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AGR-CSI53-02 · Lesson 6 · 13 clock hours · practice

Rough Sketching: Measurement Techniques & Hand-Drawn Diagrams

Objective. Students will produce a complete rough sketch of a staged indoor scene using baseline and triangulation measurements accurate to ±0.1 ft, with a north arrow, title block, legend, and all five evidence items correctly located.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why the Rough Sketch Matters

Before a single measurement is entered into a CAD program or a formal diagram is drawn, investigators produce a rough sketch — a freehand, hand-drawn record of the scene created while they are still standing in it. The rough sketch is a legal document. Courts have accepted rough sketches as evidence, and errors you record in pencil may follow a case for years. Accuracy to within ±0.1 ft (about 1.2 inches) is the professional standard for indoor scenes.

A complete rough sketch contains five required elements: a title block, a north arrow, a legend, room dimensions, and the located positions of all evidence items. Missing any element makes the sketch incomplete for court purposes and for the IAI Certified Crime Scene Investigator exam.

Safety Before You Measure

Even in a staged training room, practice the habits that protect you on real scenes. Wear nitrile gloves when handling any item that may be treated as evidence. Keep tape measures flat on the floor — a tape stretched across a walkway is a trip hazard; call out "tape down" so partners know before they step. Never move evidence markers to make measurement easier. If a measurement requires you to kneel or reach into a confined space, check for sharp objects first.

The Title Block

The title block sits in the lower-right corner of every sketch. It must include: case number, date and time, location (address and room), your name and badge or student ID, and the names of any witnesses to your measurements. Fill it in before you draw anything else — if the sketch is separated from its notes, the title block is the only identifier.

The North Arrow

Indoor scenes rarely have a true north wall, so investigators designate one wall as "north" for the purpose of the sketch and note that designation clearly (e.g., "Architectural North — east wall of Room 104"). Place the north arrow in the upper-right corner. Every measurement relationship in the sketch is oriented to this reference, so choose it before the first tape goes down.

Baseline Measurement

The baseline method establishes a straight reference line — usually one wall or a string stretched across a room — and measures each evidence item by dropping a perpendicular from the item to the baseline. You record two numbers for each item: (1) how far along the baseline the foot of the perpendicular falls, and (2) how far the item is from the baseline (the perpendicular distance).

When to use it: Baseline works best in long, narrow spaces such as hallways, or when evidence items are arranged roughly parallel to one wall.

How to run a baseline: Lay the 100 ft fiberglass tape along the reference wall from a fixed corner point (your "0" end). For each evidence item, hold the 25 ft steel tape at the item and walk it perpendicular to the fiberglass tape until it crosses the tape at a 90° angle. Read both tapes. Record as: Item 1: 14.3 ft along baseline, 3.7 ft from baseline.

Triangulation

Triangulation locates an item by measuring to it from two fixed, known points — almost always two corners of the same wall. The intersection of those two distances uniquely fixes the item's position on your sketch.

When to use it: Triangulation is preferred for open rooms, outdoor scenes, or any evidence item that is not conveniently near a wall.

How to triangulate: Select two permanent reference points — Corner A and Corner B of one wall. Hook the steel tape at Corner A and extend it to the evidence item; record that distance. Without moving the item marker, hook the tape at Corner B and measure to the same item; record that distance. On the sketch, swing arcs of each distance from each corner; the arcs cross at the item's correct location.

Professional practice: Use triangulation as a check on baseline measurements for critical items (a body position, a weapon) whenever possible. Two independent methods that agree to ±0.1 ft give your measurements legal credibility.

The Legend

The legend is a small table or key on the sketch face that identifies every symbol and abbreviation you use. Number evidence markers (E1, E2 …) in the legend and describe each item: E1 — spent cartridge case, brass, 9 mm. If you use a symbol for a door swing or a window, define it. Reviewers who were not present at the scene must be able to read your sketch without asking you to explain it.

Scale Notation

A rough sketch is not drawn to scale, but you must still note "Not to Scale" (NTS) prominently. This prevents anyone from measuring lines on your sketch and assuming those distances are real. The actual distances live in your recorded measurements, not in the length of the lines you drew.

Recording Discipline

Call out measurements to your partner in a consistent format: direction first, value second — "Along baseline, 14 ft 4 inches" rather than "14 and a half, along the wall." Your partner repeats the value back before writing it. This two-person verification caught errors before they reach the sketch. Record in feet and tenths of a foot (14.3 ft), not feet and inches, to match the ±0.1 ft accuracy standard and to make later CAD entry straightforward.

Completing the Sketch Under Exam Conditions

For the IAI CCSI exam and the State Evidence Tech Cert, you are expected to demonstrate both baseline and triangulation methods and to produce a sketch with all required elements. The 10-item checklist used in this course mirrors the documentation standards those credentials test. Aim for 9 of 10 on every practice sketch so that meeting the standard is a habit, not a scramble.

