Position & Origin
This term names the location in space where the victim was when bloodshed occurred, determined by intersecting projected trajectories of blood droplets.
Area of Origin
Bloodstain pattern formed only by gravity; drops are usually circular when they fall at 90°.
Passive bloodstain.
Drops about 4 mm or larger are typically categorized as this velocity class.
Low velocity
True or False: A perfectly circular bloodstain indicates the droplet struck the surface at a 90° angle.
True.
The procedure that uses a pole, strings, and measured impact angles to find the area of origin is commonly called the ____ method.
String method (pole-and-string method).
Name two types of transfer patterns and give a short example of how each might be created at a scene.
Trail (dripping from an object) — e.g., bleeding person walking; Swipe (object moving through wet blood) — e.g., shoe moving through blood and leaving a smear.
What velocity class usually produces very small drops less than 1 mm, often seen with gunshot wounds?
High velocity.
Write the formula used to calculate the angle of impact from the measured width and length of a bloodstain (use Sin notation).
sin(Angle) = width / length.
In the "wife" case study, bloodstain analysis showed the victim was originally on this body position when the spatter occurred.
Prone (on her stomach).
Which projected pattern is characterized by bubbles in the drops and is commonly produced when blood is expelled from the mouth or nose?
Expiratory (expirated) pattern
Explain why caution is needed when inferring weapon type solely from drop size or velocity.
Because drop size is influenced by many variables (distance, surface, secondary impact, clotting, multiple mechanisms) and multiple velocities may coexist; weapon inference requires corroborating evidence
Show the calculation steps (no numeric values needed) to find the angle of impact for a drop: measure width, measure length, and then the trigonometric step.
Steps: measure width (w) and length (l); compute ratio r = w/l; use inverse sine: Angle = sin^-1(r).
Define "area of convergence" and explain how it differs from "area of origin."
Area of convergence: area where angles of bloodstains meet; Area of origin: location in which person was located based on the angles of impact and areas of convergence
Explain the difference between forward spatter and back spatter, and give one crime-scene example of each.
Forward spatter: blood propelled away from source (e.g., exit wound, blood on target in front of impact). Back spatter: blood traveling back toward the source of force (e.g., blood on shooter, interior of firearm, clothes of person near entry wound).
Given a mixed scene with low-, medium-, and high-velocity stains, describe an investigative approach to determine which stains correspond to which mechanism of bloodshed.
Separate stains by measured diameter ranges and morphology, map spatial distribution, analyze context with scene and victim wounds, and use sequence/overlap and transfer patterns to attribute mechanism
An investigator measures a drop: width = 2.0 cm, length = 4.5 cm. Calculate the angle of impact (show work and final angle rounded to nearest degree).
Calculation: ratio = 2.0 / 4.5 = 0.444... Angle = sin^-1(0.444) ≈ 26.4°, rounded ≈ 26°.
Describe the full step-by-step method you would use at a crime scene to go from individual bloodstain measurements to determining the three-dimensional area of origin (include measurements, tools, and reasoning).
Steps: identify and select impact stains; measure length and width for each stain; compute angle of impact via sin^-1(width/length); draw back-projected straight lines from each drop at their calculated angles to find area of convergence; Use this info to determine the area of origin
List and describe at least four distinct projected bloodstain patterns (e.g., cast-off, arterial spray) and explain what aspect of the mechanism (pressure, motion, source) produces each pattern.
Cast-off: blood flung from a moving object (weapon) producing linear arcs; Arterial spray: high-pressure spurting from a severed artery, producing large rhythmic spurts with vertical arcs; Expiratory: low-pressure, bubbly pattern from mouth/nose; Projected spurt: ejection under pressure producing directional spray. (Mechanisms: motion/centrifugal force, cardiac pressure, respiratory pressure, vascular pressure.)
Discuss how surface texture (smooth vs. porous) interacts with droplet velocity to change the resulting stain morphology (spines, satellites, and edge characteristics). Use physical reasoning (cohesion, surface tension, absorption).
On smooth surfaces, cohesion preserves round shapes and produces clear satellites/spines for higher-energy impacts; on porous surfaces, absorption blunts edges and reduces satellite formation; higher velocity increases fragmentation and satellites while porous surfaces absorb and produce irregular edges.
Explain how you would determine the area of convergence.
Determine impact angles for stains, find intersection region for convergence; then use string method or trigonometric projection to find height.