Photogrammetry in Forensics Primer

When one tries to imagine how many digital images are captured each day around the world to preserve some type of visual data for accident or crime scenes, it quickly becomes apparent that there are practical uses for photogrammetry in forensics and reconstruction. Historically, the Remote Sensing and Mapping industries have been the largest users of photogrammetry; however, there has been an adoption of this technology in areas such as architecture, forensics, archeology and film. Photogrammetry is the science of taking geometric measurements from photographs. “Photogram” is a photograph and “metry” is the science of measuring. Since a photograph takes images from the 3D world and projects it on a flat 2D image plane, we lose the depth information. However, photogrammetry can be viewed as the process by which we do the reverse. By knowing some information about the camera that took the photographs and by having two or more photos of the same object from a different perspective, we can gain some 3D information back. In some cases, it is possible to take measurements from a single photo as well, but there are some criteria that need to be met.The principles of photogrammetry date back several hundred years. Modern day photogrammetry is still a developing science and the majority of people are unaware of what photogrammetry is and what it can do for them. As a result, many are not taking advantage of capturing and preserving geometric data in their photos and others are simply not aware that they can extract 3D measurement data from their existing photos.There are different approaches to a photogrammetry analysis based on the number and quality of photos. Each approach has its advantages in accuracy and simplicity as well as what can actually be output for visual presentation. In a properly documented scene for which photogrammetry has been taken into account. All details can be recreated in 3D and the accuracy of measurements can be simply stated as being limited by the size of photos taken and by how well the objects appear in the photos.What most people responsible for taking photos at accident or crime scenes don’t know, is that they could be capturing entire sets of 3D measurement data by simply adjusting their techniques to make their photos “photogrammetry friendly”. In most cases, the methods required to capture data is only a matter of a few extra minutes and a few extra photos at the highest possible resolution.Unfortunately, it is sometimes months afterwards when an Accident Reconstructionist or Insurance Adjuster looks at a photo and says, “Darn, I wish we had measurements for that”. In some cases there could be a piece of evidence that appears in a photo that was not initially measured because it was not deemed important or it was simply overlooked.Regardless of the situation, photogrammetry can be used in practical applications such as:• blood spray patterns• Skid mark measurements• Measuring the height of a suspect in a security video• Crush measurements from a damaged vehicle.• Measuring distances between two objects.Of course, it is not always possible to take accurate measurements from any photo or set of photos. There are some criteria that need to be met. Each case is specific because of all the various types of variables involved in taking photos in different situations.It is important to speak with a photogrammetry expert and provide them with all the photos for review to be entirely certain. In most cases, a reputable photogrammetry expert will provide an initial consultation on the quality and applicability of the photos to a photogrammetric analysis at no charge. The list below can be used as a quick check to determine if a photogrammetric analysis might be possible. 1. There are several photos of the object to be measured from varying camera angles.2. There are objects in the scene that have known reference dimensions or dimensions that can obtained after the photos were taken (e.g. there is a table in a photograph for which the length, width and height can be obtained).3. The object to be measured is clearly visible in the photos and has distinguishable features (i.e. the object should not be blurred or out of focus).4. The size of the object to be measured needs to be sizeable relative to the entire photograph (e.g. a skid mark that shows up as a tiny mark on a photograph way off in the distance, cannot be measured with any decent accuracy).5. Photos were taken with the same camera and same focal length (i.e. zoom) setting.The above is simply a quick check and not having met one or more of the above criteria does not mean a photogrammetric analysis is impossible. Each case is specific and must be reviewed by an expert to determine whether or not it is applicable.Taking photographs of accident and crime scenes is such a practical and common activity that it is a wonder why people are not taking advantage of capturing geometric data in their scenes by simply making changes in their methodology of taking photos (alas, a subject for another article).However, in many cases, the photographic evidence that exists may be sufficient to obtain measurements from and just because no one had documented or measured a feature at a scene, doesn’t mean it is a dead end. Photogrammetry can bring great value to lawyers and accident reconstructionists who need the advantage of “better information” in a court case or accident reconstruction report. Having accurate data that the opposing council may not have considered or simply ruled out as impossible to obtain can often be the difference in winning or losing a case or settlement.