SfM Photogrammetric Techniques Applied in the Building Archaeology Works of the Old Cloister of the Monastery of San Francisco from the 16th Century (Cazalla de la Sierra, Seville)

The cloister from the old monastery of San Francisco (Cazalla de la Sierra, Seville) has 12 been suffering a series of remodelling and transformations from its original construction, in the 16th 13 century, to the current day. Thus, a study of building archaeology needed to be accomplished by 14 using photogrammetric techniques by SfM (Structure from Motion) and laser scanning or TLS (Ter- 15 restrial Laser Scanning) that ensure a geometric exactitude and high resolution of the facings sur- 16 veying. For that, over 500 images were taken for the 4 existing facings (about 78 lineal metres) from 17 which a photogrammetric model was obtained of over 50 million polygons; as well as a cloud of 18 over 40 million points from the laser scanning. It can be concluded that by using the techniques of 19 SfM, the task of documenting, analysing and studying the facings of the historical buildings in order 20 to establish its evolutional process, gains, not only in precision and exactitude, but also opens the 21 possibility to go further by obtaining products that are capable in the labour of conservation, resto- 22 ration and protection of the historical heritage, as well as the generation of 3D virtualizations, 23 planned for the diffusion

Photogrammetry (SfM) vs Terrestrial Laser Scanning (TLS) for archaeological excavations: mosaic of Cantillana (Spain) as a case study

The discovery of a Roman mosaic from the 2nd century AD in Cantillana (Seville) generated interest and the need for exhaustive documentation, so that it could be recreated with real measurements in a 3D model, not only to obtain an exact replica, but with the intention of analyzing and studying the behavior of two main geomatics techniques. Thus, the objective of this study was the comparative analysis of both techniques: near object photogrammetry by SfM and terrestrial laser scanner or TLS. The aim of this comparison was to assess the use of both techniques in archaeological excavations. Special attention was paid to the accuracy and precision of measurements and models, especially in altimetry. Mosaics are frequently relocated from their original location to be exhibited in museums or for restoration work, after which they are returned to their original place. Therefore, the altimetric situation is of special relevance. To analyze the accuracy and errors of each technique, a total station was used to establish the real values of the ground control points (GCP) on which the comparisons of both methods were to be made. It can be concluded that the SfM technique was the most accurate and least limiting for use in semi-buried archaeological excavations. This manuscript opens new perspectives for the use of SfM-based photogrammetry in archaeological excavations

Line-based Robust SfM with Little Image Overlap

International audienceUsual Structure-from-Motion (SfM) techniques require at least trifocal overlaps to calibrate cameras and reconstruct a scene. We consider here scenarios of reduced image sets with little overlap, possibly as low as two images at most seeing the same part of the scene. We propose a new method, based on line coplanarity hypotheses, for estimating the relative scale of two independent bifocal calibrations sharing a camera, without the need of any trifocal information or Manhattan-world assumption. We use it to compute SfM in a chain of up-to-scale relative motions. For accuracy, we however also make use of trifocal information for line and/or point features, when present, relaxing usual trifocal constraints. For robustness to wrong assumptions and mismatches, we embed all constraints in a parameter-less RANSAC-like approach. Experiments show that we can calibrate datasets that previously could not, and that this wider applicability does not come at the cost of inaccuracy