ABSTRACT:Structure-from-Motion (SfM) photogrammetry is a flexible and powerful tool to provide 3D point clouds describing the surface of objects. Due to the easy transportability and low-cost of necessary equipment with respect to laser scanning techniques, SfM photogrammetry has great potential to be applied in harsh high-mountain environment. Here point clouds and derived by-products (DEM's, orthoimages, Virtual-Reality models) are needed to document surface morphology and to investigate dynamic processes such as landslides, avalanches, river and soil erosion, glacier retreat. On the other hand, from both the literature and the direct experience of the authors, there are some technical issues that still deserve thorough investigations. The paper would like to address some open problems and suggest solutions, in particular on regards of the photogrammetric network design, the strategy for georeferencing the final products, and for their comparison within time. The discussion is documented with some examples, mainly from surveying campaigns at the Forni Glacier in Italian Alps.
<p><strong>Abstract.</strong> This paper describes the use of some tool to help training of photogrammetry for applications in the field of landslide and slope stability assessment and monitoring. These tools have been used in classes of the MSc on Civil Eng. for Risk Mitigation at Politecnico di Milano university, Lecco (Italy). The first tools are hardware facilities. The first one consists of a ‘Landslide Simulator,’ where shallow landslides may be reproduced at small scale. Simulations are also used here for active-learning purpose. In particular, here the use of digital images to obtain multi-temporal information is presented. The second tool is a ‘Rock face 3D Modelling Simulator.’ This is used by students to learn how a photogrammetric block should be designed in order to reconstruct rock slopes using Structure-from-Motion photogrammetry. The last to tools are software packages (CloudCompare and LIME) devoted to point cloud analysis (including change detection/ deformation analysis) and advanced visualization, respectively. The combination of these tools together with datasets from either lab and the real field, has been successfully tested to provide efficient training to students in an active-learning fashion.</p>
La ville de Nîmes est un territoire particulièrement exposé au risque inondation, et notamment aux crues torrentielles. Si aucun cours d'eau majeur n'est présent dans sa zone urbaine dense, ce sont les cadereaux, fossés de garrigues, qui sont en charge de la collecte du ruissellement et traversent la ville. Les évènements méditerranéens, des bassins versants de taille réduite (au plus 30 km2) et de nature karstique ainsi qu'une configuration en piémont sont autant de facteurs à l'origine de ruissellement potentiellement conséquent avec débordements majeurs sur la ville, de type crues-éclairs. Suite aux inondations du 3 octobre 1988, la ville de Nîmes a lancé une politique ambitieuse de prévention du risque inondation, incluant un aménagement important des cadereaux. Dans cette démarche, elle se dote en 2004 d'un système d'alerte et d'aide à la gestion de crise permettant de prévoir le risque inondation : ESPADA (Evaluation et suivi des précipitations en agglomération pour devancer l'alerte). L'objectif est de prévoir les débits des cadereaux à échéance maximale 1 h 30, en exploitant un modèle hydrologique intégrant une représentation fine du fonctionnement karstique. Dans le cadre de son Programme d'actions de prévention des inondations (PAPI), la ville de Nîmes a débuté en 2014 la modernisation du système ESPADA avec pour objectif d'améliorer la qualité des données d'entrée du système temps réel (pas de temps 5 min). Un réseau radio de collecte fiabilisée des capteurs hydrométéorologiques de la Ville a été mis en place, assurant également la collecte de postes du SPC-GD (Service de prévision des crues du Grand Delta). Parallèlement, un outil radar d'observation et prévision de pluie au pixel 500 m, échelle adaptée aux contraintes hydrologiques locales, a été développé, incluant une calibration de la donnée par exploitation des données des postes pluviographiques disponibles. Dans le cadre de la seconde étape de cette modernisation, le modèle de prévision du système est retravaillé afin de fonctionner en continu au pas de temps 5 min. La propagation est gérée via un modèle hydraulique 1D. Par ailleurs, les données limnigraphiques sont utilisées en temps réel afin de réajuster les valeurs de débits et les niveaux des bassins de rétention. De même, une interface en ligne est en cours de développement, permettant une utilisation efficace des outils par l'ensemble des acteurs de la gestion de crise. À terme, il est prévu de mettre à disposition de la population une partie des informations associées au système ESPADA via une interface de type Vigicrues.
ABSTRACT:The application of image processing and photogrammetric techniques to dynamic reconstruction of landslide simulations in a scaleddown facility is described. Simulations are also used here for active-learning purpose: students are helped understand how physical processes happen and which kinds of observations may be obtained from a sensor network. In particular, the use of digital images to obtain multi-temporal information is presented. On one side, using a multi-view sensor set up based on four synchronized GoPro 4 Black ® cameras, a 4D (3D spatial position and time) reconstruction of the dynamic scene is obtained through the composition of several 3D models obtained from dense image matching. The final textured 4D model allows one to revisit in dynamic and interactive mode a completed experiment at any time. On the other side, a digital image correlation (DIC) technique has been used to track surface point displacements from the image sequence obtained from the camera in front of the simulation facility. While the 4D model may provide a qualitative description and documentation of the experiment running, DIC analysis output quantitative information such as local point displacements and velocities, to be related to physical processes and to other observations. All the hardware and software equipment adopted for the photogrammetric reconstruction has been based on low-cost and open-source solutions.
<p><strong>Abstract.</strong> The application of Structure-from-Motion photogrammetry with ground-based and UAV-based camera stations can be effectively exploited for modeling the topographic surface of Alpine glaciers. Multi-temporal repeated surveys may lead to geometric models that may be applied to analyze the glacier retreat under global warming conditions. Here the case study of Forni Glacier in the Italian Alps is presented. Thanks to the integration of point clouds obtained from the independent photogrammetric processing of ground-based and UAV blocks of images (captured on 2016), a complete 3D reconstruction also including vertical and sub-vertical surfaces has been achieved. This 3D model, compared to a second model obtained from a ground-based photogrammetric survey on September 2017, has been exploited to understand the precursory signal of a big collapse that might have involved tourists and hikers visiting the glacier ice tongue during summer. In addition to some technical aspects related to the acquisition and processing of photogrammetric data of glaciers, this paper highlights how Structure-from-Motion photogrammetry may help evaluate the risk of collapse in Alpine glaciers.</p>
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