Marco Dubbini

Evaluating Multispectral Images and Vegetation Indices for Precision Farming Applications from UAV Images

Unmanned Aerial Vehicles (UAV)-based remote sensing offers great possibilities to acquire in a fast and easy way field data for precision agriculture applications. This field of study is rapidly increasing due to the benefits and advantages for farm resources management, particularly for studying crop health. This paper reports some experiences related to the analysis of cultivations (vineyards and tomatoes) with Tetracam multispectral data. The Tetracam camera was mounted on a multi-rotor hexacopter. The multispectral data were processed with a photogrammetric pipeline to create triband orthoimages of the surveyed sites. Those orthoimages were employed to extract some Vegetation Indices (VI) such as the Normalized Difference Vegetation Index (NDVI), the Green Normalized Difference Vegetation Index (GNDVI), and the Soil Adjusted Vegetation Index (SAVI), examining the vegetation vigor for each crop. The paper demonstrates the great potential of high-resolution UAV data and photogrammetric techniques applied in the agriculture framework to collect multispectral images and evaluate different VI, suggesting that these instruments represent a fast, reliable, and cost-effective resource in crop assessment for precision farming applications.

Combining nadir and oblique UAV imagery to reconstruct quarry topography: methodology and feasibility analysis

ABSTRACT The feasibility of unmanned-aerial-vehicle-based photogrammetry was assessed for the reconstruction of high-resolution topography and geomorphic features of quarries by nadir and off-nadir imagery. The test site was a quarry located in the rural area of Turi (Bari, southern Italy). Two processing scenarios were created. Nadir images were initially used, and images acquired with off-nadir angles were added. An accurate set of ground control points (GCPs) were surveyed for both georeferencing purposes and validation processes. In the reconstruction of the surfaces, an accuracy of a few centimeters was achieved in the final positioning of point clouds representing the main geometries of quarry environment. However, greatest differences were found along the edges or the lines characterized by sudden slope changes. To better understand such results, some characteristic quarry shapes depicted by both the scenarios were compared to those surveyed by a total station used as an independent benchmark technique. It allowed to define the benefits introduced by the joint use of nadir and oblique images in the delineation of quarry shapes, surface discontinuities and better descriptions of sub-vertical walls. Beside the evaluation of benefits introduced by use of oblique cameras, the effectiveness of the proposed methodology was also discussed with alternative technologies. Unmanned aerial platforms represent an effective solution, with the need for few accurate GCPs.

Recent approaches in geodesy and geomatics for structures monitoring

AbstractMonitoring the time history of structures and infrastructures has always been an important area of application of geodetic and geomatic methods. Here we shall concentrate on the item of monitoring ancient buildings because, beyond its intrinsic interest, it providesn a good illustration of some of the most recent monitoring techniques. Identifying the overhang, progressive changes of inclination, differential movements of the structure and detailing the study of structural elements are just some examples of the many fundamental and necessary information for structural engineers. Those data are required to study and analyze the behavior of a structure with the purpose to assess the stability. Looking at the several methods offered by Geomatics, laser scanning appears to be the best technology to provide an effective 3D solution to those requirements. Surveying by means of a terrestrial laser scanner, allows to detect a huge number of 3D information with high accuracy in a relatively short time and high accuracy. Just analyzing the point clouds, interesting information along with useful products can be obtained in order to draw some considerations about the investigated structure. This research aims at suggesting a new philosophy for using 3D models in a diagnostic perspective in order to study structures along with their actual dimensions, their stability and so on. Traditionally, indeed, laser scanning is chosen for artistic and architectural studies and the resulting three-dimensional model represents what often is of concern. Furthermore, the use of more classic techniques, such as total station and digital leveling, and LST is fundamental as an integrated approach for the monitoring of ancient buildings. The integration of different techniques allow a redundancy of observation and the possibility to verify the results obtained form independent techniques as is shown throughout some experimental applications.

An Integrated Procedure to Assess the Stability of Coastal Rocky Cliffs: From UAV Close-Range Photogrammetry to Geomechanical Finite Element Modeling

