Using laser scanning technology to extract data from video evidence
By combining video evidence with point clouds generated by laser scanners, forensic investigators can accurately measure positions, distances and heights of objects and people in 3D
As useful as video evidence is to police investigations, it does have its limits. Video images are displayed in 2D (two dimensions), making it difficult to accurately assess the moving positions, distances and heights of objects and people on screen. Video is also limited by the position of the camera that shot it. Images behind the camera and too far off to the sides don’t get recorded.
These and other visual limits can be overcome when police video is overlaid on top of point clouds captured by portable stationary laser scanners.
Using a technique known as Light Detection and Ranging (LiDAR), this system works by setting up a laser scanner at the crime scene. The rotating head of the LiDAR laser scanner emits and then measures and records the distances between itself and objects and surfaces in the scanning space in three dimensions. Depending on the measuring parameters – down to millimeters or less between each succeeding laser beam point – the resulting 3D image that results can be astonishingly detailed in its resolution. The sum of these measurements is known as the point cloud.
Capturing a true 3D view
Like the video shot from one position, measuring a room using a laser scanner in just one position will deliver limited results.
“But if you move the scanner around, you can generate point clouds from various angles that, when combined will give you an accurate 3D map of the space,” said Bryon O'Neil, President of the IAFSM (International Association of Forensic and Security Metrology) and a criminalist with the Clackamas County Sheriff’s Office’s Criminal Reconstruction and Forensic Technician Team in Oregon. “The laser scanners can also take color digital photos, which means you can combine the point clouds and photos to create truly accurate 3D renderings of incident scenes.”
A powerful combination
The purpose of combining police video with point clouds is to provide accurate 3D references for images that were shot in 2D. When the third dimension is added, it is possible to determine where people and objects were in physical space – and how their positions, directions and speeds changed as the incident under investigation progressed.
“Let’s say you have a car crash that happens in front of a convenience store’s video camera,” said O’Neil. “When officers go to document the scene, they create a point cloud (or clouds) using a laser scanner that is aligned with the location and height of the video camera that shot the accident footage. Back at the station, the video and the point cloud(s) can be combined to create an accurate 3D recreation of what happened as it happened.”
Laser scanners are also useful for capturing the finest details of crime scenes for later reference, including the recording of subtle evidence that could be overlooked during the initial investigation. They are also effective for trajectory analysis when investigators are trying to determine where bullets fired within a space originated from.
Then there’s the power of combining 3D video records with 3D printers: “It is possible to laser scan two cars that have been involved in a collision with each other, and then use the data to 3D print accurate scale models of the cars,” O’Neil said. “You can then take the two 3D car models and fit them together, to see how they actually interacted with each other.”
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Laser scanners are powerful tools that capture the finest details of crime scenes that traditional photographs and videos may leave out.
A potent tool in court
By itself, video evidence is a potent tool in court. Combine it with 3D point clouds and the video’s level of potency goes through the roof.
“You can use video overlaid on point clouds to create 3D incident scenes that the jury can walk through while wearing virtual reality goggles,” said O’Neil. “In this way, the prosecutor can walk the jury through the video evidence as if they were actually on the scene themselves.”
“They say a picture is worth a thousand words. Well, a 3D picture/video that you can move around in and look at different things is worth much more than that.”
This said Bryon O’Neil doesn’t recommend using point cloud-enhanced video in every case. “You only need to use this tool when it can answer questions in a case that can’t be reliably answered by other means,” he said.
Easy to use
As impressive and effective as laser scanners can be at crime scenes, the equipment itself is easy to use. An investigator simply erects the laser scanner where they want it and presses the scan button. The scanner then generates and records the point cloud.
The only variables to consider are the degree of detail in the point cloud measurement – the more points to be measured, the more time required – and the capabilities of the computer being used to process the point clouds. Given the multi-gigabit size of the point cloud files and the processing power required, high capacity/high functioning laptops are recommended.
This said, combining video and point clouds gives investigators a much-needed edge in solving crimes and collecting evidence that stands up in court. It is truly the next big step in modern police work.