Skip to content

Striation-Overlay Comparator

Run the Striation-Overlay Comparator MicroSim Fullscreen

About This MicroSim

When a tool scrapes across a surface, the microscopic nicks and wear on its edge drag along and cut a pattern of fine parallel lines called striations. Those striations are individual characteristics — unique to that one tool. A firearms-and-toolmark examiner lines up a questioned mark against a test mark under a comparison microscope, sliding one against the other until the striations either line up ridge-for-ridge or clearly don't.

This MicroSim, investigators, gives you that split-field view. The crime-scene toolmark sits on top; the suspect tool's test mark sits below, across the comparison-microscope bridge. You choose one of three recovered tools, then slide its strip to align the striations. Press Score Alignment and the tool reports what percentage of the scene's striations coincide, with green guides through the matching lines. Exactly one tool truly matches — a wrong tool never climbs to a match no matter how you shift it. The panel also shows each tool's class width, so a tool of the wrong size can be excluded on class before you ever look at striations.

How to Use It

  1. Read the class characteristic (width) row first. If a tool's width differs from the scene mark, it is excluded on class — no striation match can save it.
  2. Pick a Tool from the dropdown.
  3. Drag the Shift (align) slider to slide the suspect test mark left or right until its striations line up under the scene mark's striations.
  4. Press Score Alignment. Read the coinciding lines, the score %, and the MATCH / NO MATCH verdict; green guides mark every coinciding striation.
  5. Press Reset to re-center the strip, and test the other tools. Only one will reach a match — and only after you align it.

Learning Objective

Analyze two toolmark striation patterns by aligning the suspect test mark to the crime-scene mark and scoring their correlation — excluding a tool on class characteristics and identifying the source tool on individual characteristics, just as a comparison microscope does (Bloom Level 4 — Analyze).

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/striation-overlay-comparator/main.html"
        width="100%" height="565" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 15–20 minutes Bloom level: Analyze (L4) — align and score toolmark striations to identify or exclude a suspect tool.

Warm-up. Ask investigators: "The nicks worn into one screwdriver's edge are unique to that tool. If we drag it across metal, what kind of pattern does it leave, and how could that pattern tie a specific tool to a crime scene?"

Guided questions:

  • Read the class-width row first. How can you exclude a tool before you ever slide its striations, and why is that faster than a full comparison?
  • Pick the matching tool and align it. What do the "score %" and the count of coinciding lines tell you, and why does a wrong tool never climb to a match no matter how you shift it?
  • What is the difference between a class characteristic and an individual characteristic, and which one lets you identify one specific tool?

Extension. Research the debate over the scientific validity of toolmark identification and why examiners now emphasize documented, reproducible comparisons.

References

Specification

This MicroSim was generated from a specification in Chapter 13: Firearms, Ballistics, and Toolmark Examination.

Design note: the striation strips and scoring are stylized for teaching — the alignment score is a qualitative measure chosen to illustrate class-versus-individual reasoning, not a validated statistical toolmark-match metric.