How to Measure a Crime Scene With the Baseline System

The baseline method of crime scene measurement locates each piece of evidence using two numbers taken from a single straight reference line: how far along that line you travel to reach a point directly across from the item, and how far the item sits from the line at a right angle. Those coordinates pin every item to a spot on a two-dimensional sketch, the same way X and Y values plot points on a graph. The technique needs almost no equipment, but it depends heavily on careful technique to hold up in court.

The Two Measurements That Define Every Item

Once a straight line is stretched between two fixed points at the scene, any object not sitting directly on that line is located by dropping a perpendicular from the object to the baseline. You measure the length of that perpendicular, then measure from one of the fixed endpoints to the spot where the perpendicular meets the baseline.¹ If an item sits on the baseline itself, one distance from an endpoint is enough.

That’s the whole geometry. Everything else is about making sure those two numbers are accurate and reproducible.

When the Baseline Method Is the Right Choice

The National Institute of Justice recommends the baseline approach for outdoor areas that are irregularly shaped and where no natural reference line exists, such as desert terrain or open farmland.¹ It also handles large rooms where evidence spans a long distance, and elongated outdoor scenes where fixed reference points are scarce.

Two other manual methods cover different situations. Rectangular coordinates measure from two walls that meet at a right angle and are often easier for indoor scenes with well-defined perpendicular walls. Triangulation measures the distance from each of two fixed points to the evidence item and tends to be more accurate than either the baseline or rectangular approach when fixed reference points are plentiful. The baseline method sits at the simpler end of the spectrum; its strength is versatility in open or elongated scenes rather than pinpoint precision.

Equipment You Need

The NIJ’s general sketching kit includes tape measures of varying lengths, graph paper and pencils, a small ruler or straight edge, evidence placards, and templates for scene and body outlines.² For the baseline method specifically, you’ll want:

A long measuring tape or string, typically 100 or 200 feet, to form the baseline itself. It has to be pulled taut so the reference line stays straight.
A shorter tape for the perpendicular offset measurements from the baseline to each item.
A compass to record the baseline’s orientation relative to north so the sketch can be reproduced later.
Anchors for the endpoints. Building corners, utility poles, or driven stakes work outdoors. The NIJ guide lists walls, curbs, doors, windows, and even kitchen appliances as acceptable fixed points indoors.¹
A camera for photographic documentation that supplements the sketch.
Chalk, flags, or evidence placards to identify items and measurement points at the scene.

Picking and Anchoring the Fixed Points

The baseline is only as reliable as its endpoints. If either one shifts, or cannot be relocated months later when the case goes to trial, the entire sketch loses its value. The NIJ guide notes that a wall is preferred as a fixed point when one is available.¹ Permanent structures like building corners, curbs, and street signs all work because they will still be there when someone needs to verify the measurements.

Open terrain is the hardest case. When there are no buildings, fences, or landmarks, investigators sometimes drive stakes or rebar into the ground, and those temporary markers can get removed or disturbed. Recording GPS coordinates for each endpoint creates a backup that lets the scene be reconstructed if the physical markers disappear.

Wherever you set the endpoints, the baseline should span enough of the scene that most evidence falls within a reasonable perpendicular distance. A line running along one far edge forces long offset measurements and increases angular error. Running the line through or near the center of the evidence distribution keeps offsets short.

Stretching and Orienting the Line

Once the two fixed points are chosen, stretch the tape or string taut between them. Any sag creates a curved reference line, and every measurement taken from a curved baseline is wrong. Secure each end so it will not shift during documentation.

Use the compass to determine the baseline’s direction relative to north and record that bearing on the sketch, along with the baseline’s total length and a clear description of both endpoints. That information is what lets someone else reproduce the exact reference line later.

When you lay the tape, be careful not to step on, move, or contaminate evidence. If the shortest path between your two fixed points runs directly through a cluster of items, offset the line slightly and accept longer perpendicular measurements rather than disturb the scene.

Taking the Measurements

For each piece of evidence, find the point on the baseline that is directly across from the item at a 90-degree angle. Measure from your designated endpoint to that point along the baseline. Then measure the perpendicular distance from that point out to the evidence item.¹

The NIJ guide sets an accuracy target of within one-quarter inch for all crime scene measurements.¹ Meeting that target requires disciplined technique. Hold the shorter tape at a true right angle to the baseline. Even a few degrees off perpendicular will shift the item’s plotted position on the sketch. A right-angle tool, or a second person sighting the angle, helps.

Use consistent units throughout. Mixing feet and inches with metric on the same sketch invites confusion when the sketch is analyzed or presented in court.

The Rough Sketch and What Happens After

Record every measurement on the sketch immediately, at the scene, while you are still standing next to the item. For each piece of evidence, note its number or description, its distance along the baseline from the designated endpoint, and its perpendicular offset. Writing measurements on a separate notepad and transferring them later invites transcription errors.

The sketch you draw at the scene is called the rough sketch. It does not need to be drawn to scale, but the measurements, locations, and directions must be accurate. The Federal Law Enforcement Training Center instructs that no deletions or additions should be made to the rough sketch after leaving the scene, because the sketch itself may be introduced as evidence.³

A finished drawing is later prepared to scale from the rough sketch and notes, and must be verified and approved by the officer who made the original.³ Considerable time can pass between the investigation and courtroom presentation, so the rough sketch is the permanent record that everything downstream relies on.

Errors That Undermine the Sketch

The single biggest source of error is failing to hold the offset tape truly perpendicular to the baseline. Angular error at each item compounds across a large scene. One analysis of manual crime scene documentation noted that tape-and-wheel measurements are “more susceptible to error due to surface irregularities, the ability of the user to maintain a straight course while measuring, and the skill of the practitioner to make such measurements with perfect perpendicularity to the reference baseline.”⁴

Other recurring problems include baseline sag from a tape that isn’t pulled tight, inconsistent measurement units, and endpoints chosen from objects that are not truly permanent. Each one is preventable with a few extra seconds of attention at the moment the measurement is taken.

Manual measurement is still widely taught and used even where total stations and laser scanners are available. Technology can malfunction, batteries die, and some scenes are too remote for heavy equipment. Documenting a scene with a tape measure, a compass, and graph paper is a skill that stays useful regardless of what else is in the trunk.

1
National Institute of Standards and Technology. Crime Scene Investigation – A Guide for Law Enforcement
2
National Institute of Justice. Equipment Needed for Crime Scene Investigation
3
Federal Law Enforcement Training Center. Crime Scene Sketch Student Guide
4
FARO Technologies. Incident Documentation: Laser Scanner vs Conventional Measurement