Using Drones and 3D Modeling to Survey Tibetan Architectural Heritage: A Case Study with the Multi-Door Stupa

Tibet has an average altitude of 4900 m, a complex terrain, and unique climatic conditions. The technologies used to survey this country's local architectural heritage must be portable, efficient, and versatile. Low-cost Unmanned Aerial Vehicles (UAVs) and Structure from Motion (SfM) algorithms can help satisfy these demands. Recent studies employing nadir images from low-cost UAVs and SfM algorithms have reported positive metric results (with centimeter-level accuracy) when modeling 2D objects (e.g., land, roofs, and facades). In Tibet, however, forming a complete 3D model of architectural heritage is highly preferable and doing so requires a camera network that can create nadir and oblique images with various baselines. This study compared the accuracies of surveying a Tibetan stupa using the UAV-SfM method compared with Ground Control Points (GCP) and Terrestrial Laser Scanning (TLS). The results indicated that the UAV-derived model is accurate enough for most surveying purposes (RMSE = 2.05 cm; 1/2000 of the stupa's dimension). The accuracy and completeness of the 3D model allowed Historic Building Information Modeling (HBIM) and structural deformation analysis to also be undertaken. In addition, the stupa was integrated with geographic data (terrain and infrastructure) for visualization, management, and evaluation purposes at a larger scale. Considering the low cost, portability, and completeness offered by UAV and SfM, this tool offers promise for surveying Tibet's architectural heritage.