As forensic engineers, one of the most common scenarios we face is not having a lot of information to go off of when performing our work, be it during an accident reconstruction or failure analysis. In fact, more often than not, we must rely on only photographs that are given to us by our clients that illustrate the problem that we are asked to solve. While it would be nice to have all the information given to us perfectly every time, it is simply not possible for this to happen. Therefore, one of the biggest challenges we face as forensic engineers is analysis of physical evidence based only on photographs.
This is the essence of photogrammetry – at least in the field of forensic engineering. Using the science of photogrammetry, one is able to extract more than just visual observations from photographs. In fact, photogrammetry can be used to extract 3-Dimensional data from photographs of evidence that has been long gone since the photo was taken.
One question asked of us frequently by our clients is:
“What do you make of the accident based on these photographs I gave you?”
To answer this, let’s dive into a photogrammetric analysis process based on a hypothetical scenario described below:
An accident has occurred some years ago. There are photographs of the tire marks left at the accident scene but the tire marks are long gone nowadays. In order to do a proper accident reconstruction, the lengths of the tire marks need to be determined. How do you get measurements from the tire marks if they don’t exist anymore? The answer: Photogrammetry in three main steps.
Step 1: The first step is to survey the accident site. Even though the tire marks are gone now, a survey to get the general geometry of the roadway is helpful in performing the photogrammetry. In the picture below, the tire marks are visible leaving the roadway and leading to disturbed dirt and grass off of the roadway shoulder. Based on a survey of the accident site, the road surface is defined, minus the location of the tire marks.
Step 2: Mark points on the photograph of the tire marks. Using software made especially for photogrammetry, the photograph is rectified properly and the location of the tire marks is marked by the forensic engineer. The picture below shows the markup of these tire marks.
Step 3: Export the data from the photogrammetry into a computer-aided design (CAD) program to measure the lengths of the photographs. The picture below shows what the tire marks would look like from a bird’s eye view, after importing the photogrammetry project into a CAD program. From here, the lengths of the tire marks are easily determined, allowing the forensic engineer to properly perform an accident reconstruction.
While this process has been simplified in the preceding example, in many cases photographs are more difficult to rectify and information is not as easy to determine. Therefore, it is important that you contact us with your photogrammetry questions before committing to a case based on photographs. As always, we are happy to review your case without any obligation. Let us know how we can help you!