3D Scanning of Hard-to-Reach Objects Using SfM-MVS Photogrammetry and a Low-Cost UAS

SfM-MVS (Structure from Motion -Multi View Stereo) photogrammetry is a cost-effective and versatile technique used, among other applications, for the three-dimensional (3D) modelling of archaeological heritage sites and artistic objects. Traditional photogrammetry primarily derives the calibration parameters of the camera and the exterior orientation variables from ground control points (GCPs), obtained by means of a GPS, and tie points (TPs). The SfM-MVS approach simultaneously computes both the relative projection geometry and a set of sparse 3D points. However, when these techniques must be used with difficult-to-reach objects, the process is very complex and implies the use of expensive pieces of equipment. For these reasons, this study develops a new procedure to obtain 3D scale models using of a low-cost Unmanned Aerial System (UAS). To achieve good dimensional accuracy, a scaling tool has been designed which can be carried by the aircraft and deployed on the top of object. By means of this tool it is possible to determine the object actual size, scale the 3D model and then verify the operability and quality of the measurements obtained. The new method has been validated by comparing point clouds and distances. The results showed an error of less than 1 mm in almost 90% of the point cloud and a mean relative error of 1.49%. The procedure is therefore simple, effective and allows the use of SfM-MVS photogrammetry techniques indoors or outdoors on objects, which due to their complex location, it is impossible to use scaling references and their reconstruction is only feasible with sophisticated and expensive technical means.