Articles | Volume 17, issue 7
Brief communication
18 Jul 2017
Brief communication |  | 18 Jul 2017

Brief communication: 3-D reconstruction of a collapsed rock pillar from Web-retrieved images and terrestrial lidar data – the 2005 event of the west face of the Drus (Mont Blanc massif)

Antoine Guerin, Antonio Abellán, Battista Matasci, Michel Jaboyedoff, Marc-Henri Derron, and Ludovic Ravanel

Abstract. In June 2005, a series of major rockfall events completely wiped out the Bonatti Pillar located in the legendary Drus west face (Mont Blanc massif, France). Terrestrial lidar scans of the west face were acquired after this event, but no pre-event point cloud is available. Thus, in order to reconstruct the volume and the shape of the collapsed blocks, a 3-D model has been built using photogrammetry (structure-from-motion (SfM) algorithms) based on 30 pictures collected on the Web. All these pictures were taken between September 2003 and May 2005. We then reconstructed the shape and volume of the fallen compartment by comparing the SfM model with terrestrial lidar data acquired in October 2005 and November 2011. The volume is calculated to 292 680 m3 (±5.6 %). This result is close to the value previously assessed by Ravanel and Deline (2008) for this same rock avalanche (265 000 ± 10 000 m3). The difference between these two estimations can be explained by the rounded shape of the volume determined by photogrammetry, which may lead to a volume overestimation. However it is not excluded that the volume calculated by Ravanel and Deline (2008) is slightly underestimated, the thickness of the blocks having been assessed manually from historical photographs.

Short summary
The coupling of terrestrial lidar scans acquired in 2011 and a photogrammetric model created from 30 old Web-retrieved images enabled reconstructing in 3-D the Drus west face before the 2005 rock avalanche and estimating the volume of this event. The volume is calculated as 292 680 m3 (±5.6 %). However, despite functioning well for the Drus (legendary peak), this method would have been difficult to implement on a less-well-known site with fewer images available to be collected and downloaded.
Final-revised paper