Accuracy assessment of low-cost Terrestrial and UAV-based photogrammetry for Geomatics applications in architectural and cultural heritage contexts

Abstract

In the last decades, the field of Geomatics applied to architectural and cultural heritage has benefited from some major advances. Upon their introduction, laser scanners have revolutionized the surveying world, gradually establishing as a basic tool, at first for terrestrial and later for airborne surveys. At about the same time, photogrammetry also experienced its own evolution, culminating with Structure from Motion (SfM) and MVS (Multi-View Stereo) algorithms. These algorithms generate dense 3D colour point clouds that however may not always be considered reliable. In fact, matching algorithms can be very sensitive to data collection, lighting and texture, and accuracy control is difficult. The high automation levels attainable also require caution because, while allowing for quicker modelling, control and perception of the steps to follow become looser. In terrestrial surveys, accessibility can still present challenging issues, where both Terrestrial Laser Scanning (TLS) and terrestrial photogrammetry are not viable options. Many situations do not allow acquisition of both images and TLS data, necessary to generate 3D models. A solution of this problem is a new technique for the acquisition of photogrammetric data, based on the use of Unmanned Aerial Vehicles (UAVs). Within the broader field of study of Terrestrial and UAV-based photogrammetry in architectural and cultural heritage contexts, this thesis focuses on four aspects in order to provide an operating methodology for surveys: 1-Influence of number and position of Ground Control Points (GCPs) and tie points in SfM and MVS techniques; 2-Best methods for survey assessment; 3-TLS/SfM-MVS integration; 4-Original applications in architecture surveys. Besides, introduction of UAV-based applications has been investigated in some cases. The thesis provides guidelines for low-cost terrestrial and UAV-based photogrammetry aimed at any figure involved in surveying in architectural and cultural heritage contexts. These guidelines include methodologies for accuracy checks and data integration as well as a workflow enabling survey optimization and devising original applications. Besides, integration of the research aspects has allowed to provide accuracy checks of the acquired data and integration of data from different sources, as well as accuracy controls of both each single-technique model and models obtained through technique integration.