Key terms8

Rough sketchA freehand, hand-drawn diagram of a crime scene produced at the scene; a legal document that records spatial relationships and measurements before the scene is altered.
Baseline measurementA method that establishes a straight reference line and locates each evidence item by recording its distance along the line and its perpendicular distance from it.
TriangulationA method that fixes an item's position by measuring from it to two separate fixed reference points (such as two wall corners) and intersecting the resulting arcs on the sketch.
Title blockA standardized box on every sketch that records case number, date and time, location, and the names of the measuring technician and any witnesses.
LegendA key on the sketch face that defines every symbol, number, and abbreviation used, allowing reviewers who were not present to interpret the diagram.
North arrowA directional indicator on the sketch designating which wall or direction is treated as north for the purposes of spatial orientation; may be architectural (assigned) rather than true magnetic north.
Not to Scale (NTS)A notation required on all rough sketches indicating that the line lengths drawn do not proportionally represent real distances; actual distances are the recorded measurements only.
Fixed reference pointA permanent, immovable feature of a scene — typically a wall corner — used as the known origin for measurements so that positions can be reproduced later.

Worked examples4

1. Using the baseline method, locate Evidence Item 2 (a shoe). The fiberglass tape runs along the south wall from Corner A. The perpendicular from the shoe meets the baseline at 8.5 ft. The shoe is 4.2 ft from the south wall. What two values do you record, and how do you write them?
  1. Identify the two values the baseline method requires: distance along the baseline and perpendicular distance from the baseline.
  2. Distance along baseline = 8.5 ft (read from the fiberglass tape at the foot of the perpendicular).
  3. Perpendicular distance = 4.2 ft (read from the steel tape stretched from the shoe to the baseline at 90°).
  4. Write in standard format: E2 — 8.5 ft along baseline, 4.2 ft from south wall.
Answer: Record as: E2 — 8.5 ft along baseline, 4.2 ft from south wall.
2. Using triangulation, locate Evidence Item 4 (a knife). Corner A and Corner B are the two ends of the north wall, which is 12.0 ft long. Measuring from Corner A to the knife gives 9.3 ft. Measuring from Corner B to the knife gives 6.1 ft. How do you plot the item on the sketch?
  1. Set your compass (or use a ruler as an arc guide) to 9.3 ft at the sketch's chosen scale and draw a partial arc centered on Corner A.
  2. Reset the compass to 6.1 ft and draw a partial arc centered on Corner B.
  3. The single point where the two arcs cross is the plotted position of Evidence Item 4.
  4. Label that point E4 and verify the two recorded distances are entered in the legend description.
Answer: The knife plots at the intersection of a 9.3 ft arc from Corner A and a 6.1 ft arc from Corner B.
3. A technician measures the distance from Corner A to a blood stain twice. The first reading is 11.4 ft and the second reading is 11.6 ft. Does this meet the ±0.1 ft accuracy standard? What should the technician do?
  1. Calculate the difference between the two readings: 11.6 ft − 11.4 ft = 0.2 ft.
  2. Compare the difference to the allowable tolerance: 0.2 ft > 0.1 ft.
  3. The readings do not agree within the required tolerance, so the measurement does not meet the standard.
  4. The technician must re-measure, checking that the tape is straight, taut, and reading from the same fixed point both times, then record the verified value.
Answer: The 0.2 ft discrepancy exceeds ±0.1 ft; the technician must re-measure until two readings agree within 0.1 ft.
4. A room's north wall (designated architectural north) is measured with the 25 ft steel tape in two segments: the first segment reads 16.8 ft and the second reads 7.4 ft. What is the total wall length to record?
  1. Add the two segment lengths: 16.8 ft + 7.4 ft = 24.2 ft.
  2. Confirm the arithmetic: 16.8 + 7.4 = 24.2 ft.
  3. Record the total north wall dimension as 24.2 ft in the room dimension section of the sketch.
Answer: Total north wall length = 24.2 ft.
In class: Instructor demonstrates baseline measurement and triangulation on a whiteboard diagram, then repeats the demonstration in the hallway with a tape measure and two mock evidence markers, having students call out the math. · Students work in pairs inside a staged room: one measures and calls out values, the other records on the rough sketch form; roles alternate halfway through so both students produce a complete independent sketch. · Instructor conducts a measurement verification check by comparing two randomly selected measurements from each pair's sketch against the instructor's key measurements — discrepancies greater than 0.1 ft are retaken.
Materials: 100 ft fiberglass tape measures; 25 ft steel tape measures; rough sketch forms with title block; pencils and erasers; numbered evidence markers; staged indoor room; instructor measurement key; rulers.
Check: Rough sketch scored on 10-item checklist (title block, north arrow, legend, room dimensions, five evidence items located, two measurement methods used, readable scale notation) — 9 of 10 required; any measurement error >0.1 ft requires a re-measure.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Crime scene documentation
1. Which element of a rough sketch allows a reviewer who was not present at the scene to identify what each numbered marker represents?
Crime scene documentation
2. A technician sets the zero end of a fiberglass tape at Corner A of the south wall and runs it to Corner B. Evidence Item 3 is 6.7 ft along the tape and 2.3 ft from the wall. Which measurement method is being used?
Crime scene documentation
3. In the triangulation method, what geometric event on the sketch marks the plotted position of an evidence item?
Crime scene documentation
4. Two measurements of the same distance read 5.3 ft and 5.5 ft. What must the technician do next?
Crime scene documentation
5. Why must a rough sketch be labeled 'Not to Scale'?
Crime scene documentation
6. A technician is measuring a long, narrow hallway with evidence items placed close to both walls. Which method is most efficient for this scene layout?
Crime scene documentation
7. A room's east wall is measured in two segments: 9.6 ft and 8.7 ft. What is the total length to record in the sketch's room dimension section?
Crime scene documentation
8. During a paired measurement exercise, which practice best prevents transcription errors between the measurer and the recorder?
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