Measurements of scene evidence or vehicle damage are often used in forensic analysis and reconstruction because many of the calculations used to answer questions depend on these measurements. One of the techniques experts use to accurately measure evidence such as vehicle crush damage is photogrammetry. Photogrammetry is the process of extracting measurements from photographs and it has applications in many fields such as cartography, architecture, archeology, and even medicine. In a forensic reconstruction setting, we use photogrammetry to obtain measurements of scene evidence or vehicle damage. We have even used the technique to efficiently obtain measurements from a single photograph or videos taken by police, media, witnesses, CCTV or unknown sources, when the evidence is no longer available to directly examine.
There are in-house-built and various commercial software programs available that assist us in conducting photogrammetry, and each works slightly differently, but they all do the same thing: they use engineered mathematical models and methods to transform the two-dimensional planes of a series of photographs into a three-dimensional model of the subject. Photogrammetry has been shown to be a reliable, appropriate, and efficient method for the documentation of evidence in our field, .
When we examine a vehicle and take photographs specifically for the purpose of photogrammetry, we take many high quality photographs from different elevations and angles and include coded targets (see Figure 1A below) on and around the vehicle (Figure 1B). These targets allow the expert to automatically orient the camera’s position relative to the targets and the vehicles in each image using the software. This “network” of images is analyzed using complex mathematical transformations that take into account the parameters and distortions of the camera and lens. This concept is illustrated in Figure 2 through an image that shows the location and orientation of the camera for each photo that was taken, as determined through analysis of the network of photographs and coded targets. Each small cluster of dots is one of the coded targets.
Figure 1A (left): A close-up view of one of the coded targets. Figure 1B (right): A view of coded targets placed on and around a vehicle from one of our crash tests for photogrammetry.
Figure 2: A three-dimensional view of the solved network of photographs. The outline of the damaged vehicle (black line) and the wheelbase (red line) are shown. The coded targets, location, and orientation of each camera are shown. The purple lines represent the field-of-view of one of the cameras.
The photogrammetry expert will optimize the results, adjust unknown parameters by accounting for a range, and refine the camera positions to define the uncertainties (which is an inherent aspect of a scientific process).
While photogrammetry is most accurate when conducted using photos taken by an expert with a calibrated high resolution camera and coded targets, it is possible to extract measurements from other photographs. Often, when the subject scene evidence or vehicle is no longer available, photographs taken shortly after a loss showing the vehicles at rest or roadway evidence are all of the evidence we have available. This evidence may be useful and meaningful to us. We can often extract information such as the parameters of the camera and its location relative to the environment. We can then also re-attend the scene and retake the photographs with measurement markers included, and use these new, enhanced photographs to locate the evidence as it would have been after the incident.
There are other ways of documenting forensic evidence, many of which involve the use of high-tech equipment that is capable of capturing sub-millimetre accuracy, which may be necessary in some fields. Laser scanners are available that can take measurements with sub-millimetre accuracy, but this type of equipment comes with a hefty price tag: most of them cost tens of thousands of dollars. Even after capturing thousands of data points with pin-point accuracy in minutes, this data must be expertly filtered or “cleaned” before the expert can begin to analyze the evidence. The cost of the equipment along with the additional work of filtering and analysis required to distill millions of data points makes this type of service far more expensive than photogrammetry. Similarly, a mapping drone can also be implemented in this day and age, however, the time and cost to lawfully operate the UAV in a forensic manner may also not be economical. Photographs, even when expertly taken using a known and calibrated camera, will not exactly provide the sub-millimetre of accuracy of surveying or scanning equipment; however, centimetre accuracy is sufficient in most accident scenarios.
For example, consider the photogrammetry results obtained from the crash test vehicle in Figures 1 and 2 above. Photogrammetry indicated that the vehicle sustained 22 cm of damage compared to its original shape (see Figure 3 below, which shows an outline of the vehicle from a top-down view).
Figure 3: This image shows the red outline of the damaged vehicle (from Figure 1B) super-imposed over the outline of the undamaged vehicle in blue.
The actual direct crush measurement using a tape measure was 23 cm; this difference of less than 5% is acceptable, given that any analysis of this crush measurement will also involve vehicle and impact location specific stiffness assumptions, which also include uncertainty. To deal with these assumptions and uncertainties in their analysis, experts generally work with ranges, and the resulting range used for crush damage in calculations would indicate that any additional level of measurement detail (i.e. sub-millimetre measurements) would be akin to measuring for a cut with a micrometer and then making that cut with a chainsaw.
What all this means is that photogrammetry gives us measurements that are accurate down to the centimetre, which is sufficiently accurate for reconstruction purposes where ranges are included to allow for uncertainties introduced by various assumptions. Compared to other methods of preserving and documenting evidence, photogrammetry is one of the most cost-effective, and can be applied in various scenarios, such as when we need to document evidence more thoroughly, or even when the evidence is no longer available other than through provided photos or videos.
 B. Randles, B. Jones, et al., “The Accuracy of Photogrammetry vs. Hands-on Measurement Techniques used in Accident Reconstruction,” SAE Technical Paper 2010-01-0065 (2010).
 D. Templeton, Jr., “Close-range Photogrammetry as a Routine Crash Reconstruction Tool Within the Florida Highway Patrol,” ARC-CSI Crash Conference, Las Vegas, NV, 2008.
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