The present paper explores the combination of unmanned aerial vehicle (UAV) photogrammetry and three-dimensional geomechanical modeling in the investigation of instability processes of long sectors of coastal rocky cliffs. The need of a reliable and detailed reconstruction of the geometry of the cliff surfaces, beside the geomechanical characterization of the rock materials, could represent a very challenging requirement for sub-vertical coastal cliffs overlooking the sea. Very often, no information could be acquired by alternative surveying methodologies, due to the absence of vantage points, and the fieldwork could pose a risk for personnel. The case study is represented by a 600 m long sea cliff located at Sant’Andrea (Melendugno, Apulia, Italy). The cliff is characterized by a very complex geometrical setting, with a suggestive alternation of 10 to 20 m high vertical walls, with frequent caves, arches and rock-stacks. Initially, the rocky cliff surface was reconstructed at very fine spatial resolution from the combination of nadir and oblique images acquired by unmanned aerial vehicles. Successively, a limited area has been selected for further investigation. In particular, data refinement/decimation procedure has been assessed to find a convenient three-dimensional model to be used in the finite element geomechanical modeling without loss of information on the surface complexity. Finally, to test integrated procedure, the potential modes of failure of such sector of the investigated cliff were achieved. Results indicate that the most likely failure mechanism along the sea cliff examined is represented by the possible propagation of shear fractures or tensile failures along concave cliff portions or over-hanging due to previous collapses or erosion of the underlying rock volumes. The proposed approach to the investigation of coastal cliff stability has proven to be a possible and flexible tool in the rapid and highly-automated investigation of hazards to slope failure in coastal areas.

New Geodetic and Gravimetric Maps to Infer Geodynamics of Antarctica with Insights on Victoria Land

In order to make inferences on the geodynamics of Antarctica, geodetic and gravimetric maps derived from past and new observations can be used. This paper provides new insights into the geodynamics of Antarctica by integrating data at regional and continental scales. In particular, signatures of geodynamic activity at a regional extent have been investigated in Victoria Land (VL, Antarctica) by means of Global Navigation Satellite System (GNSS) permanent station observations, data from the VLNDEF (Victoria Land Network for Deformation control) discontinuous network, and gravity station measurements. At the continental scale, episodic GNSS observations on VLNDEF sites collected for 20 years, together with continuous data from the International GNSS Service (IGS) and Polar Earth Observing Network (POLENET) sites, were processed, and the Euler pole position assessed with the angular velocity of the Antarctic plate. Both the Bouguer and the free-air gravity anomaly maps were obtained by integrating the available open-access geophysics dataset, and a compilation of 180 gravity measurements collected in the VL within the Italian National Program for Antarctic Research (PNRA) activities. As a result, new evidence has been detected at regional and continental scale. The main absolute motion of VL is towards SE (Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr) with a pattern similar to the transforms of the Tasman and Balleny fracture zones produced as consequence of Southern Ocean spreading. Residual velocities of the GNSS stations located in VL confirm the active role of the two main tectonic lineaments of the region, the Rennick–Aviator and the Lillie–Tucker faults with right-lateral sense of shear. The resulting VL gravity anomalies show a NW region characterized by small sized Bouguer anomaly with high uplift rates associated and a SE region with low values of Bouguer anomaly and general subsidence phenomena. The East and West Antarctica are characterized by a different thickness of the Earth’s crust, and the relative velocities obtained by the observed GNSS data confirm that movements between the two regions are negligible. In East Antarctica, the roots of the main subglacial highlands, Gamburtsev Mts and Dronning Maud Land, are present. The Northern Victoria Land (NVL) is characterized by more scattered anomalies. These confirm the differences between the Glacial Isostatic Adjustment (GIA) modeled and observed uplift rates that could be related to deep-seated, regional scale structures. Remote Sens. 2018, 10, 1608; doi:10.3390/rs10101608 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1608 2 of 27

Micro-Gravity Measurements in Northern Victoria-Land, Antarctica: A Feasibility Study

Within the Italian Antarctic research programme repeated GPS-measurements are being carried out at well installed points in the area of Northern-Victoria-Land (NVL), Antarctica, called VLNDEF: Victoria Land Network for Deformation Control. Although the deformations obtained over a period of 4 years are quite small we consider micro-gravity measurement a suitable complement for geodynamic research. Since the German Federal Institute for Geosciences and Natural Resources (BGR) is active in that area as well, until the year 2010 performing nine expeditions comprising geological and geophysical work, we used the offer to join expedition GANOVEX X during the season 2009/2010. With three gravimeters 13 points of the Italian network near the German station Gondwana were observed. The advantage of such measurements is that they do not require topographic corrections. Thus, the instrumental resolution and the measurement conditions, resp., are the limits for the resolution and accuracy of the measurements. In order to receive a reliable data base we used three well calibrated gravimeters in parallel and repeated the measurements several times. Local effects due to changes of the ice cover and snow fall are below the achieved measurement accuracy, because the points are so exposed that local effects are negligible. Although the elevation differences up to more than 2,700 m are quite strong, the results reveal the feasibility of these measurements in that area: The obtained standard deviations are in the order of ±10 to ±20 μGal.