Abstract
A stability investigation based on Digital Outcrop Models (DOMs) acquired in emergency conditions by photogrammetric surveys based on Remote Piloted Aerial System (RPAS) was conducted on an unstable rock slope near Gallivaggio (Western Alps, Italy). The predicted mechanism of failure and volume of the unstable portion of the slope were successively verified on the DOMs acquired after the rockfall that effectively collapsed the May 29th, 2018. The comparison of the pre- and post-landslide 3D models shows that the estimated mode of failure was substantially correct. At the same time, the predicted volume of rock involved in the landslide was overestimated by around 10%. To verify if this error was due to the limited accuracy of the models georeferenced in emergency considering only the Global Navigation Satellite System/Inertial Measurement Unit (GNSS/IMU)-information of RPAS, several Ground Control Points (GCPs) were acquired after the failure. The analyses indicate that the instrumental error in the volume calculation due to the direct-georeferencing method is only of the 1.7%. In contrast, the significant part is due to the geological uncertainty in the reconstruction of the real irregular geometry of the invisible part of the failure surface. The results, however, confirm the satisfying relative accuracy of the direct-georeferenced DOMs, compatible with most geological and geoengineering purposes.
Highlights
In recent years, the application of remote sensing techniques for the development of Digital Outcrop Models (DOMs) and the analysis of unstable rock slopes has been continuously increasing (e.g. [1,2,3,4,5,6,7])
As shown by [9,10], due to the recent development in Remote Piloted Aerial System (RPAS), RGB camera performance and reduced costs, the RPAS-based Digital Photogrammetry (RPAS-Digital Photogrammetric (DP)) seems the best solution for high steep slope analysis, because it permits one to acquire data with similar resolution and cheaper instrumentation compared to the use of a laser scanner [11], among other things avoiding the effects of occlusion that affects the terrestrial remote sensing techniques [10]
A 3D Direct-Georeferenced model (DOMs) was developed during the emergency, before the rockfall, in a relatively short time, without measuring Ground Control Points (GCPs), but using only the positions of the photographs registered by the RPAS onboard Global Positioning System (GPS);
Summary
The application of remote sensing techniques for the development of Digital Outcrop Models (DOMs) and the analysis of unstable rock slopes has been continuously increasing (e.g. [1,2,3,4,5,6,7]). As shown by [9,10], due to the recent development in Remote Piloted Aerial System (RPAS), RGB camera performance and reduced costs, the RPAS-based Digital Photogrammetry (RPAS-DP) seems the best solution for high steep slope analysis, because it permits one to acquire data with similar resolution and cheaper instrumentation compared to the use of a laser scanner [11], among other things avoiding the effects of occlusion that affects the terrestrial remote sensing techniques [10]. In large, dangerous and scarcely accessible areas, high accuracy topographic surveys can take a long time (e.g., few days) and slow down the analysis and, in emergency condition, DOMs can be georeferenced using only the information recorded by the RPAS onboard Global Navigation Satellite System/Inertial Measurement Unit (GNSS/IMU) instrumentation without the GCPs acquisition survey This approach is referred to as direct-georeferencing (DG)
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