Assessing Vehicle Profiling Accuracy of Handheld LiDAR Compared to Terrestrial Laser Scanning for Crash Scene Reconstruction

Technical Note: Validation of Recon-3D, iPhone LiDAR for bullet trajectory documentation

Bullet trajectory documentation is an important part of shooting reconstructions. Manual methods are quite common, using a protractor and angle gauge, but more advanced methods using laser scanners and photogrammetry are also used for documentation. Past studies have shown that using terrestrial laser scanners (TLS) can reduce the variability in trajectory documentation. In 2020, Apple Inc. released the iPad Pro & iPhone 12 Pro with a Light Detection and Ranging (LiDAR) sensor, which effectively placed a low-cost laser scanner in mobile devices. In May of 2022, an iOS application focused on forensics called Recon-3D was released. Recon-3D uses Apple's LiDAR sensor and fuses it with photogrammetry to produce 3D point cloud data in the e57 format. The purpose of this research was to compare the accuracy, repeatability, and quality of 3D data, using Recon-3D to the Faro Focus S350 laser scanner in bullet trajectory documentation. As a first test, twelve trajectory rods were installed on a wooden panel in a controlled, indoor environment, in order to optimize the best settings for scanning trajectory rods. Subsequently, a more practical exercise was run outdoors with a vehicle containing five trajectory rods. The mean horizontal angle error for the twelve trajectory rods installed on the wooden panel was 0.14° with a standard deviation of 0.30°. The mean error for the vertical angle was - 0.05° with a standard deviation of 0.25°. For the outdoor vehicle test, the mean error for the horizontal angle was 0.03° with a standard deviation of 0.28°. The mean error for the vertical angle was 0.22° with a standard deviation of 0.36°. These errors are similar to other studies which utilize the terrestrial laser scanner with mean errors well below 1°. Although further work is required to make Recon-3D a more robust application, preliminary results are promising and in line with previous studies where the terrestrial laser scanner has been used. Thus, Recon-3D appears suitable to document trajectory rods when used in shooting reconstructions.