Application de deux méthodes de télédétection rapprochée à l'étude des escarpements rocheux instables : la photogrammétrie et la scannerisation laser
Today, studying an unstable cliff requires on-site fracture measurement by means of a clinometer compass. From accessibility and safety point of view, this method is far from perfect. Teledetection techniques provides an alternative for studying the cliffs morphology without such in-place measurements. Two methods seams beforehand particularly well suited. Photogrammetry on the one hand leads to the computation of the XYZ position of a point starting from its 2D position in two photographs taken from slightly different point of view. Numeric cameras have brought great improvements to this robust technique. Laser scanning on the other hand, consist in computing the position of a large number of points on the wall by measuring the time used by a laser impulse to reflect on the cliff. These two methods are subject two different limitations in terms of resolution, precision, on-site acquisition time and required computing power. Photogrammetry and laser scanning have been used on four sites to test their efficiency on real unstable cliffs. The Rocher du Midi zone (Vercors, 38, France), has been first studied with two laser scans acquired with a very grazing point of view from the top of the unstable wall. It has also been photographed with a large field silver camera from an helicopter. The Ravin de l'aiguille (Chartreuse, 38, France) has been studied through two laser scans realized ahead from the wall. The first one at grazing angle and the other one almost from a front point of view. These acquisitions were possible thanks to the crest line specific geometry. A photographic cover has also been realized from a numeric reflex on helicopter board. The Rocher de la bourgeoise (Vercors, 38, France), has been measured by means of a laser scanning on helicopter board and by mean of pictures taken at the same time from a numeric camera. The Gorges de Paganin (06, France) have been studied with three laser scan from the bottom of the cliff. To be able to use laser scan data from these four sites, the concept of solid image developed by the Turin Polotecnico has been reimplemented in the ImageJ software. Thus, add-ons have been easily developed for each measurement usefull for the ex-ploitation. Comparing the results obtained by this technique with those obtained with photogrammetry or direct on-site measurement have proved, with several restrictions, the efficiency of both teledection methods. Teledetection in general and the solid image concept in particular provide a noticeable added value compared to on-site measurements because they can be used as a support to complementary geophysic measurements or they can help the computation of a full geometric model preliminary tostability simulation. Despite these good results, improvements could still be obtained to exploit at bestthe laser scan and the photogrammetry techniques.