoid nearly circular stains (close to 90° impact) — they give poor directionality.
- If a stain appears deformed, skip it or note extra uncertainty.
Measure each stain
- For each selected stain record:
  - Length (long axis, L)
  - Width (short axis, W)
- Use digital calipers and magnification for tiny stains.
- Record measurements in a table (stain number, length, width).
Compute impact angle for each stain
- Compute the ratio: R = W / L (R < 1).
- Impact angle: θ = arcsin(R). Use the inverse sine (arcsin) on a scientific calculator.
- Record θ for each stain.
- Check angles for consistency. If one angle is very different, re-measure (small-stain measurement error is common).
Determine the 2-D point/area of convergence on the target surface
- For each stain, draw a line down the long axis (through the center of the stain) to represent the line-of-flight projection on the surface.
- Extend those centerlines until they intersect on the plane; the cluster/intersection region is the point/area of convergence (2-D origin projection).
- Tape the pattern to stabilize it while drawing and extending lines.
Find the vertical height (3-D area of origin)
- Option A — Physical string method:
  - Tape one end of a string at the leading edge (start of the stain in the travel direction) of each stain.
  - Set the string to the previously calculated impact angle θ relative to the surface plane using a protractor.
  - Stretch the string to a wall/backstop/tripod and tape it there.
  - The spatial intersection of the strings indicates the area of origin. Measure the vertical distance from the target plane (convergence center) up to that intersection to get height.
  - Notes: use a far enough backstop so strings can cross; dental floss works as a substitute for thin string.
- Option B — Trigonometric calculation:
  - Measure the horizontal distance d between each stain’s projected line-on-surface and the 2-D convergence point.
  - Use the angle θ: h = d * tan(θ).
  - Compute h for multiple stains and average to estimate height.
  - The presenter noted sine-based approaches can be used depending on geometry, but the common vertical-height formula uses tangent.
Finalize results and troubleshoot
- Inspect consistency: angles and heights should cluster. Outliers likely indicate measurement error or stains from a different event.
- Re-measure small or suspect stains if angles/heights are inconsistent.
- Document everything in a chart to keep measurements organized.

Common pitfalls and practical tips

Small stains are difficult to measure — use a magnifier and calipers.
Prefer elongated stains (impact angle ≲ ~60° recommended) for reliable angle calculation.
If one stain’s angle is “wacky” compared to the rest, re-measure it.
Tape the sample so it does not move while drawing lines or attaching strings.
The method is inherently approximate. Physical stringing gives a visual 3-D intersection; trigonometry gives a calculated height — both are complementary.

Key equations

Ratio: R = W / L
Impact angle: θ = arcsin(R)
Height from horizontal distance d to convergence: h = d * tan(θ) (compute per stain, then average)

Speakers / sources featured

Single demonstrator/presenter (unnamed) — instructor showing and narrating the procedure.
Background music present in the video (no other speakers identified).