Underground photogrammetry. Lessons learned from digitising environments in total darkness

Abstract. This paper presents lessons learned in the digital documentation of subterranean and lowlight heritage environments, with a case‐study on the four historic forts dating from the Italian Fascist period, located in Val Divedro (Verbano-Cusio-Ossola Province, Piedmont, Italy). We employed photogrammetric techniques - adapted to total darkness conditions - within disused alpine fortifications, transforming these once inaccessible environments, completely devoid of natural light, into high-resolution and photographic textured 3D models. Key challenges included lighting design (or absence thereof), stabilising camera rings in wet or uneven terrain, dealing with darkness, narrow corridors, and deep vertical shafts. and ensuring sufficient overlap and coverage for accurate photogrammetric reconstruction. The outcome demonstrated that, despite the inherent challenges of working in total darkness, the photogrammetric system achieved unexpectedly excellent results. This success is particularly significant considering the strong variability of illumination caused by the limited light output of a single portable flash source. Even under these conditions, the workflow produced decent image orientation, consistent surface reconstruction, and colour accurate texturing, proving that photogrammetry can remain effective and precise when properly adapted to extreme low light environments. While laser scanning, LiDAR or SLAM systems are generally considered more orthodox solutions for environments without light, the photogrammetric approach was chosen here for its capacity to produce true-to-life, colour-accurate visualisation of surfaces, a key element for assessing conservation state and enhancing public dissemination. From these 3D models, four digital virtual tours were also created, providing remote access to sites that remain physically inaccessible due to safety conditions and lack of viable paths.