On the use of semi-georeferenced photogrammetric dense point clouds in the investigation of rock mass discontinuity properties

This study investigates the potential and limitations of using partial image orientation in Structure from Motion (SfM) photogrammetry to assess geometric properties of rock mass discontinuities investigated under various conditions. The photogrammetric point clouds were produced from images taken with a low-cost camera. An arbitrary (local) coordinate system was established by aligning a leveled 3D box with all axes oriented to the geographical North. Consequently, the need for terrestrial surveys to obtain ground control points was eliminated as the translation parameters required for photogrammetric image orientation could be disregarded in the proposed method. The investigations were conducted at various experimental sites to measure discontinuities in rock masses with diverse structural properties. The discontinuity properties such as orientation, persistence, weathering, aperture, filling, roughness, and waviness were measured by applying traditional scan-line surveys. Traditional orientation measurements and photogrammetric point cloud values were compared across different rock masses and discontinuity conditions. The results indicated that using a smartphone for image capture and a prismatic scale box for partial absolute orientation produced highly accurate point cloud data for characterizing rock mass discontinuities. Additionally, a new method, LCP + LSPF (Least Cost Path + Least Square Plane Fitting), was introduced for measuring partially closed-trace discontinuities. This method was found to be essential for sedimentary formations, primarily characterized by bedding planes. Moreover, it became evident that as the level of structural blocking increased and the interlocking of rock fragments decreased, the LCP + LSPF method was crucial for accurately representing rock masses, especially when considering Geological Strength Index (GSI) values.