ENHANCING UAV-SfM 3D MODELS ACCURACY OF UNIQUE HERITAGE INFRASTRUCTURES. CASE OF ISABEL II DAM, ALMERIA, SPAIN

In recent years, resources and computer tools have devolved to allow the production of three-dimensional (3D) models that have become great sources of data for the reconstruction of elements of forgotten heritage. The main advances in the field of 3D modelling are related to terrestrial laser scanning (TLS) and unmanned aerial vehicle (UAV) photogrammetry. Thanks to the development of image processing techniques, such as structure from motion (SfM) and multiview stereopsis (MVS), combination of both techniques offers effective and cost-efficient photogrammetric process to obtain a high-resolution data set from photographs taken from different points of view. The goal of this paper is to study the effect of the number and distribution of GCP, and the inclusion of oblique photographs on the accuracy of the 3D points clouds yielded by UAV photogrammetry. To achieve this purpose, 18 UAV photogrammetric projects, combining number and distribution of GCPs and inclusion of oblique images to the nadir images, were developed, and the accuracy, precision, and distance respect to a reference cloud were estimated. Results concluded that when only nadir images are used, the best results were found with nine GCPs placed at each of the four corners of the object under study, four in the midpoint of each line that joins two consecutive corners, and another one in the midpoint. Maintaining this arrangement of GCP and adding 12% of oblique images with respect to the total of zenith images, with an angle of 20 degrees with respect to the nadir, improves the precision of the 3D point cloud generated by the photogrammetric project. If it is not possible to place the GCPs in this arrangement, the best option is to place them at the top of the dam.