The first step in using this footage is simply discovering its existence. By knowing about the possibility of the footage's existence, an investigator can begin the process by surveying those areas and establishments likely to have cameras. Once a potential camera and footage source is found, the next step is to determine if the camera captured footage during the time in question. Often, cameras are mounted simply for deterrence, which means they would not have been active or functional during the time in question.

One important thing to note is that most video surveillance systems capture footage in a “loop sequence” — the camera overwrites older footage at a specified time interval, which can sometimes be as short as six hours. Once the footage has been overwritten, it is gone forever and is not available for analysis. For this reason, it is critical to determine if footage is available as quickly after the incident as possible, and work to secure the footage before it is overwritten.

Once the footage has been obtained, the next step is to determine if the footage has any value in reconstructing the event. Did it record during the correct timeframe? Was it aimed towards the area of the event, and did it capture the actual event? Although it is rare to see a case where the actual event was captured in its entirety, even peripheral information can be exceptionally powerful in helping determine what happened, or perhaps more importantly for the defense, what did not happen.

In order to see what the footage holds, it is of course necessary to be able to view it, and there are a few technical considerations involved in this step. Video of this type is typically recorded in one of two manners: onto tape or digitally. If the video was recoded to tape, then the physical tape must be obtained so a copy can be made for viewing. In this event, the major consideration is the type of tape — VHS, S-VHS, BetaCam, Hi-8, or Digital Video — and finding a player that can play the style of tape.

If the footage was recorded digitally, additional considerations come into play. Typically, digitally recorded footage is written not to a physical tape, but rather to a hard drive, like those found in desktop and laptop computers.

Rather than obtaining a physical copy of a tape, the investigator will obtain a computer file or series of files. These can be copied to a typical computer storage device such as a floppy, CD, or DVD. In addition to the computer files of the footage, it may be necessary to obtain a viewer as well. Viewers are software programs that are often customized to the video system used, and they display the captured footage. Some systems are advanced and offer an array of tools to assist in the viewing and analysis of the footage. In any event, both the footage itself and the software must be obtained and forwarded to any experts who will view and analyze the footage.

So you've discovered a potential source of video footage, determined that the camera was functional, and obtained the requisite tape copy or digital files and software. Now to the fun part: analyzing the footage and using it to help make your case. Now it's time to delve into the more technical aspects involved in analyzing the captured footage.

Video Processing

First the video is viewed to locate the footage captured near the time of the event. Typically I will review the video footage multiple times in order to get a sense of the quality of the footage and the type of information it holds. Where is the camera looking? Is the view clear or grainy? What portion of the event or event area does the camera record?

Once the initial viewing is complete and the portions of interest are determined, the footage is digitized. The video is separated into individual frames or pictures, which can be enhanced digitally to improve clarity and picture quality. These still frames each depict a viewed area at a single point in time — a series of sequential “snapshots” of the area.

The rate at which the camera records the video must be determined in order to use the sequence of frames as a timer. The recording rate, or “refresh rate,” determines the smallest time interval that can be analyzed for reconstruction. This is one of the most powerful aspects of the footage, as each snapshot or frame records the scene at a given time, with a consistent interval between frames. This quality of the captured footage allows the expert to use the frame intervals as a timer, providing a critical piece of information for analyzing factors such as speed and acceleration.

Typically, today's security cameras record at a rate of around 1.5Hz or 1 new picture every 2/3 of a second. Handheld video cameras typically record at a rate of 30Hz, or one new picture every 1/30 of a second (30 snapshots every second). Using a 1.5Hz security camera as an example, events that are seen by this camera and are visible in the footage two frames apart occurred 1.33 seconds apart (2/3 x 2). This method can be used to determine the time between any event or object that is recorded by the video, such as the passing of pedestrians or traffic signals in the case of a camera mounted on a city bus.

After the individual frames are digitized and enhanced and the recording rate is determined, the event scene is surveyed with a total station or preferably, a 3-D scanning laser. The measurements are used to create a 3-D working model of the scene, providing distances between objects seen in the video. Coupling these distances with the elapsed times determined from the refresh rate, it is now possible to calculate the speed, position, and acceleration of any object seen in the footage, as well as that of the vehicle carrying the camera (in cases of a vehicle-mounted system). The working model can then be used as the basis for a compelling 3-D animation of the event, showing the relative positions and motions of all objects seen in the footage.

