Unmanned aerial vehicle and structure-from-motion photogrammetry for three-dimensional documentation and digital rubbing of the Zuo River Valley rock paintings

Three-dimensional (3D) documenting and rubbing of rock art is aimed to produce descriptive and analytic graphic documentation with metric scale and geospatial information for archaeological analysis. Although the integrated surveying technologies of aerial photogrammetry, close-range photogrammetry, and laser scanning have been widely used to generate the 3D models of archaeological sites, the implementation of these technologies in complex surroundings with steep terrain, such as riverside cliffs, remains challenging. In this study, we present an unmanned aerial vehicle (UAV)-based structure-from-motion (SfM) photogrammetry approach to obtain the 3D geospatial data of rock paintings. A field data collection approach for high-resolution multiperspective images using UAV equipped with a high-resolution camera is implemented together with high accuracy ground control points (GCPs). An appropriate flowchart of multi-view stereo (MVS) photogrammetry processing is designed including multi-view dense matching, bundle adjustment (BA) and metric rectification for the orthoimages production of the rock paintings. A digital rubbing approach based on the orthoimages is proposed to obtain the geometry contents, including symbols and characters, of the rock paintings. The Huashan rock paintings, which are located on the vertical faces of the cliffs that line the course of Zuo River, is taken as a case study. Our proposed approach can obtain orthoimages of the Huashan rock paintings with 2 mm resolution. The reality-based 3D model can reach the absolute accuracy of 5 mm. Clear, exact, and blur-free metric rubbing images of the Huashan rock paintings are produced, which are useful for the research and preservation of Zhuang culture.