Structure from Motion technique of proximal-sensing airborne data for 3D reconstruction of extraction sites

Digital photogrammetry based on Structure from Motion (SfM) algorithms nowadays represents a low-cost, valid and rapid technique for the tridimensional detection and mapping of large areas of particular geologic-geomorphologic, natural and environmental interest. Products derived from the photogrammetric processing, such as tridimensional point clouds, digital surface and terrain models (DSM and DTM), and high-resolution orthophotos, allow to study and monitor the territory and its productive activities. In this case study we used images captured from a small aircraft, agile and extremely inexpensive, for the 3D reconstruction of extraction sites and zones of geo-structural importance. The aircraft, called RadGyro (property of the CGT Group, University of Siena), is able to deal with complex flight plans and low altitudes in a way to make possible the detailed detection of desired features even in very complex areas. An example of this complex situation is represented by the test area here presented, located in the marble district of the Apuan Alps, the biggest and most exploited European extractive basin. The aircraft is equipped to acquire series of photos in an independent manner, using two digital cameras characterized by a full frame sensor and 35 mm fixed lens. Moreover, five GPS antennas and an inertial navigation system (INS) allow associating to each frame the position and the angular orientation of the camera. Flying at an altitude varying from 100 m to 400 m above ground level, the cameras can detect about 50 km2 and 200 km2, respectively, with a ground resolution of 3.5 cm and 10.0 cm. The exterior orientation of images can be improved, if necessary, acquiring some ground control points through traditional topographic survey. Info about the exterior orientation, associated to each frame, is therefore used to finalize the photogrammetric processing, using appropriate software. Structure from Motion techniques and algorithms allow to detect millions of "tie points" necessary for the accurate alignment of photos. The photogrammetric processing, then, is finalized by the creation of 3D point clouds, DSM, DTM and orthophotos. These products are used in this case study for the detailed analysis of the marble quarries morphology and geology and, through a multitemporal acquisition, for the assessment of the volumetric variations related to the extraction activities. Moreover, we want to point out, finally, how the RadGyro is able to provide data for environmental researches thanks to the following additional equipment: an hyperspectral camera in the range from 400 nm to 1000 nm, a thermal camera (7500 -1300 nm) and a gamma ray spectrometer equipped with detectors.