1,720,986 research outputs found
Single frequency GPS monitoring of glacial displacements: the case of Grand Jorasses glacier
Full text linkIl contributo del GPS al monitoraggio del rischio glaciale: la sperimentazione sul seracco delle Grandes Jorasses
No full textVelocity estimation of Grandes Jorasses glacier in the framework of GlaRiskAlp project
No full textThe Grandes Jorasses hanging glacier, located at 4100 m asl above a populated area, is monitored by Fondazione Montagna sicura since 2007, on account of the Geological Service of the Aosta Valley Autonomous Region. The serac dynamic shows a periodic trend of mass increasing, interrupted by a collapse: The collapse is preceded by an exponential acceleration that lasts few days, as revealed in 1997-98 by Prof. M Funk (ETH Zurich). By tracking the velocity of the ice mass it is therefore possible to forecast its breakdown. At the early stage of the project the native monitoring system consisted of poles with prisms placed on the glacier surface, monitored by an automatic total station (motorized total station) sited at the valley bottom. During bad weather conditions the system could loss measurements, so alternative monitoring systems were designed, installed and tested in the framework of the GlaRiskAlp project (Alcotra program). In particular the new monitoring systems consisted of a low cost, single frequency, GNSS wireless sensors network, able to trace continuously (H24) the receivers positions to measure the displacement in real time, and a seismometer at the top of the glacier was placed in order to follow the seismic activity (that is proportional to the velocity). Furthermore, in order to better characterize the mass evolution, photogrammetric activities were carried out to reconstruct the glacier surface. Prototypes, results and outlooks of these monitoring systems for Grandes Jorasses glacier will be presente
Photogrammetric digital surface model reconstruction in extreme low-light environments
Full text linkDigital surface models (DSM) have become one of the main sources of geometrical information for a broad range of applications. Image-based systems typically rely on passive sensors which can represent a strong limitation in several survey activities (e.g., night-time monitoring, underground survey and night surveillance). However, recent progresses in sensor technology allow very high sensitivity which drastically improves low-light image quality by applying innovative noise reduction techniques. This work focuses on the performances of night-time photogrammetric systems devoted to the monitoring of rock slopes. The study investigates the application of different camera settings and their reliability to produce accurate DSM. A total of 672 stereo-pairs acquired with high-sensitivity cameras (Nikon D800 and D810) at three different testing sites were considered. The dataset includes different camera configurations (ISO speed, shutter speed, aperture and image under-/over-exposure). The use of image quality assessment (IQA) methods to evaluate the quality of the images prior to the 3D reconstruction is investigated. The results show that modern highsensitivity cameras allow the reconstruction of accurate DSM in an extreme low-light environment and, exploiting the correct camera setup, achieving comparable results to daylight acquisitions. This makes imaging sensors extremely versatile for monitoring applications at generally low costs
Analysis of low-light and night-time stereo-pair images for photogrammetric reconstruction
Full text linkRockfalls and rockslides represent a significant risk to human lives and infrastructures because of the high levels of energy involved in the phenomena. Generally, these events occur in accordance to specific environmental conditions, such as temperature variations between day and night, that can contribute to the triggering of structural instabilities in the rock-wall and the detachment of blocks and debris. The monitoring and the geostructural characterization of the wall are required for reducing the potential hazard and to improve the management of the risk at the bottom of the slopes affected by such phenomena. In this context, close range photogrammetry is largely used for the monitoring of high-mountain terrains and rock walls in mine sites allowing for periodic survey of rockfalls and wall movements. This work focuses on the analysis of low-light and night-time images of a fixed-base stereo pair photogrammetry system. The aim is to study the reliability of the images acquired over the night to produce digital surface models (DSMs) for change detection. The images are captured by a high-sensitivity DLSR camera using various settings accounting for different values of ISO, aperture and time of exposure. For each acquisition, the DSM is compared to a photogrammetric reference model produced by images captured in optimal illumination conditions. Results show that, with high level of ISO and maintaining the same grade of aperture, extending the exposure time improves the quality of the point clouds in terms of completeness and accuracy of the photogrammetric models
Indirect UAV strip georeferencing by on-board GNSS data under poor satellite coverage
