Innovative Reality Capture Solutions Provide Comprehensive Scene Mapping in Washington Homicide Investigation
When the Washington State Patrol’s (WSP) Crime Laboratory had to map the scene of a stray-bullet shooting that killed 23-year-old Alyssa Christine Smith on June 16, 2013, it was one of the largest and most challenging crime scene reconstructions they had ever encountered. For help in solving this complex forensic mapping challenge, the WSP turned to the Leica Geosystems Public Safety Group (PSG). The team selected the following scene mapping tools to ensure accurate scene mapping.
Intelligent Unmanned Aircraft System: The Leica Aibot
The Leica Aibot is an autonomously flying hexacopter specifically designed for demanding tasks in surveying, industrial inspection, agriculture and forensic mapping. Equipped with a high level of artificial intelligence, this UAV reaches virtually any target and can independently capture high resolution images and videos. With the data produced by the Aibot, the WSP was able to generate orthophotos, 3D models, and high-density point clouds with great accuracy.
Proven 3D Laser Scanning: The Leica ScanStation
Although the Aibot UAS was ideal for mapping the majority of the crime scene, mapping the wooded backyard of the Smith property where the Aibot could not “see” the ground from the air required a terrestrial 3D laser scanner. Leica ScanStations are survey-grade laser scanners, the kind recommended by the National Institute of Justice for crime scene and crash scene reconstruction. The ScanStation is used by surveyors and engineers worldwide for critical measurements. The system has an on-board traversing routine which allows users to set up over carefully selected points they establish in the field. Once the height of the instrument is measured they can then backsight another point, and foresight to a third point, thus creating a network of survey control.
The scan data collected from any point in the network is registered on-board the ScanStation in the field and can be used immediately upon download in the office. This traverse capability proved critical to the overall mission plan as a strategy and work plan was developed.
Currently the WSP CSRT deploys three ScanStations and has recently added to its fleet several Leica BLK360 imaging scanners for the collection of demonstrative (as opposed to scientific) evidence. Leica ScanStations are capable of measuring to a range of 120 meters up to over one kilometer, depending upon the model. For this crime scene, a Leica ScanStation P20 was used for data collection along the riverbank where the shooter had fired and the backyard of the Smith home where Alyssa was fatally wounded.
Efficient and Effective Ground Control: The Leica GS14 GNSS Smart Antenna
One of the witnesses to the shooting identified Zylstra’s approximate firing position along the riverbank. Eight hundred yards away, the Smith family had planted a small rose garden in their backyard to memorialize the spot where Alyssa was struck down. These two positions defined the alignment for the UAV to fly for the mapping mission. While the Aibot’s software can create point clouds (in post-production using photogrammetry techniques) without ground photo control targets, what was needed here was a way to merge the point cloud developed from the air with the point clouds created by the ScanStation on the ground. Part of the solution came in the form of four-inch-square, white bathroom tiles that were placed 50 feet on either side of, and along, the alignment, and also atop the 60-penny nails driven into the ground to establish the survey control networks in the Smith backyard and at the riverbank.
The other part of the solution was the Leica GS14 GNSS smart antenna. Leica’s GS14 is a GNSS powered real-time kinematic (RTK) antenna used as a lightweight and easy to use “rover” for high-accuracy surveying and mapping. When used with an RTK base station, the GS14 delivers 3D accuracy of 1-2 centimeters, making it the perfect tool for one-person surveying of the photo control targets and key features that could be seen in the photos taken from the air. The GS14 requires real-time corrections to deliver centimeter level accuracy, and these were provided by SmartNet.
A newer GNSS smart antenna introduced after this investigation, the GS18 T, is now the standard for public safety due to its high accuracy, fast positioning, ease of use, immunity to magnetic disturbances, and calibration free “tilt” operation.
Reliable and Repeatable RTK GNSS Corrections: The SmartNet Network
HxGN SmartNet is a network of permanent GPS and GNSS receivers whose combined data are used to generate RTK corrections for a rover such as the Leica GS14 or any other brand. SmartNet provides subscription users with robust, traceable and repeatable network RTK corrections at centimeter-level accuracies tied to a common datum such as was needed for this project. With over 1,300 stations covering almost a billion acres in 44 states and 8 provinces, SmartNet stations provide coverage for most of the state of Washington. A station happened to be located just a few miles south of the crime scene near Ferndale.
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