Crime scene reconstruction normally starts ideas of what happened during the crime and then moves to an analysis of the evidence at the scene. It focuses on gathering as much data and evidence to form a valid hypothesis. The hypothesis can then be subjected to various tests to prove or disprove the overall interpretation of the reconstruction. Once the reconstruction is formalized a theory can be determined in support of the reconstruction. There are three types of crime scene reconstructions. They are specific incident reconstruction, specific event reconstruction, and specific physical evidence reconstruction.
Specific incident reconstruction involves reconstructing a crime scene where an accident or incident occurred. This will be needed during such incidences as traffic accidents or homicides. The purpose is to identify the types of evidence that can be associated with these incidents. Using specific event reconstruction, the sequence of events or timelines can be established. This form of reconstruction looks at how all of the pieces of the puzzle fit together. With specific event reconstruction the sequence of events can be determined. The final type of reconstruction is specific physical evidence reconstruction.
This involves evidence such as blood and bullets. Through reconstruction of blood splatter, it can be determined where the shooter was standing during a homicide. It will also help identify the location of the bullet if it is exits the body of the victim. Capturing the crime scene is an important part of the crime scene reconstruction process. Typical methods include sketching the crime scene using graph paper and a pencil or taking photographs from a digital camera. Both of these methods do provide a snapshot of the crime scene for preservation but, they do not capture the scene in its entirety.
A sketch will note measurements of physical evidence in their relation to the victim’s body or to such items as furniture and doorways. However, it is completely relying on the investigator to supply accurate measurements and identification of the physical evidence. Using 3D technology, the entire crime scene can be analyzed for accurate measurements at anytime. An advantage that 3D technology has over other methods of crime scene reconstruction is that it can preserve the crime scene in a moment in time. This is vital if the scene is in a populated area and needs to return to its natural state as soon as possible.
Think about a crash scene involving two vehicles on an expressway in Los Angeles during rush hour that resulted in a fatality. The time it takes to clear the scene is a very important variable when collecting the evidence. Under these conditions, there may be evidence that goes unnoticed by an investigator that is sketching the scene. Not to mention the time that it takes to do physical measurements. Using 3D technology can allow the investigator to collect the data and have confidence that nothing will missed. In order to capture a crime scene in its entirety multiple scans of the scene must be considered.
The collection of data comes from only the viewpoint of the investigator. Consider capturing the image of an automobile. If we were to stand at the front of the automobile we would not be of the automobile to in order to collect a complete image. One tool used to collect 3D images from a crime scene is a calibrated digital camera. It uses a technology called stereo photography. Standard photographs are only 2-dimensional representations of what you see. 3-dimensional photographs are taken from two perspectives. Because we have two eyes, we will need two perspectives on a scene.
By forcing each eye to see only one photograph, i. e. the left eye sees the left photograph and the right eye sees the right photograph, your brain will reconstruct the depth information from the two pictures and you will see a 3D image (3dphotography, 2010). The use of calibrated digital cameras allows the viewer to see the image as it would have been seen by the individual taking the pictures. When multiple photographs are combined, a reconstruction of the scene is created. Another tool that is used to collect 3D images from a crime scene is a 3D scanner or laser. A 3D scanner is known for high-accuracy and long range.
Most 3D scanners can collect data from 900 feet away. It can operate in bright sunlight or total darkness, indoors or out. The built-in digital camera allows the measured 3D data (known as a “point cloud”) to automatically be mapped creating a 3D rendering of the scene (3D Forensic Mapping, 2010). The 3D scanner quickly digitizes a scene using both panoramic photography and 3D laser scanning which is the process of making millions of highly accurate measurements in Just a few minutes. The result is an accurate 3D representation of he scene from which any measurement can be made, even long after the scene has been vacated.
Whether using a calibrated digital camera or a 3D scanner, it is time to create a 3D model of the data. The models are assembled in 3D animation software. This is when the data that was collected at the crime scene is put into the software. In the case of the using a calibrated digital camera, each pixel is assigned a coordinate. The coordinate is made up of XYZ; where X is an Easting coordinate, Y is a Northing coordinate, and Z is the elevation. The pixels or coordinates are then lotted on a 3 dimensional grid. If we consider a sketch that is typically performed at a crime scene, it is laid out on a piece of graph paper.
The investigator assigns a certain measurement between squares and then plots all the relative items of the crime scene at the respected distance. This would be considered a 2 dimensional drawing where only X & Y are plotted. In a 3D plot, it includes the Z value. This gives the 3 dimensional model its depth. So, the multiple photographs taken with the calibrated digital camera are overlaid, assigned a coordinate, and modeled into a 3D image. The 3D scanner is not much different from a calibrated digital camera, although it uses an infrared laser to collect data points instead of pixels.
The hardware then assigns coordinates to each data point and the software plots them. A 3D scanner can collect as many as 100,000 data points per second (Oberle, 2009). This creates a huge advantage over using digital camera which can only collect upwards of 8 million pixels with each photo (Patterson, 2010). It would only take a scanner Just over a minute to surpass a digital camera in resolution. At this point, the “scene” is ready for review. With the combination of photo-like images the software will allow the viewer to spin the images 360-degrees.
Looking at the computer screen, you will be able to enter the crime scene as if you were actually there. This can be copied and viewed by anyone with access to the 3D software. It becomes a crime scene, the more likely evidence will not go unnoticed. When reviewing the crime scene reconstruction, measurements can be achieved right from the office. Because each data point is assigned a coordinate, the distance formula can be used to calculate distances between two points. The software includes an algorithm that can quickly calculate the distance between any points selected by the user.
Therefore, determining specific physical evidence reconstruction such as blood splatter is made possible back at the office. In blood spatter evidence, the measurements will help calculate the mass of each drop from the size of its stain, and use this to calculate its maximum potential velocity. Air drag would tear apart a droplet if it travelled faster than this limit (Marks, 2010). With that information, and an angle of impact estimated from the shape of the stain, the software projects a ealistic trajectory backwards in time to locate the origin of the blood spatter.