A Primer on the Tools of Crime Scene Analysis

It has been my experience that crime scene investigators tackle their jobs in a methodical manner (taking photographs, searching for and collecting evidence, taking measurements, and so on). Too few, however, involve themselves in an equally important aspect of their jobs: discovering what it all means. Some view their responsibility at a crime scene as merely the documenter of facts. Interpretation and analysis are left to someone else. But, depending on the size and structure of their agency, they may be called upon to provide an analysis of the crime scene or an opinion on the basis of their experience and training. These investigators are in a unique position to re-create the crime, because they have seen the evidence and scene firsthand. They don't have to work on assumptions made regarding the work of another person.

It is not always necessary to create a three-dimensional model of the crime scene. Often, a good crime scene diagram or photographs can be used to illustrate a theory. However, each crime scene should be photographed and measured with the assumption that a reconstruction will be made. With little additional effort, the crime scene investigator can put together a credible interpretation of how the crime was committed. Computer programscan help by doing a lot of the calculations and reconstruction work. You don't have to be an engineer to visualize your theories or to test alternative proposals. The engineering is done by the computer.

A reconstruction does not prove a theory. It merely allows for the possibilities suggested by the theory. A reconstruction can disprove a theory by showing or demonstrating how it was not possible for certain things to happen as suggested or interact with other elements as described.

There are basic considerations for compiling the material and information needed to put together a crime scene reconstruction and analysis. These include photography, crime scene diagrams, accurate measurements and scale, known reference points, and specialized computer software.

Photograph

This is the last time the crime scene will ever appear as you found it, so document it thoroughly. The photographs need to accomplish five things: supply documentary evidence; verify witnesses' testimony; supply the source material for expert analysis and reconstruction; record the scene with your point of view, advancing your analysis; and re-create the scene for the jury.

Most professional 35 mm and medium format cameras will do an acceptable job. If you are using digital photography, try to use a camera that records at a resolution of three megapixels or better. Experiments I have conducted on behalf of the New Jersey Division of Criminal Justice showed that this resolution will provide the detail necessary to verify your reconstruction.

For three-dimensional rendering, shoot four overlapping angles. Each photograph should include about twenty percent of the previous recorded image. This will cover the subject sufficiently for a computer reconstruction. Additionally, take straight on (perpendicular) shots of the subject, which will be used to verify the reconstruction. Overhead shots may be acquired using commercially available devices that allow you to hoist your camera over the scene. Larger scale depictions mayinvolve the cooperation of the fire department or utility companies with bucket trucks.

Hold the camera level for the verification shots. The use of a tripod will make it easier to critically establish level and plumb references in the final photos. Maintaining critical level and plumb alignments will reduce oblique perspectives that will affect the measurements taken from the photographs (Figures 1 and 2).

It is important to use a lens of the same focal length for all your reconstruction photos. Photogrammetry (taking measurements from the photographs), if necessary, will be easier if a consistent focal length is used. Photogrammetry software can be used to verify the crime scene diagram measurements to create three-dimensional models from photographs. Photogrammetry software can also provide measurements of objects or distances not determined during the investigation.

Crime Scene Diagram

Most investigators photograph the scene first. You should then have an idea of the content of your photos and be able to determine the key elements that need to measured. This will lend itself to easier interpolation of objects in the scene, if needed.

Your rough sketch needs to be orderly and legible. It may be some time before you will need to use it for a reconstruction or to prepare a clean copy for court. Additionally, it may be discoverable. You do not want to give the defense any reason to question the accuracy of your work.

Use graph paper. The grid will help you maintain proportion, scale, and straight lines. An architect scale will help you quickly establish a properly proportioned drawing.

Start your rough sketch by establishing the outer perimeter of the scene. You can then subdivide the sketch into appropriate parts (e.g., rooms, paths, driveways, streets, landscaping features, etc.). Locate the windows, doors, and openings within the walls of the building. Next, place large items, such as furniture, cars, or bodies, within the scene. You may need to create additional diagrams that deal with small details that focus on portions of the overall crime scene.

Take consistent measurements. If you decide to measure to the center of an object as opposed to an inside or outside surface, do so in all cases. Use the same measuring device for all measurements taken at a single scene. This will ensure that proper spatial relationships are retained. Use a retractable steel measuring tape of 25-30' in length. The wider blade is usually more rigid than the narrower, shorter lengths. This size will allow you to measure almost any interior scene and accomplish the task single-handedly.

Triangulate to locate items within the scene. Unless you have references in the scene that are perfectly aligned, triangulation is the only way to accurately place items in the scene. Triangulation is accomplished by measuring an object from two different established points. Measure the distance between the two reference points to complete the triangle.

If you are not a good freehand artist, take instant photos of complicated details. Record the measurements directly on the photos with a fine tip permanent marker.

Scale

Take measurements of objects to be rendered in three dimensions. Use the “tailoring” method. (Measure the object as though you were making it a new set of clothes that need to fit perfectly.) Guesswork is not an element of a good and defensible reconstruction. Although you may be using a computer to create your final diagram, you still need thorough and accurate input, which will come from your rough sketch.

