Combining nadir and oblique UAV imagery to reconstruct quarry topography: methodology and feasibility analysis

Abstract

The feasibility of unmanned-aerial-vehicle-based photogrammetry was assessed for the reconstruction of high-resolution topography and geomorphic features of quarries by nadir and off-nadir imagery. The test site was a quarry located in the rural area of Turi (Bari, southern Italy). Two processing scenarios were created. Nadir images were initially used, and images acquired with off-nadir angles were added. An accurate set of ground control points (GCPs) were surveyed for both georeferencing purposes and validation processes. In the reconstruction of the surfaces, an accuracy of a few centimeters was achieved in the final positioning of point clouds representing the main geometries of quarry environment. However, greatest differences were found along the edges or the lines characterized by sudden slope changes. To better understand such results, some characteristic quarry shapes depicted by both the scenarios were compared to those surveyed by a total station used as an independent benchmark technique. It allowed to define the benefits introduced by the joint use of nadir and oblique images in the delineation of quarry shapes, surface discontinuities and better descriptions of sub-vertical walls. Beside the evaluation of benefits introduced by use of oblique cameras, the effectiveness of the proposed methodology was also discussed with alternative technologies. Unmanned aerial platforms represent an effective solution, with the need for few accurate GCPs

An Integrated Procedure to Assess the Stability of Coastal Rocky Cliffs: From UAV Close-Range Photogrammetry to Geomechanical Finite Element Modeling

“…However, the geomechanical model is more sensitive to a close reconstruction of surface features and their orientations rather than the accuracy in the absolute positioning of the point cloud. In a previous paper, the present authors verified the ability to accurately map natural landforms with sub-vertical development by the combination of nadir and oblique images by adopting the same equipment and survey strategy [50].…”

The Use of Nadir and Oblique UAV Images for Building Knowledge

“…The choice of these parameters, medium quality in particular, was dictated by the results achieved on the check point and by the need for a compromise between a reasonable processing time and an accuracy level compatible with the 1:2000 scale of the survey [35].…”

“…This choice was dictated by the impossibility of placing GCPs at different heights, such as on the roofs of buildings, as suggested by scientific literature, for reasons of privacy, since the buildings are privately owned. The good distribution of GCPs is of high importance not only for the image orientation, but also for the prevention of block deformation effects that may result from the remaining systematic errors in the camera calibration [35,36]. The coordinates of the points were determined using a GNSS RTK survey in the ETRF2000 datum.…”

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