CRIME SCENE MEASUREMENT

The accuracy of a crime scene measurement &/or sketch depends on the purpose of the measurements. (See article on precision vs accuracy.) A sketch is usually drawn from an overhead perspective, not 3D, or side views, in order to accurately show distances and relationships of objects. Photographs are usually used to give side view representation and models are constructed to give a 3D view. One measures the scene in order to draw inferences from it. These inferences can be made while at the scene or later, with a recreated scene or a drawing. The drawing also allows the jury to draw inferences at trial.

There are three basic techniques for making horizontal &/or vertical measurements at a scene: triangulation, polar coordinates and rectangular coordinates. Each can be elaborated upon or combined in a particular scene. All measurements begin from a fixed, or known, starting point(s) so that they can be recreated if necessary. The orientation of this starting point(s) to North, or to some other reference direction is desirable.

 

TRIANGULATION - Two fixed points in a scene are selected and the distance between them measured. Either distance or angular measurements are then made to objects in the scene from each of these two points. Dashed lines indicate measuring the lengths of base line and the two sides. Solid lines indicate measuring the angles the two sides make with the base line.

 

POLAR COORDINATES - Measure both the distance and direction (angle) an object is from a known reference point. For example, 40 feet from the edge of a house in a direction of 15 degrees east of north.

 


The angle can be measured with either a large protractor or an optical device such as a transit or a compass. The protractor technique with a 360 degree protractor is useful for underwater scenes. The starting point must be fixed so it can be reproduced. Its distance from the corner of the building is indicated in the drawing.


RECTANGULAR COORDINATES - Base Line The simplest form of rectangular coordinate system. Using a straight line between two known points, items are measured along the line and perpendicular from the line. Inside or outside of a house, this line can be a straight wall. Outdoor scenes can use a string or long measuring tape as the reference or base line.


RECTANGULAR COORDINATES Grid - Measure the distance of items from two perpendicular base lines. This technique is particularly appropriate in a room with perpendicular walls or outdoors with perpendicular streets.

 

 

The SURVEY is an extension of the polar coordinate system. Beginning from a known point and known direction, a distance and azimuth are measured to an intermediate point. From that point a distance and azimuth are measured to a second intermediate point, etc. Items can be measured using triangulation (ex. "x") or polar coordinates (ex. "y") from the intermediate points.

Continuing to measure to intermediate points until one arrives back at the origin is called closing the survey. By closing the survey one can calculate the error included. Extensive outdoor scenes, such as along a winding road, may require this type procedure. With practice one can do a satisfactory survey for most investigative purposes with a lensatic compass. Measurements for calculating trajectories may require more sophisticated equipment.

 

There are two approaches to measuring features such as door and window openings in a house -- interval and continuous. That is, one can measure the features in the drawing below as a set of intervals. If the interval approach is used, a check measurement of the total distance should be made.

Or, the features can be measured continuously by placing a tape measure in front of the house starting at one corner as in the following drawing.

The continuous approach leaves less chance for blunders and a better chance of catching them if they do occur. Since one may not be able to get too close to all of the features due to plants, etc. one may wish to measure certain features more precisely as a check or because those features are of particular interest.