Diagramming a crime or crash scene can be a painstaking process, always preserving the possibility of missing a critical item of evidence or getting a measurement wrong. By relying on a 3D scanner, the crime scene investigator can produce a much more detailed record in far less time.
What the 3D scanners are able to capture and generate help give jurors and those in court an impactful visual of the scene and helps provide the spatial reference and at times can help stop trials from even going to court.
Hardware and mechanics
A modern laser scanner largely automates the process of recording the scene and everything in it. The basic technique begins by placing the scanner, mounted on a sturdy tripod, more or less in the middle of the area to be recorded. The scanner projects a vertically-spinning laser beam onto the surrounding landscape, recording points as reflected off every object in view. The scanner rotates 360 degrees in the horizontal axis as it scans, moved by a motor mount slaved to the scanner itself.
As the laser light is reflected back to the scanner, the time required for the light to make the trip there and back is recorded and converted into a distance measurement, using the speed of light as a constant. This is similar to the way that traffic LIDAR is used to get the range to a target vehicle. This is called a “time of flight” calculation.
At the same time, the scanner records the angle of the emitted light, the angle of the reflection, and the distance calculation to triangulate and establish x, y and z coordinates for that data point. This creates a fixed point in space for that point, relative to the scanner.
Point clouds
By recording these data points and displaying them graphically, the laser scanner system creates a “point cloud” that recreates the shapes of all the objects in the scanner’s view. Most scanners can record everything in their line-of-sight view from around 0.6 meters to 1,100 ft. (or more) away. The effect is to see a three-dimensional view of the area from the perspective of the scanner location, with a resolution of thousands of data points per inch. The scanners accumulate this data very quickly, recording between 10,000-100,000 data points per second.
When there are obstructions between objects of interest and the scanner, those areas hidden to the scanner will not be recorded. The image is fleshed out by moving the scanner to another location where the area of interest is not obscured, and repeating the scan. Software typically furnished with the scanner will stitch together the images, using GPS coordinates recorded at the time of the scan to register the combined data point sets.
Most scanners used for crime and crash scene reconstruction incorporate conventional digital cameras, so that the point clouds are superimposed onto the photographs. This results in a 3D representation of the area, recorded digitally and transferable as with any other digital file.
Analyses and calculations
Analysts can zoom in on any feature, determine distances and angles between points, and make calculations of relevant factors, such as vehicle force vectors and bullet flight paths. The images can be rotated and viewed from any angle, providing perspectives that would be impossible with a conventional set of photographs.
Representations of crime and crash scenes recorded by laser scanners are much more detailed and accurate than a human diagrammer could produce. They are more persuasive to judges and juries who have been “educated” by TV and movie depictions of forensic analyses, and who have come to expect high-tech evidence in the courtroom.
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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.
