Desde01 janeiro 2014
CentroCentro de Sistemas de Informação e de Computação Gráfica
Goncalves, JA; Bastos, L; Madeira, S; Magalhaes, A; Bio, A;
INTERNATIONAL JOURNAL OF REMOTE SENSING
Regular monitoring is essential to understand coastal morphodynamics and anthropic as well as natural impacts, at different temporal and spatial scales. A stereoscopic video-based terrestrial mobile mapping system, three airborne digital photography systems (mounted on a small manned airplane, a fixed-wing UAV and a multi-rotor UAV, respectively) and airborne LiDAR were compared in terms of: system features, such as range, autonomy, acquisition and operating costs; information supplied, its type and precision; and constraints to system applicability in coastal topographic surveys. Systems differed in resolution, efficiency, and applicability. The terrestrial and UAV-based systems provided the most accurate 3D data, being particularly suited for small-scale, high-resolution surveys. UAVs were easy to deploy, but limited by weather condition, particularly wind speed. Observations from a plane were most efficient and suited for larger areas. Airborne systems had the advantage of being less (UAV) to non-invasive (plane) and thus suitable for the monitoring of sensitive areas (e.g. dunes) and/or areas with difficult access. Systems should be chosen according to the specific survey aims, spatial scale, and local conditions, taking into account their applicability and cost-benefit ratios. They may complement each other to provide a comprehensive picture of coastal morphology and dynamics at different scales.
Yan, WL; Bastos, L; Madeira, S; Magalhaes, A; Goncalves, JA;
PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING
A simple and inexpensive POS for application to light airborne platforms was developed, and a modified Kalman Filtering was designed to integrate GNSS/IMU/Image data, which uses the relative orientation information to improve the accuracy of the exterior orientation parameters. The precise relative orientation of conjugate images was obtained using a SIFT/SFM matching algorithm. The relative exterior orientations were transformed from the camera frame to the navigation frame before they were used as external update information of the Kalman Filter. Combining all the relative orientation information retrieved from images, the Kalman Filter can give an improved output for the exterior orientation parameters. Airborne result from the tests of one straight strip shows that the heading accuracy of a GNSS/MEMS-IMU was improved from 0.26 degrees to 0.10 degrees, and the result from one closed strip shows that heading accuracy was improved from 1.11 degrees to 0.85 degrees and roll accuracy from 0.54 degrees to 0.43 degrees.
Bio, A; Bastos, L; Granja, H; Pinho, JLS; Goncalves, JA; Henriques, R; Madeira, S; Magalhaes, A; Rodrigues, D;
Journal of Integrated Coastal Zone Management
Coastal zones are naturally dynamic and mobile systems exposed to natural factors (river flows, waves and storms) as well as human interventions that continuously reshape their morphology. Erosion phenomena related to extreme weather events and sediment scarcity are common, threatening buildings and infrastructures, as well as beaches, ecosystems and valuable wetland; conditions that pose challenges to coastal security and defence. Regular monitoring of coastal areas, assessment of their morphodynamics and identification of the processes influencing sediment transport are thus increasingly important for a better understanding of changes and evolutionary trends in coastal systems. This demands a multi-disciplinary approach involving researchers with expertise in coastal processes and state- of-the-art observation technologies. In this paper state-of-the-art surveying methods for an efficient quantification of changes in coastal environments are described and evaluated, and two NW-Portuguese case studies are presented. Survey methods included: topographic surveys based on terrestrial videogrammetric mobile mapping and aerial photogrammetry; sub-tidal bathymetry with sonar imagery using an Autonomous Surface Vehicle (ASV); as well as field observations, with sediment sampling and beach characterisation. In the first case study, erosion/accretion patterns in the Douro estuary sand spit were analysed, considering its breakwater, river flow, wave and wind effects. Prior to the construction of a detached breakwater, the spit's morphodynamics was related to extreme river flow events, wave and wind conditions; afterwards the spit stabilized its shape and increased its area and volume. In the second case study the coast of Vila Nova de Gaia was broadly analysed, including the shoreface, foreshore and dunes, the characterization of major features and a short-period analysis of installed dynamics. Results obtained from field data, topographical surveys and numerical wave models were combined for an erosion risk assessment, using a methodology specifically developed for the study area. Both monitoring programs achieved their proposed objectives and provided valuable information to the local authorities, as gathered and processed information constitutes a valuable database for coastal planning and for ICZM purposes. They demonstrate the potential of several approaches, supported by advanced technologies, for the study of complex coastal morphodynamic processes.
Madeira, S; Yan, WL; Bastos, L; Goncalves, JA;
MEMS Inertial Measurement Units are available at low cost and can replace expensive units in mobile mapping platforms which need direct georeferencing. This is done through the integration with GNSS measurements in order to achieve a continuous positioning solution and to obtain orientation angles. This paper presents the results of the assessment of the accuracy of a system that integrates GNSS and a MEMS IMU in a terrestrial platform. We describe the methodology used and the tests realized where the accuracy of the positions and orientation parameters were assessed using an independent photogrammetric technique employing cameras that integrate the mobile mapping system developed by the authors. Results for the accuracy of attitude angles and coordinates show that accuracies better than a decimeter in positions, and under a degree in angles, can be achieved even considering that the terrestrial platform is operating in less than favorable environments.
Madeira, S; Goncalves, J; Bastos, L;
JOURNAL OF COASTAL CONSERVATION
A Mobile Mapping System (MMS) is composed by a set of sensors placed on a mobile platform whose main objective is to obtain attitude and position over time of a referential in the platform together with the image of the objects surrounding the platform. The acquired data allows to obtain georeferenced positions of object points captured by the object sensors. This paper describes a methodology to automatically extract accurate Digital Terrain Models (DTM) of sand beach areas using a Terrestrial Mobile Mapping System (TMMS). The main goal of the presented TMMS is to obtain accurate 3D models of sand beaches. A very important consequence of this will be the abbility to track spatial and temporal changes in coastal geomorphology. The presented methodology has several advantages over more classical approaches: the overall cost, flexibility for regular surveys or at the most convenient epoch, such as after storms. Besides one of the major constraints in Mobile Mapping, that is poor GNSS (Global Navigation Satellite Systems) observation conditions, is usually not present in these areas. The TMMS uses two progressive colour video cameras and can work with any type of direct georeferencing system, which obvoiusly has impact in the final accuracy of the derived DTM. For the results herein reported, a dual frequency GNSS receiver and a low grade type of IMU (Inertial Measurement System) were used. The moving platform where the system was installed is a moto quad. The developed methodology for the DTM generation relies on an automatic choice of terrain points whose three-dimensional coordinates are calculated using a presented image matching technique. A cloud of points obtained in a nearly regular grid pattern is the base for the DTM. The proposed methodology was applied on a sector of the Vila Nova de Gaia Coast, in northern Portugal, and the results presented here show that it is possible with this methodology to produce DTMs with a precision of a few centimetres.
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