Unmanned aerial vehicle (UAV) platforms are nowadays a valuable source of data for inspection, surveillance, mapping, and 3D modeling issues. As UAVs can be considered as a low-cost alternative to the classical manned aerial photogrammetry, new applications in the short- and close-range domain are introduced. Rotary or fixed-wing UAVs, capable of performing the photogrammetric data acquisition with amateur or SLR digital cameras, can fly in manual, semiautomated, and autonomous modes. Following a typical photogrammetric workflow, 3D results like digital surface or terrain models, contours, textured 3D models, vector information, etc. can be produced, even on large areas. The paper reports the state of the art of UAV for geomatics applications, giving an overview of different UAV platforms, applications, and case studies, showing also the latest developments of UAV image processing. New perspectives are also addressed.

UAV for 3D mapping applications: a review

Autonomous underwater vehicle (AUV) navigation and localization in underwater environments is particularly challenging due to the rapid attenuation of Global Positioning System (GPS) and radio-frequency signals. Underwater communications are low bandwidth and unreliable, and there is no access to a global positioning system. Past approaches to solve the AUV localization problem have employed expensive inertial sensors, used installed beacons in the region of interest, or required periodic surfacing of the AUV. While these methods are useful, their performance is fundamentally limited. Advances in underwater communications and the application of simultaneous localization and mapping (SLAM) technology to the underwater realm have yielded new possibilities in the field. This paper presents a review of the state of the art of AUV navigation and localization, as well as a description of some of the more commonly used methods. In addition, we highlight areas of future research potential.

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AUV Navigation and Localization: A Review

Landslides represent major natural hazards, which cause every year significant loss of lives and damages to buildings, properties and lifelines. In the last decades, a significant increase in landslide frequency took place, in concomitance to climate change and the expansion of urbanized areas. Remote sensing techniques represent a powerful tool for landslide investigation: applications are traditionally divided into three main classes, although this subdivision has some limitations and borders are sometimes fuzzy. The first class comprehends techniques for landslide recognition, i.e., the mapping of past or active slope failures. The second regards landslide monitoring, which entails both ground deformation measurement and the analysis of any other changes along time (e.g., land use, vegetation cover). The third class groups methods for landslide hazard analysis and forecasting. The aim of this paper is to give an overview on the applications of remote-sensing techniques for the three categories of landslide investigations, focusing on the achievements of the last decade, being that previous studies have already been exhaustively reviewed in the existing literature. At the end of the paper, a new classification of remote-sensing techniques that may be pertinently adopted for investigating specific typologies of soil and rock slope failures is proposed.

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Remote Sensing for Landslide Investigations: An Overview of Recent Achievements and Perspectives

This paper represents the result of the IAEG C35 Commission “Monitoring methods and approaches in engineering geology applications” workgroup aimed to describe a general overview of unmanned aerial vehicles (UAVs) and their potentiality in several engineering geology applications. The use of UAV has progressively increased in the last decade and nowadays started to be considered a standard research instrument for the acquisition of images and other information on demand over an area of interest. UAV represents a cheap and fast solution for the on-demand acquisition of detailed images of an area of interest and the creation of detailed 3D models and orthophoto. The use of these systems required a good background of data processing and a good drone pilot ability for the management of the flight mission in particular in a complex environment.

The use of unmanned aerial vehicles (UAVs) for engineering geology applications

In the photogrammetry field, interest in region detectors, which are widely used in Computer Vision, is quickly increasing due to the availability of new techniques. Images acquired by Mobile Mapping Technology, Oblique Photogrammetric Cameras or Unmanned Aerial Vehicles do not observe normal acquisition conditions. Feature extraction and matching techniques, which are traditionally used in photogrammetry, are usually inefficient for these applications as they are unable to provide reliable results under extreme geometrical conditions (convergent taking geometry, strong affine transformations, etc.) and for bad-textured images. A performance analysis of the SIFT technique in aerial and close-range photogrammetric applications is presented in this paper. The goal is to establish the suitability of the SIFT technique for automatic tie point extraction and approximate DSM (Digital Surface Model) generation. First, the performances of the SIFT operator have been compared with those provided by feature extraction and matching techniques used in photogrammetry. All these techniques have been implemented by the authors and validated on aerial and terrestrial images. Moreover, an auto-adaptive version of the SIFT operator has been developed, in order to improve the performances of the SIFT detector in relation to the texture of the images. The Auto-Adaptive SIFT operator (A2 SIFT) has been validated on several aerial images, with particular attention to large scale aerial images acquired using mini-UAV systems.

