7.6: Measuring, Sketching, and Diagramming

One essential process in any crime scene investigation is measuring, sketching, and diagramming the scene. This process is often ignored when trained crime scene technicians are not called out to the scene. Diagramming a crime scene is the process by which a two-dimensional or three-dimensional visual representations of the crime scene. These visual representations become an important part of preserving a record of the scene for detectives, bloodstain and shooting scene analysts, crime scene reconstruction analysts, and, perhaps most importantly, for the courtroom proceedings. These diagrams must be highly detailed and accurate measured or mapped in order to present a fair representation of the scene in a way that it can be understood by the defendant, witnesses, attorneys, judge, and jury. It can also be a visual representation of the scene that can be relied upon to verify eyewitness testimony in court; certainly, in a manner that surpasses that of a hastily drawn rough sketch.

The most common sketches and diagrams used in crime scene processing are: the ‘bird’s eye view’ where you are viewing the scene as if you were looking down from the ceiling; the ‘elevation view’ where you are looking at a side view such as a door or wall; the ‘exploded view’ where it looks like you opened a room as if you laid the walls flat on the ground; and the ‘plot plan’ or ‘aerial view’ where you must document a crime scene that encompassed several city blocks or even miles of roadways. Measurements should be recorded in feet and inches.

Figure \(\PageIndex{1}\): Bird's eye view diagram. Created with ScenePD 6. Author's collection

Figure \(\PageIndex{2}\): Exploded View Diagram. Created with ScenePD 6. Author's collection

Figure \(\PageIndex{3}\): Aerial view diagram with legend and compass rose. - Created with ScenePD 6. Author's collection

Orientation, Compass Rose, Legend, Title Block, and Datum Point

There is disagreement in the crime scene investigations community on how the diagram technician should orient their sketch and diagram; whether it should be as viewed by the technician or with north always being at the top of the page. Your author is of the opinion that in both indoor and outdoor scenes, the diagram should be oriented as viewed by the technician. The true compass direction should be represented by a compass rose on the diagram. Either method is acceptable provided the true compass direction is included on the diagram. The legend provides useful information to the viewer to identify items of evidence or important features within the scene. If dimension lines complicate the overall view of the diagram, the measurements can be included within the legend. If the dimension lines are removed from the diagram, it is important to include a scale for reference. The title block contains the information pertinent to the diagram including the location of the scene, the date and time of the incident, the case number, and the name of the diagram technician. Finally, the datum point allows a starting point from where the measurements were taken. This datum point should also be captured as an image to assist anyone who is tasked with reconstructing the scene.

Measuring Methods - Baseline, Rectangle and Triangulation Methods

The baseline method of collecting measurements is likely the most useful. In order to use this method, you must have at least one fixed point (datum) that will not change (fire hydrant, telephone pole, light pole, the corner of a building, etc.) and a compass heading. If you do not have a fixed reference point in a large field or wooded area, you can use a portable GPS unit and place a stake or marker at the GPS coordinates. Roll out a fiberglass measuring tape along this line and leave it in place. This line becomes your baseline or ‘X’ axis. Facing away from your fixed point, you may have items of evidence laying either to the left or right of the line. Those items laying to the left can be measured back to the baseline on a ‘-Y’ axis at a 90-degree angle, and those items lying to the left can be measured back to the baseline on a ‘+Y’ axis at a 90-degree angle.

Figure \(\PageIndex{4}\): Baseline measurement method diagram. Created with ScenePD 6. Author's collection

Remember, the purpose of measuring and sketching the crime scene is so that any person can go back to the scene and recreate it with accuracy. Therefore, it is critical to have a compass heading for the baseline.

The rectangular measurement method is best for items of evidence that are found in areas that have a defined shape and fixed points, such as a shed, room, prison cell, bathroom, yard, etc. Run two fiberglass or metal tape measures along two adjoining walls or fences so that they meet at the corner. The corner will be the ‘0’ datum for both tape measures. Designate one tape measure as the ‘X’ axis and the other as the ‘Y’ axis. Then measure from various noted points on the object back to the axis’ and record the measurements. Items that have a shape will need multiple measurements; whereas items with an irregular shape, or are too small to measure multiple points, such as a fired cartridge casing, are measured to center of the object.

