By Mike Cunningham, May 22, 2015
When scanning a crime or crash scene, investigators need to not only maintain the scene in as pristine a condition as possible but also work efficiently so as not to cause serious public inconvenience. These two considerations—evidence preservation and time-savings—are what make scanning without targets so attractive in public safety applications.
Targetless scanning has always been possible with Leica Cyclone 3D point cloud processing software, but increased speed and efficiency at the scene required a tedious and time-consuming cloud-to-cloud registration routine back at the office.
That is no longer the case. With the launch of Leica Cyclone 9.0 in fall 2014, targetless registration is easy. In fact, the new intuitive automatic alignment and visual registration features of Cyclone 9.0 (and higher) have made targetless scanning the preferred method for public safety users, and Leica’s Public Safety Group certified training curriculum has been updated to reflect this efficient method.
By following these five targetless scanning tips, you can protect the integrity of the scene while maximizing efficiency in the field and the office:
No. 1: Prepare a scan plan that includes easily recognizable features.
When planning a project, survey the scene to determine the best locations to set up the scanner. As always, you want to choose scanner positions that will provide good fields of view of all sides of the evidence. Additionally, when scanning without the use of targets, you will need to identify and include easily recognizable features, e.g., distinct land characteristics or man-made structures, within the scene that will aid the software’s visual alignment process. (Note: Very flat fields or heavily treed areas may not be good candidates for targetless scanning if they lack recognizable features.)
No. 2: Ensure that each pair of scans has sufficient overlap.
Cyclone 9’s new automatic alignment and visual alignment registration capabilities are optimized when pairs of scans share a good amount of data. When making your scan plan, make sure each scan station has a 20 percent or greater overlap with the previous station. Since Leica ScanStations feature excellent long-range capabilities, this typically can be accomplished easily in the field, but it is a factor that should be considered when walking through the scene and developing your scan plan.
No. 3: Utilize the benefits of the dual-axis compensator.
Leica ScanStations have a built-in real-time dual-axis compensator. This important component is sometimes referred to as a bubble-level, but in a Leica scanner, it is much more than a simple level.
While leveling the scanner at each set-up position is a task users sometimes tell me they loathe, the dual-axis compensator is a critically important part of a laser scanner. If the dual-axis compensator is not properly adjusted at any point during the scanning process, it could lead to problems during scan registration, and most importantly, it could ultimately affect the quality of the measurements produced.
Leica’s compensator constantly monitors in real-time both axes of the vertical axis tilt to ensure the scene is recorded in an accurate manner. If level issues arise at any point in time during scanning, e.g., tilt caused by settling of the ground, etc., the user is immediately alerted to a potential problem and can quickly take corrective action.
Utilizing the real-time dual-axis compensator will aid the targetless scanning process and be of great benefit when offering the measurements produced as scientifically sound evidence in court.
No. 4: Adjust software preferences.
When Cyclone 9 is initially installed on your computer, the program’s default settings take effect. However, you have the option to adjust these preference settings to suit your own needs and match the capabilities of your computer’s performance.
When using a targetless scanning workflow, customization of the default registration settings can aid the software’s auto alignment and visual registration processes. Simply select [Edit] on the toolbar; choose [Edit Preferences] from the list of options; and click the [Registration] tab to adjust the following settings:
- Max Search Distance is how far Cyclone looks to find overlapping data. This number may be increased.
- Subsampling Percentage is the amount of data used in the optimization operation. The default setting of 3 percent is fine when data has plenty of overlap or when using a slow computer. However, increasing this number to 30 percent is useful for targetless registrations.
- Max Iterations is the number of times the optimization algorithm repeats. By default, this is set to 100. Increasing this value to 300 has been shown to improve the root mean square.
No. 5: Include a calibrated twin-target pole and a registration diagnostics report for validation.
Laser scanner users in the public safety realm have a unique need for validation because the data collected by the scanner and processed by the software must be proven capable of producing scientifically valid measurements to aid conclusions.
Since the conclusions will be presented in court during expert-witness testimony, a foundation must be established for the admissibility of such evidence. Part of this quality-assurance process is validating the equipment and measurements to a traceable standard such as Leica Geosystems’ twin-target pole. This laser scanning artifact is available calibrated by the National Institute of Standards and Technology (NIST) for this exact purpose.
Although targets are obviously not needed for registration in a targetless scanning workflow, the use of a twin-target pole is still recommended to provide a method for validation. You will not acquire the targets in the field. You’ll simply set the pole within the scanner’s field-of-view and ensure it is captured with good resolution. By doing so, the NIST-traceable twin-target pole will help you meet forensic accreditation standards and establish that the measurements are suitable as scientific evidence.
To further enhance the validity of the data, Cyclone users can easily produce a registration diagnostics report and save it to the case file. The report, which is available in the [Registration] menu, lists important information, including absolute mean error and whether the dual-axis compensator continuously maintained the scanner in a level position during the scene-scanning process.
For more information about high-definition 3D laser scanning solutions for public safety applications, please contact us.
To learn how targetless scanning helped investigators from the Wichita Police Department Crime Laboratory document the challenging scene of a tragic multifatality airplane crash, follow this link.
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About the author: After a 26 year career, Mike Cunningham retired from the New York City Police Department in 2012 as a Detective 1st Grade and the senior ranking Investigator in the Crime Scene Unit. In addition to his many years of CSI experience, he was a forensics instructor for the NYPD and is a Certified Instructor for Department of Homeland Security course “Advanced Forensics for Hazardous Environments” and “Integrated Response to WMD Incidents.” As a contractor for the U.S. State Department, he served as an International Police Instructor for “Forensic Examination of Terrorist Crime Scenes” delivered to U.S. anti-terrorism partner nations. Mike served his country with distinction and professionalism for ten months at Ground Zero in the aftermath of September 11th. He is an IAI Certified Crime Scene Investigator and a New York State Certified Police Instructor.