Module 2: Bloodstain Pattern Analysis - Stain Shape and Vector Correlation

Introduction to Stain Shape and Vector Correlation

  • Focus on the examination of blood stains found at crime scenes.

  • Introduces the principle of stain shape and vector correlation.

Main Principle Overview

  • General Principle: The shape of certain blood stains provides indicators of both the direction of deposition and the spatial origin.

Sub Principles

  1. Directionality

    • Defined as the determination of the direction in which blood droplets were traveling at the moment of impact on a surface.

    • **Characteristics of Stains:

      • Circular or elliptical shape present.

      • May feature spines, scallops, or satellite stains.

    • Role of Morphology:

      • The shape and features help define the direction of travel at impact.

    • Specifics of Spatter Stains:

      • Individual spatter stains will establish impact direction based on their shapes.

      • Directional attributes include:

        • Long axis of the stain

        • Creation of scallops, tails, and satellites, present on the side opposite the initial contact point.

    • Visualization of Droplet Origin:

      • By evaluating the directionality of multiple stains, it is possible to visualize the general area where the blood droplets originated.

      • The concept of reverse vectors indicates convergence of stains at a two-dimensional area known as the pattern's convergence point.

Impact Angle

  1. Impact Angle

    • Definitions and Characteristics:

      • The collapse of a droplet creates a stain that maintains either a circular or elliptical shape.

      • There is an empirical relationship between the ratio of the long and short axes of the stain and the impact angle at which the droplet struck the target.

    • Components of Stain:

      • Each stain has:

        • Major axis (long axis)

        • Minor axis (short axis)

    • Empirical Relationships:

      • Historically established relationships show that angular measurements correlate with stain ratios, thus allowing us to approximate the impact angle.

    • Visualization of Geometry:

      • A right triangle can be visualized using:

        • Diameter of the droplet

        • Path of the droplet

        • Area on the target where the droplet first impacts

      • The internal angle of the lines formed in the triangle (angle ACB) is the same as the angle of impact (denoted as angle I).

    • Trigonometric Relationship:

      • The major axis of the blood stain correlates to the hypotenuse of the triangle.

      • The minor axis of the blood stain corresponds to the side opposite the angle of impact.

      • This relationship allows for angle determination using trigonometric functions.

Practical Application

  • Measuring Stains:

    • The process involves:

      • Measuring the long and the short axes of the stain (excluding tails, scallops, or satellites).

      • The ratio obtained from dividing the short axis by the long axis will yield a value of one or less.

      • Use a calculator to determine the inverse sine of this ratio to find the angle of impact.

  • Association with Bullets:

    • Similar relationships can be applied to defects created by bullets.

Shape and Angle Correlation

  • Stains can be categorized based on their shape and corresponding impact angles:

    • More Circular Stains:

      • Indicate impact angles between 70° and 90°.

    • Bare Claw Shaped Stains:

      • Indicate impact angles ranging from 40° to 60°.

    • Elliptically Shaped Stains:

      • Indicate acute angle impacts of 30° or less.

Area of Origin and Three-Dimensional Analysis

  • The combination of impact angle and directional angles leads to a corollary known as Area of Origin:

    • This concept suggests that the impact and directional angles defined for multiple stains at a scene can help in pinpointing the stains' origin in three dimensions.

    • When defining both angles for several well-defined stains, the resulting vectors converge in three-dimensional space, indicating the area from which the blood droplets originated during the event.

  • Stringing Technique:

    • Utilizes directionality, pattern convergence, and area of origin to deduce event occurrences, allowing for theoretical reconstruction of movement and position during a crime.

    • Physically or virtually orienting strings in line with impact and directional angles can isolate the area of origin.

    • Advances in technology now allow software to assist in this reconstruction process, optimizing the analysis beyond physical stringing.