Full text linkThe so-called Real Time Kinematic (RTK) option, which allows one to determine with cm-level accuracy the Unmanned Aerial Vehicles (UAV) camera position at shooting time, is also being made available on medium- or low-cost drones. It can be foreseen that a sizeable amount of UAV surveys will be soon performed (almost) without Ground Control Points (GCP). However, obstacles to Global Navigation Satellite Systems (GNSS) signal at the optimal flight altitude might prevent accurate retrieval of camera station positions, e.g., in narrow gorges. In such cases, the master block can be georeferenced by tying it to an (auxiliary) block flown at higher altitude, where the GNSS signal is not impeded. To prove the point in a worst case scenario, but under controlled conditions, an experiment was devised. A single strip about 700 m long, surveyed by a multi-copter at 30 m relative flight height, was referenced with cm-level accuracy by joint adjustment with a block flown at 100 m relative flight height, acquired by a fixed-wing UAV provided with RTK option. The joint block orientation was repeated with or without GCP and with pre-calibrated or self-calibrated camera parameters. Accuracy on ground was assessed on a fair number of Check Points (CP). The results show that, even without GCP, the precision is effectively transferred from the auxiliary block projection centres to the object point horizontal coordinates and, with a pre-calibrated camera, also to the elevations
UAV BLOCK GEOREFERENCING and CONTROL by ON-BOARD GNSS DATA
Full text linkUnmanned Aerial Vehicles (UAV) are established platforms for photogrammetric surveys in remote areas. They are lightweight, easy to operate and can allow access to remote sites otherwise difficult (or impossible) to be surveyed with other techniques. Very good accuracy can be obtained also with low-cost UAV platforms as far as a reliable ground control is provided. However, placing ground control points (GCP) in these contexts is time consuming and requires accessibility that, in some cases, can be troublesome. RTK-capable UAV platforms are now available at reasonable costs and can overcome most of these problems, requiring just few (or none at all) GCP and still obtaining accurate results. The paper will present a set of experiments performed in cooperation with ARPA VdA (the Environmental Protection Agency of Valle d'Aosta region, Italy) on a test site in the Italian Alps using a Dji Phantom 4 RTK platform. Its goals are: a) compare accuracies obtainable with different calibration procedures (pre- or on-the-job/self-calibration); b) evaluate the accuracy improvements using different number of GCP when the site allows for it; and c) compare alternative positioning modes for camera projection centres determination, (Network RTK, RTK, Post Processing Kinematic and Single Point Positioning)
Influence of Block Geometry Configuration on Multi-Image Dense Matching
Full text linkDigital photogrammetry is a widespread surveying technique in different fields of application due to its flexibility, versatility and cost-effectiveness. Despite its increasing automation and simplicity, a proper image block design is crucial to ensure high standards of performance and accuracy. Studies on camera network design have been largely dealt with in the scientific literature with reference to image orientation process, while they are still poor on dense matching. This paper investigates the influence of different block geometry configurations on multi-image dense matching. Starting from the same orientation solution, dense matching was performed considering different combinations of number of images and base length distance between the first and the last image within a strip. The raster Digital Elevation Models (DEM) resulting from each sequence of images were compared with a reference DEM to assess accuracy and completeness. The tests were conducted using different cameras and at various test sites to assess different survey conditions and generalize the findings. The presented results provide some operational guidance on block geometry optimization to maximize the accuracy and completeness
A MONTE CARLO SIMULATION STUDY ON THE DOME EFFECT
Full text linkA dome-shape deformation has been found to affect the photogrammetric surface reconstruction in several real and simulated experiments. Its origin has been recognised in inaccurate estimation of the camera parameters and many papers already concentrated on conditions to avoid its development, especially as far as block design is concerned. This paper presents a Monte Carlo simulation to investigate surface reconstruction elevation errors in UAV (Unmanned Aerial Vehicle) photogrammetric blocks. The simulation tests are designed to find out the effect of block shape, camera axis inclination, side-lap, cross strips addition and block control by GCP or GNSS-assisted on the extent of the deformations. The main findings are: i) that GNSS-assisted blocks are generally more robust compared to GCP-controlled ones; ii) that, in GNSS-assisted blocks, unless a mix of nadiral and inclined strips is present, at least one fixed GCP must be provided; iii) that cross strip can conveniently be slimmed to save flight time and processing time; iv) that the effectiveness of GNSS deteriorate as the block shape slims out
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