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Performance Analysis of the SIFT Operator for Automatic Feature Extraction and Matching in Photogrammetric Applications

MEMS inertial sensors are widely used for navigation applications where size, weight, power and cost are key sides, such as autonomous vehicular control and pedestrian navigation. Otherwise, if there is no doubt that MEMS technologies represents an interesting turning point for low cost inertial-based sensors and applications, nevertheless it is absolutely true that, in order to obtain good positioning accuracies, it is necessary to investigate very well the behaviour of these MEMS sensors and realize special test calibrations, both in static and kinematic conditions. In order to evaluate the potentialities (and the limits) of these sensors, comparative tests have been realized considering MEMS inertial sensors with different characteristics and different performances, First of all, a static calibration of the sensors has been made, in order to compare the bias values and their stability with respect to the time. In particular, an Allan-variance analysis and a modified six position static test were carried out for each sensor, preserving carefully the same environment conditions for all the tests. After the lab tests, the performances of all the sensors were compared in a field kinematic test, integrating their data with a GPS solution.

Performances comparison of different MEMS-based IMUs

The Network Real Time Kinematic (NRTK) positioning is nowadays a very common practice not only in academia but also in the professional world. Since its appearance, over 10 years ago, a growing number of people use this type of positioning not only for topographic applications, but also for the control of vehicles fleets, precision agriculture, land monitoring, etc.. To support these users several networks of Continuous Operating Reference Stations (CORSs) were born. These networks offer real-time services for NRTK positioning, providing a centimetric positioning accuracy with an average distance of 25-35 kms between the reference stations.

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Achievable positioning accuracies in a network of GNSS reference stations

The analysis and prevention of natural hazards is one of the aims of the scientific community and those who are involved in managing the environment and territory. Terrestrial LiDAR and photogrammetric techniques are effective methods of acquiring environmental data and of integrating them in digital products. These techniques have shown remarkable effectiveness when the area of interest is limited. In this paper, the authors have attempted to resolve this problem by presenting an experimental vehicle that allows the extensive use of these techniques to survey rock faces. These procedures were then applied during a measurement campaign, which was conducted along a major international roadway (the Simplon Pass) near the Divedro Valley.

Rock face surveys using a LiDAR MMS

Over last twenty years, positioning with low cost sensors has developed rapidly in the U.S.A. and in Europe at both a commercial and academic research level. In this respect, the Geomatics Research Group of LEGEND at the Politecnico di Torino has also designed and developed a universal system for Mobile Mapping, called LCMMS (Low Cost Mobile Mapping System), which uses only low-cost sensors and instrumentation. It is possible to find results of the various experiments conducted by this group in the bibliography (De Agostino, 2009 and Piras et al., 2008), where the integration and use of inertial equipment (3 IMU - Inertial Measurements Units) and GPS instruments (GPS 3 u-Blox + 1 Leica 1230+) were considered. This work is a continuation of the above described research but without the use of inertial equipment and with only L1 low cost GPS instruments, except a Leica 1230+, used only to obtain the control of the trajectory with a high precision tool. The use of this type of instrumentation introduces a number of problems, such as the accuracy that determines the position of the rover: the true goal of this work is to improve the quality of positioning and the control of the same quality. In order to solve this type of problem and to test the possibility of using L1 low cost GPS receivers for mobile mapping, the Research Group has developed a software written in MATLAB language, which allows the single epoch network kinematic positioning of a certain number of rovers in reference to a mobile master, experiments that, to the Authors knowledge, have not been carried out before. To improve but also to control the quality of positioning the idea is to insert, in addition to the measurement equations, for every age, the constraint equations of the distances between the antennas. A mobile mapping test was conducted to validate this algorithm in order to understand the accuracy that can be achieved with this low cost instrument under various satellite visibility conditions, in terms of urban obstructions

Mass-market L1 GPS Receivers for Mobile Mapping Applications: A Novel Approach

The present work shows the results that have been obtained using a low-cost pedestrian system made up of a GPS receiver and an inertial platform. This positioning system could be used by rescue teams to locate hazard zones and escape routes. Our study shows how multiple sensors can be used in a non-traditional way: the inertial platform is used as an odometer (step counter) in which the magnetometers allow the estimation of gyroscope drifts. The GPS receiver has instead been used to correct the bias when the GPS observations are sufficiently reliable. We have in particular focused on the estimation of sensor errors and on the reliability of the system, which is the main problem of this equipment.

Mattia De Agostino

Papers

Performances comparison of different MEMS-based IMUs

Achievable positioning accuracies in a network of GNSS reference stations

Rock face surveys using a LiDAR MMS

Mass-market L1 GPS Receivers for Mobile Mapping Applications: A Novel Approach

A GNSS/INS-based architecture for rescue team monitoring