Techniques to Evaluate Terrestrial Laser Scanner Errors
Bala Muralikrishnan, Prem K. Rachakonda
Forensic photography has been one of the most important tool for investigators over many decades. They preserve the history of the scene and provides a way for investigators to revisit the scene. However, it is difficult to ascertain any objective measurements with high confidence from such photographs. Terrestrial Laser Scanners (TLSs) offer a way for investigators to revisit the scene and enable them to measure many features such as a trajectory of a bullet, track width of a vehicle etc. Because they also capture intensity data, they offer a way to reconstruct the photographs in any desired orientation from the measured data. TLSs do suffer from some issues that can cloud the veracity of the measurements. Their construction and data analysis procedures could introduce errors that have to be characterized. To standardize the measurements from TLSs, the Dimensional Metrology Group (DMG) at the National Institute of Standards and Technology (NIST) has been working with TLS manufacturers to develop a new documentary standard for these class of instruments. The existing ASTM E2938- 15 standard for 3D imaging systems only addresses the performance of these instruments in the ranging direction, however, most forensic scans are used for measurements in many directions. The new proposed standard addresses the issues with the performance evaluation of these instruments for any spatial distance measurement. NIST is a major contributor towards this effort and possesses unique expertise for this task due to our prior experience with laser trackers that could be applied for evaluating TLSs. We will discuss the errors and accuracy of TLSs and other efforts to characterize the performance of TLS systems to ensure that they meet the needs of the forensics community.