Fusion of multimodal three-dimensional data for comprehensive digital documentation of cultural heritage sites

Geometry, appearance and context are essential aspects to capture in the digital documentation of cultural heritage sites. Geometry must be accurate and should provide a level of precision necessary for quantified diagnostics. Visual appearance should capture the “as-is” state, while site specific context is important for correlation, interpretation and analysis. Light detection and ranging (LiDAR) has established itself as the premier laser scanning modality for the acquisition of trusted geometry, while photogrammetry techniques like structure from motion (SfM) are used to construct visually compelling models. A common challenge of these line-of-sight techniques is that the imaging equipment must be systematically moved throughout the target environment to assure that the data captures the entire target and allows for the removal of occlusions in the final model. By combining terrestrial and airborne imaging techniques using unmanned aerial vehicles (UAV), also frequently referred to as drones, it is possible to streamline the acquisition of the target data sets. This paper discusses the fusion of full resolution three-dimensional data streams generated from laser scanning, ground based photogrammetry and drone based photogrammetry. Maintaining full resolution of the data sets allows for diagnostic analysis of very subtle deformations and defects like erosion and cracks. In a presented case study in Mexico, terrestrial laser scanning serves as a geometric scaffold that the photogrammetry data is registered to in order to generate a holistic model of a one hectare site containing two historic structures. The laser scanning and photogrammetry data sets have sufficient overlap to enable fusion, and more importantly the individual sets can supplement each other, providing geometry, photorealism and context that the other set lacks.