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Capturing Snow Impressions
Police officers are asked to wear many hats. At any given moment they may be expected to function as a law enforcement officer, marriage counselor, social worker, paramedic, animal management officer, detective and, yes, crime scene investigator. Of course, most agencies only budget to provide officers with the bare minimum standards to perform each task, if that.
Not surprisingly, a lot of officers are reluctant to perform evidence collection tasks they are unfamiliar with and don’t get to practice very often. Evidence collection is hard enough. Even some criminalists aren’t up to speed on proper collection techniques, so how can anyone expect our street officers to do any better? One such area that presents problems for officers is the documentation of footwear impressions in snow.
A number of ways exist to document and collect footwear impressions. There are many things to consider, such as temperature, snow pack composition and materials being used. This article won’t cover all aspects of snow pack or footwear photography and casting, but it will provide the officer with a simple and cost-effective way to capture footwear impressions for use in the laboratory.
If your department utilizes an on-call criminalist, you may want to call him if the evidence on scene warrants it. This may be the result of complexity or quantity of evidence rather than the crime type. However, if you stand with the majority of police officers and sheriff’s deputies who don’t have such resources readily available, then these techniques may be of great value to you.
Photographing in Context
Footwear examiners are often frustrated by limited photographs depicting the impression within the scene. It is not enough to simply photograph an impression. Even if examiners are provided with an excellent photograph of the impression, they may be limited in associating that impression with any portion of the scene. Was it found on the front lawn or in the backyard? Was it on the public sidewalk or the driveway outside the point of entry? These are the questions that proper photographs attempt to answer.
Good mid-range photographs show relationships between items of evidence (that is, how close the impression was to the point of entry and which way it was pointing). When photographing snow impressions, try using a placard or simply taking a mid-range photograph showing the location of the impression relative to a recognizable landmark in the scene.
Photographing the Impression
Most officers are only provided with a small point-and-shoot digital camera, which is not ideal for photographing three-dimensional impressions. In a perfect world, the officer would utilize an SLR-style camera with a tripod elevating the camera to a parallel plane above the impression.
SLR cameras are also equipped with a hot shoe, allowing the photographer to attach a flash cord offering a number of oblique lighting options to the officer. Ideally, the flash unit could be activated at various elevations and orientations to the impression, thereby highlighting its three-dimensional aspects.
The reality for most officers, however, is that such a camera setup is simply not available. So the choice then becomes to either take a photograph with the point-and-shoot camera, or take no photo at all. Photography is an important step in the documentation process for one simple reason: It serves as a backup should anything go wrong in the casting process. While the use of a point-and-shoot camera is less than ideal, there are a few methods which can improve the quality of your photos.
Proper Scale in the Proper Plane
Most officers know to use a scale in their photographs. This is nothing new. But many officers aren’t aware of some critical issues relevant to snow impressions. First and foremost, you need to use a proper scale. A straight edge ruler or bureau scale 6 inches to 12 inches in length is preferable.
Second, and more importantly, the scale needs to be placed on the same plane as the impression. This means the ruler must be level with the bottom of the impression, not the top of the snow next to the impression. When the scale is placed on top of the snow, distortion occurs.
Footwear examiners require 1:1, or life-sized, photographs of the impression in order to conduct a comparison with the known shoe. The distortion created by the placement of the scale above the impression may make this task very difficult, if not impossible. Great care must be observed when placing the scale at the same level as the bottom of the impression so snow does not fall into the impression, thereby obscuring important details.
Highlighting the Impression
Snow is both monochromatic and highly reflective, making contrast photography difficult at best. Images of footwear impressions often appear burned out in photographs because the flash unit strikes the impression at a perpendicular angle. This may be unavoidable with most point-and-shoot cameras.
Highlighting is a process by which colored spray paint is applied to the impression. While primer gray or black work best, any dark-colored paint can be used in this process. To highlight an impression, the officer sprays the paint above the impression, allowing the paint to fall into the mark. Spraying the paint directly on the impression may damage or destroy important features.
Several applications should be made from different directions around the impression. By applying a faint layer of paint to the impression, the three-dimensional aspects will be highlighted and more easily photographed.