Case Example

The following example from a recent case illustrates how an accident can be reconstructed accurately and with proper foundation using the video from a bus-mounted video camera.

A Los Angeles city bus collides with a bicyclist while making a right turn into an intersection. The rider is crushed under the bus and suffers major injuries. The defense alleges that the bike hit the bus and that the bus was acting in accordance with traffic and safety rules. An accurate reconstruction using specific tools is needed to determine who is at fault.

3-D Laser Scanner The 3-D laser scanner is an advanced class of survey instruments that is used to remotely measure surface geometry of sites and objects with extraordinary completeness, accuracy, and speed. Unlike traditional surveying tools that are used to record certain, selected points within a scene, a 3-D laser scanner automatically blankets the scene with millions of closely spaced point measurements. The resultant “point cloud” is used to create extremely accurate 3-D models of everything in the scene that the laser “sees,” including colors, so that every road stripe, sidewalk crack, and object — down to the leaves on every tree — is captured and added to the 3-D model. No other method of scene or object measurement comes close to the level of accuracy demonstrated by the 3-D laser scanner. Scans are usually done from several different vantage points in order to capture geometry for the entire scene or object, with a typical scan taking between five and 20 minutes.

Laser-assisted Photogrammetry In our case example, the velocities and trajectories of both bicycle and bus be determined and synchronized in order to reconstruct the accident. The video camera located on the bus had taken several pictures of the stationary objects located on the sidewalk. Using the 3-D laser scanner, the entire scene was recreated “virtually” in the 3-D working model. The working model provided the exact locations of the objects seen by the onboard video camera and photogrammetry techniques were used to determine velocity and acceleration of the bus at each frame. This determination would not have been possible were it not for the video footage and the accuracy of the analysis was maximized by using the 3-D laser scanner. Reconstruction parameters for the bicycle were derived by the expert.

As is often the case, the camera did not film the actual impact. However, since velocity and trajectory of the bus were determined for three seconds prior to impact via analysis of the video, the point of impact could be derived. The derived motion parameters for bicycle and bus were imported into the 3-D working model and used to determine that the bus did not stop in the intersection before turning into the crosswalk, as was required by law and testified to by the bus driver. The working model demonstrated that, had the bus stopped at the intersection before it made the turn, the accident would have been avoided.

Animation Analysis The results of the analysis are illustrated graphically in a real-time computer animation, depicting the motions of the bus and bicycle up to and including impact. In this case, and in keeping with the theme of the reconstruction methodology, the attorneys used videotaped depositions to record the testimony of the bus driver as she recounted her actions leading up to the accident. In her testimony, the bus driver claimed that she had in fact stopped in the intersection prior to passing through it, and had properly looked in both mirrors and cleared the way prior to entering the intersection.

From the laser-based analyses performed with the onboard video camera footage, it is apparent that the driver did not act as she testified. The discrepancy between her testimony and the demonstrated facts of the reconstruction was exploited and highlighted using a split screen to show the animation. On one side of the screen, the videotaped deposition testimony displayed the bus driver recounting her actions. As she described her actions step by step, the other half screen showed the animated results of the analyses and highlighted each discrepancy between her recorded testimony and the facts as determined by the working model.

The effect was a very powerful and compelling impeachment of the bus driver, effectively rendering her testimony as false. The case settled in favor of the plaintiff.

Exploiting the Evidence

Reconstructing accidents that have little in the way of physical evidence is always a challenge for adjusters, attorneys, and experts alike. Laying a solid and proper foundation requires the input of data related to the event. Illustrating the validity and fidelity of the reconstruction requires corroborating evidence. Both can be obtained through the analysis of videotape footage if it is available. In today's increasingly watched and recorded society, video footage has become more common and provides a powerful new tool in the field of reconstruction.

Craig Fries is president of Precision Simulations, Inc. He can be reached at 866-339-7378, [email protected].