What Public Safety Agencies Need to Know About Scan Data Before Investing in a Laser Scanner
When public safety agencies invest in laser scanning technology, it’s often with the intention of using the scan data in a court of law to offer opinions as to what took place at a scene. But not all laser scanners—and point clouds—are created equal. When comparing laser scanners from different manufacturers, it’s critical to look beyond the surface. The following are three important points to keep in mind when performing your due diligence.
1. Not all scan data can qualify as scientific evidence
In order for laser scan data to be admitted in court as scientific evidence, the laser scanner itself must meet the standards of a scientific instrument. Without proven accuracy within the system, the data—and the resulting analyses—can, quite frankly, be wrong.
The Daubert Standard requires a known error rate, so when defending your methodology on the witness stand, you’ll need to to testify as to the accuracy of your scanner. If the manufacturer you choose does not publish the scanner’s 3D positional accuracy, your data and exhibits, including bullet trajectory analysis, crash reconstruction, crush analysis, etc., can only be considered demonstrative or a visual aid.
2. Overall 3D accuracy is composed of several factors
More than a dozen different kinds of errors factor into a laser scanner’s overall 3D positional accuracy. 3D positional accuracy includes horizontal and vertical angle accuracy, spot size, range accuracy and more.
If a manufacturer provides only one specification, e.g., ranging error (which is measure of precision and should not be confused with accuracy), instead of the scanner’s overall 3D accuracy, the scanner does not meet the threshold of a scientific instrument.
Additionally, keep in mind that accuracy in ideal conditions is not the same as accuracy in adverse conditions like extreme cold and heat. A scanner must have proven accuracy throughout its entire operating temperature range.
3. All point clouds are not created equal
Let’s say that you scan the exterior of a building from beyond 50 feet. A highly accurate, long-range scanner will show a flat surface. Less accurate, short-range scanners may show undulations, or a ripple effect, on the surface. This phenomenon is apparent when the data is viewed in 3D as pictured below.
In some applications, you might have the luxury of averaging the 10,000 or so points that make up the building’s façade to remove the ripples and call it a best-fit analysis of a wall without showing the inaccuracy, or inefficiency, of your laser scanner.
But on the public safety side, you don’t have that luxury. Everything you scan is evidence, so every data point must be accurate. You cannot delete, filter or average the points to determine what the façade is because that façade could very well contain evidence critical to the investigation, e.g., a bullet impact. If you have to average out those 10,000 points to produce a flat surface on the wall, you cannot accurately determine what is truly there and what is not. (Related: 7 Key Questions to Ask Before Buying a Laser Scanner for Your Agency)
All models of Leica ScanStation laser scanners are scientific instruments. The RTC360 laser scanner also meets the standards of a scientific instrument with a published 3D positional accuracy. Importantly, Leica Geosystems’ laser scan data has been admitted as scientific evidence in U.S. courts.
Leica Geosystems publishes comprehensive specifications that include the accuracy of the range measurement, horizontal and vertical angles and the overall 3D position at both 50 m and 100 m. Leica guarantees this accuracy throughout the scanner’s temperature threshold (minus 4 F/20 C up to 122 F/50 C on a ScanStation, and minus 5 C to 40 C on an RTC360).
When comparing various manufacturers’ laser scanners, it’s critical to look beyond the surface. With these three important considerations in mind, your agency can investigate laser scanning technology with confidence, knowing what it takes for scan data and the resulting analyses to be accepted as scientific evidence in court.
About the Author: Dietrich Evans is a geospatial LiDAR specialist with more than 20 years of public- and private-sector experience. In the 1990s, he worked for the Department of Defense at the U.S. Army Yuma Proving Ground where he developed firing tables for heavy artillery in the field and became a DOD-certified geodesist with Top Secret clearance. Dietrich went on to become a pioneer in the exploration and development of laser scanning applications in a variety of fields, including historical preservation. In 2004, Dietrich launched a high-definition surveying services and consulting business, 3D Laser Imaging Inc. Over 10 years, he completed nearly 500 projects in a wide range of applications, including bomb-squad work, arson investigation, ballistic coefficients, and shooting reconstruction—all utilizing Leica Geosystems laser scanning technology. Today, as U.S. West account manager, Dietrich devotes two decades of hands-on experience, skill and knowledge to Leica Geosystems’ Public Safety Group customers.