A word of caution: The application of dark-colored paint will absorb solar radiation, i.e., heat. If the impression is sprayed during daylight, then the officer should cover the painted impression with cardboard prior to casting. If the impression is not covered, it may quickly begin to melt under direct sunlight, even if the ambient air temperature remains low.
I have observed impressions becoming completely obliterated in as little as 20 minutes under direct sunlight, even with freezing temperatures. Highlight-ing impressions does not affect your ability to cast with the method described below.
Whenever I train officers, a common question eventually comes up: Do I have to cast an impression if I’ve taken a picture of it? The answer is a resounding “Yes!” Most footwear examiners actually prefer a well-made cast to a well-made photograph for one simple reason: A cast will almost always provide more information to the examiner than a photograph.
Casting in soil is relatively straightforward, and a lot of officers are familiar with that process. Typically, it involves the mixing of powdered dental stone with water in a plastic bag. Using this method in snow, however, may damage the impression.
A number of casting techniques exist for snow impressions, but most are too complicated and impractical for the street officer. Officers usually only carry limited amounts of dental stone, if any at all. This snow casting method utilizes essentially the same materials used in casting soil impressions and, with a little practice, can be mastered rather quickly.
This method has been referred to as the dry casting method. It works best in wet, heavy snow, but it can produce good results in virtually any type of snow pack. Once the initial photographs have been taken with a proper scale, you can proceed to cast the impression. With the dry casting method you’ll need dental stone, water and preferably a pastry sifter, which can be purchased at your local Wal-Mart for about $5.
The first step is to evaluate the amount of moisture in the impression. Traditional dental stone casting involves the proper mixing of dental stone powder and water. Too much water and the cast may be too brittle, not enough and it may set up before you get it poured. Water content is just as important in the dry casting method, though our estimates are somewhat subjective. Suffice it to say, a slushy impression will require less water from you than a dry, powdery impression.
Once you are ready to cast, you must first coat the impression with a thin layer of dental stone powder by using the pastry sifter. Evenly fill the bottom of the impression to a depth not exceeding 1/4 inch. Allow a few minutes for the dental stone to absorb the moisture in the impression. This will be evident by a slight darkening of color in the casting powder.
If the powder appears dry after a few minutes, you can spray the casting powder lightly with water. A small plastic spray bottle works well for this.
If water beads begin to form on the casting material, you have too much water; stop the application of water and allow the cast to dry. After drying for about 10 to 15 minutes, repeat the above casting process. Continue layering the dental stone until you reach a depth of about ½ inch and allow it to dry for about 15 minutes. This layering process is designed to protect the impression surface from the liquid dental stone you will apply next.
Without such a base layer in place, the application of liquid dental stone could easily damage or destroy the impression. Dental stone actually heats up during the curing process, and the heat alone can melt the impression detail before the dental stone sets up. Once the base layer is established, the rest is very straightforward. Mix a bag of dental stone with water. Typically this will involve approximately two pounds of dental stone mixed with about 12 ounces of water.
I recommend that the water be cooled in the snow for several minutes before mixing with the dental stone. Many officers are familiar with this mixing method commonly done in a plastic bag. Once the mixture reaches the proper consistency (described by many as having the viscosity of pancake batter), it can be poured over the layered dental stone already in the impression.
Dental stone cures at a rate commensurate with temperature. The colder the temperature, the longer it takes to cure. That being said, the cast should be ready to lift in about an hour, even in freezing temperatures. Casts can usually be freed from the snow pack with little effort. Place the cast, impression side up, in a safe storage container, like a copy paper box top or similar device, and allow complete drying before packaging. It is important to be as careful with the cast as possible until it is fully dried—usually after 24 hours.
Putting It All to Use
Snow impressions are nothing to be feared. As in all things, practice is critical in establishing a comfort level. But many officers have proven they can effectively photograph and cast snow impressions with very little effort. The key to effectively collecting this evidence is having the proper materials and being familiar with their use. Practice in using this technique will likely pay big dividends to your cases in the future.
Tom Adair is a retired Senior Criminalist with the Westminster, CO Police Department. He is a board-certified footwear examiner with the International Association for Identification. You can reach him at firstname.lastname@example.org.
Published in Law and Order, Nov 2009
Rating : 9.7
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