Figure \(\PageIndex{5}\): Rectangular measurement method diagram. Created with ScenePD 6. Author's collection

The triangulation method can be used for items of evidence that have a distinctive shape and for items that do not have a distinctive shape but are found in an area that has a defined shape and fixed points. This is likely the most accurate measurement of the three methods listed provided a compass direction is given for each leg of measurement. Items with a distinctive shape will have at least two three-point (triangle) measurements, although three would be preferred. Items without a distinctive shape will have at least one (triangle) measurement to the center of the object. Just as with the rectangular method, you must first determine the fixed points and stretch your tape measures so that they meet in the corner with both, or all three, starting at the ‘0’ datum, then take three-point measurements back to the object.

Figure \(\PageIndex{7}\): Triangulation measurement method diagram. Created with ScenePD 6. Author's collection

There may be occasions where the fixed points for your crime scene are too far away to be useful for establishing a baseline or have a distinct boundary for either the rectangular or triangulation methods, you may have to combine the baseline and triangulation methods as illustrated below. You will need a very long tape measure and a compass. Sink metal stakes at the datum points should the fixed feature be removed. Establish GPS coordinates for each datum point as well for future reference.

Figure \(\PageIndex{8}\): Triangulation on a Baseline Diagram. Created with ScenePD 6. Author's collection

The Rough Sketch

Your author will be the first to admit that he is not an artist. He can manage something just marginally better than a stick figure. This fact does not excuse him from trying to create at least a rudimentary sketch of the scene. These rough sketches can be extremely useful when writing reports and for creating the final sketches that will be submitted for courtroom proceedings. They are especially helpful in shooting scenes where multiple fired cartridge casings remained in the scene. Often times it is difficult to appreciate the relationship of the shell casings to the other objects in the scene if you are reliant only on the photographs taken of the scene. Photographs are not good at depicting the distance between evidence items. A sketch, especially a bird’s eye view sketch, with proper measurements is far more useful. It is, however, imperative that the diagram technician retains the rough sketch with their field notes as they are discoverable evidence. It is best to scan the field notes and rough sketches into PDF files and save them in electronic form.

Your author recommends the reader find a suitable place with fixed features or walls. Place a kitchen knife in the center of the area and practice measuring and diagramming the scene. Once you have recorded the measurements and photographed the position of the knife, pick up the knife, hand it to a fellow student or friend, along with your diagram, and challenge the friend to put the knife back in the scene exactly the way it was found. Use the baseline, rectangular, and triangular methods to see which method was the most successful.

The Future of Crime Scene Diagramming

Technology is the master of all things including crime scene diagramming. Engineers have long had the ability to conduct 3-dimensional Global Positioning System (GPS) scanning of the topography of land to assist them with the design and construction of roadways. It was therefore a foregone conclusion that some bright crime scene technician had the brilliant idea of “borrowing” the scanner used by their public works department and applied it to a large crime scene. Enter the Leica Scan and FARO 3-D software and hardware GPS scanning and diagramming systems. These systems capture data by emitting laser beams to selected datum points, which create an accurate mapping of the area by collecting millions of data points that produce a point cloud imaging of the scene. Proprietary software allows the technician to create a diagram of the scene using the point cloud imaging that provides a precise representation of the scene. The benefits of GPS scanning systems are that the scans are extremely accurate, and they produce very detailed diagrams of the scene when using the proprietary software that converts the point cloud data into a visible 3-D diagram. The drawbacks are that they are bulky devices that require large tripods for the laser-scanning units, and the technician must be trained to input the datum points, which can be time consuming. They may not be the best method for diagramming small scenes. The point cloud imagery is unrecognizable without the additional software needed to create the diagram. The devices can be cost prohibitive for small agencies, although leasing options and grant funding are available. Some environmental conditions like rain, standing water, or high winds can adversely affect the quality of the data being collected. Your author can see the benefits of these devices but recommend highly to every agency that employs them to be sure to train your technicians in the tried-and-true methods of diagramming, because there always needs to be a plan ‘B’ when technology fails.