A fire that originated inside a restaurant in a strip mall became the subject of litigation almost three years after the date of the fire. An adjacent tenant in the mall was pursuing a claim against the restaurant owner seeking recovery for his losses.
Subsequent to the fire, but before we were retained by the adjacent tenant’s law firm, the property had been rebuilt. Consequently, it was impossible for us to perform a site inspection, but we were able to review photographs taken by the responding fire department’s arson investigator.
Every investigator will acknowledge that his level of confidence in his own conclusions is usually higher when he has had the opportunity to actually examine the site and the physical evidence. Nonetheless, good investigators can often develop strong conclusions from photographic evidence alone, as long as other available evidence (e.g., eyewitness testimony, building records, fire personnel observations) corroborates the findings.
One technique that can be especially helpful for analyzing historical photos is photogrammetry. In the subject case, we wanted to determine if an oven’s exhaust duct was properly installed (i.e., did it have sufficient clearance between the duct’s hot walls and the building’s wood beams and joists). We used photogrammetry to assess the dimensions of both items.
Photogrammetry is the art and science of determining geometric properties of objects from photographic images. The two images on this anecdote page give examples of a photogrammetric analysis conducted as part of our strip mall fire investigation. In both images, the superimposed green lines represented known dimensions and the superimposed red lines represented unknown dimensions. By computing the relative lengths of the red and green lines, a multiplier was obtained and the unknown dimensions were determined from the known dimensions.
In the first photo above, the known dimension (green line) was a 4-inch electrical junction box, and the unknown dimension (red line) was the width of the square exhaust duct (determined by the analysis to be 24 inches).
In the second photo, the known dimension (green) was a 16-inch cement block, and the unknown dimension (red) was the distance between joist hangers (also determined to be 24 inches).
The photogrammetric results were ultimately compared to eyewitness statements and burn patterns to form a sound conclusion about the origin and cause of the fire.
The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2013)