First tests of Imaging rover techniques for the geomatic restitution of a sinkhole: the test site known as “Pozzo del Merro"

The sinkhole known as “Pozzo del Merro” possesses unusual, if not unique, geological and natural features, providing an important opportunity to study the geological and hydrological characteristics of the area, and at the same time to preserve an ecosystem inhabited by species that, as far as we know, are found nowhere else. The site has been the object of numerous studies carried out by scholars from a wide range of disciplines (Floris et al. 2014), but a true morphological survey has not been performed until now due to logistical difficulties that include the lack of adequate measuring positions and the difficulty of establishing a clear view between points. The availability of an imaging rover (Trimble V10) an innovative photogrammetric multifocal instrument integrated with a GPS/GNSS double frequency receiver suggest to perform the survey to experiment this completely new technology, comparing and assessing with more consolidated methodologies. The instrument tested consists of an integrated camera system that captures a 60 MP 360-degree digital panoramas; each acquisition is performed with a total of 12 calibrated photogrammetric cameras – seven panorama and five downward-looking. The GPS accuracy (H 8 mm + 1 ppm, V 15 mm + 1 ppm; - Trimble (2016)). With this configuration, after a single photogrammetric acquisition, the position of the instrument is given, but not its azimuth, so more stations are needed. The photogrammetric automatic algorithm has extracted almost completely the parts of sinkhole without foliage cover that represents a very small area with respect to the whole sinkhole, reporting accuracy mostly lower 5-6 cm. compatible with declared accuracy for similar distances; the survey did not identify secondary emerged cavities or tunnels that had been hypothesized on the basis of those found above water. Nonetheless, due to the specific morphology of the site, the presence of other undiscovered morphological characteristics cannot be ruled out. The Imaging rover instrumentation showed that the accuracy is similar to laser scanner, the imaging rover has a wider vertical field of view, it is logistically much more versatile and it can be of strategic importance in such sites with complex morphologies. On the other hand Imaging rover has no capabilities of penetrating foliage that is a drawback, particularly in some periods of the year.