Photogrammetric Point-Cloud Replicability When Documenting Forensic Archaeological Scenes under Variable Lighting Conditions

Forensic archaeological scenes involving human skeletal remains in wooded environments can be challenging to document utilizing close-range photogrammetry (CRP) due to the complex nature of outdoor scenes. Previous research has demonstrated that changing lighting conditions can negatively affect three-dimensional (3D) model quality. The purpose of this research was therefore to test the impact of variable lighting on the replicability of 3D point clouds using CRP in a wooded environment. One scattered scene was created using a composite human skeleton and several clothing items. The scene was photographed three times during one day to capture changing lighting conditions: at 9:45 am (Model 1), at noon (Model 2), and at 2 pm (Model 3). Photographs were collected freehand from multiple view angles using a Sony α7 III camera with a fixed wide-angle lens, and the models were processed using Agisoft Metashape Professional. All three models achieved a total scale bar error of less than 1 mm and therefore met the accepted standards for crime scene mapping best practices. The dense point clouds were then analyzed using CloudCompare to assess point-cloud replicability between model pairs. The Multiscale Model to Model Cloud Comparison (M3C2) tool was used to calculate signed distances between point-cloud pairs. Also, histograms that display these point-to-point deviations were generated for each comparison, and a Gaussian distribution was fitted to each histogram. Deviations between point clouds were minimal, indicating that CRP-generated point clouds are replicable under changing lighting conditions, as well as other environmental variables, such as mild wind conditions and complex ground surfaces. Thus, it is recommended that forensic archaeologists incorporate CRP into their documentation protocol.