Plumb and Level

Establish a plumb line (vertical). Establish a level line (horizontal). You will need these references to verify aspects of your reconstruction. As previously mentioned, use things referenced in your photos. Photograph plumb and level references at 90°. Photograph referenced objects once the camera has been leveled and is perpendicular to the subject. This will eliminate distortions such as converging vertical lines (Figure 1). The further you are from the subject, the less the visual distortion will be.

Creating Human Models

In addition to the measurements taken of the crime scene, you will also need measurements of the people depicted in the reconstruction. Stature and weight can play important roles in evaluating what appened at the crime scene. The physical dimensions of the suspect may be obtained from his arrest record. However, these measurements should be verified. It may be necessary to obtain a court order to accomplish this. Dimensions for the victim, if deceased, may be provided by the medical examiner.

You should always take full body photographs of the victim and suspect. With just their height measurement, you'll be able to interpolate the proportions of their torso, arms, and legs.

Record clothing size. This may be a guide to body type. However, be aware that it is currently fashionable for some young people to wear clothes that are loose fitting or baggy. Their clothing and shoes may be a size or two larger than their true size.

Use photo editing software to check the accuracy of your models against photographs of the depicted subjects. The layers feature of the software allows for the overlay of two images. The transparency feature allows one image to be seen through another. This allows you to view the accuracy of your model against a photograph of the actual person.

Software

You will need four types of software to achieve a successful re-creation of the crime scene:

• Image editing software will be used for alignment, verification, and archiving.
• Drafting and modeling software will be used to create the two- and three-dimensional diagrams and models of the crime scene (Figures 3, 4, and 5).
• Photogrammetry software can be used to create threedimensional models from photographs and to allow accurate measurements to be obtained from photographs.
• Presentation software will display the re-creation. The slide show and transition effects make switching from crime scene photographs to crime scene reconstruction easy.

Not all software is created equal. When choosing the programsto use for the reconstruction, keep the following in mind:

• Acquire software programs that are compatible. Programs that use common file formats will allow for smooth transitions when importing or exporting projects. This will also permit you to bring your created file into other programs for advanced rendering or special effects.
• Stay away from proprietary formats. Your work is discoverable. If the defense cannot make use of the work you created, you may not be able to use it in court. A scientifically acceptable process must be repeatable. Other experts need to be able to verify your methods. A judge could issue an order requiring you to make your equipment and software available to the defense if your software is not generally available and
• your format is unique to the program used.
• Older versions of programs can be as useful as newer or upgraded software but much less expensive. Unless there has been a major revision to the software, such as compatibility with new operating systems, it may not pay to upgrade or buy the latest version. Think about what you need to accomplish, then choose the version that meets those needs.
• Acquire software that is easy to use. If there is a steep leaning curve to the program, it does not matter how wonderful it is. For any program to be useful, people must not be intimidated by it. The more people are willing to use a program, the more proficient they become at exploiting its full potential. Remember, the people doing the reconstruction are usually crime scene investigators, not computer geeks.

Conclusion

Reconstruction and analysis are logical extensions of the crime scene investigator's craft. Those who concentrate solely on reconstruction analysis can only do so while relying on the efforts of the crime scene investigator. Through experience and training, a crime scene investigator can develop an eye for what is needed in order to interpret the meaning of what he or she collected and photographed.

For more information, please contact:


Lt. Robert J. Garrett
PO Box 2413
Edison, NJ 08818
editor@njiai.org

Appendix

The following is a list of some of the commercially-available software that can assist you with your reconstruction efforts:

• AutoSketch & AutoCAD by Autodesk, www.autodesk.com, for creating two- and threedimensional diagrams.
• 3D StudioMax by Discreet, www.discreet.com, for creating three-dimensional models and scenes from diagrams.
• Photomodeler Pro by Eos Systems, www.photomodeler.com, for creating three-dimensional models from photographs.
• Poser by Curious Labs, www.curiouslabs.com, for creating human models.
• Photoshop by Adobe, www.adobe.com, for verifying models as wells as archiving the final images.

Figure 1
Holding the camera level and perpendicular to the
subject will help eliminate oblique perspectives.

Figure 2
Oblique perspectives should be avoided.

Figure 3
A 2-D diagram of a small house.

Figure 4
A 3-D rendering of a small house.

Figure 5
Photograph and 3-D model.

*From the Journal of Forensic Identification Vol. 53, No. 6, November/December 2003
The Official Publication of the International Association for Identification
"Reproduction of the Journal of Forensic Identification, in whole or in part, for noncommercial, educational use is permitted provided proper citation of the source is noted. Reproduction for any other use is prohibited without prior written permission. Requests for permission may be addressed to the editor (of the Journal of Forensic Identification -- jfieditor@theiai.org)."

This paper was presented at the 88th annual educational conference of the International Association for Identification held in Ottawa, Canada, July 6-11, 2003.

Article posted: August 20, 2007