A hybrid photogrammetry approach for archaeological sites: Block alignment issues in a case study (the Roman camp of A Cidadela)

Photogrammetry is a cost-effective and versatile technique used for the threedimensional (3D) registration of archaeological heritage sites. Managing datasets of heterogeneous images in terms of camera type, elevation platform, position or acquisition time can now be addressed by structure from motion (SfM) software via bundle adjustment in a single block based on collinearity principles. This development enables new possibilities with regard to data completeness assurance for 3D documentation, even for complex sites with occlusive elements and hidden areas. However, hybrid photogrammetry in large datasets often requires multiple photogrammetric blocks that must be processed individually and subsequently aligned to obtain a unified point cloud. In this paper, we discuss the steps required to homogenize the information and the methods used to perform block alignment in these cases. A case study of low-altitude aerial photogrammetry with several cameras and platforms is presented for the Roman camp of A Cidadela in NW Spain as a representative example of an archaeological site that is difficult to survey using a single photogrammetric platform. The relatively large expanse of the area and the fact that it is partially covered by a protective structure constitute an ideal framework for the fusion of multiplatform imagery. The most accurate digital surface model (DSM) was obtained via point-based method fusion, during which subsets are aligned based on automatically extracted tie points (TPs) between the dense point clouds; however, point-based method fusion is very time consuming. When hardware capabilities allow, conducting the process in a single block is preferable, which is a noticeably more accurate procedure than independent block fusion.