Method for orthorectification of terrestrial radar maps

International audienceThe vehicle-based PELICAN radar system is used in the context of mobile mapping. The R-SLAM algorithm allows simultaneous retrieval of the vehicle trajectory and of the map of the environment. As the purpose of PELICAN is to provide a means for gathering spatial information, the impact of distortion caused by the topography is not negligible. This article proposes an orthorectification process to correct panoramic radar images and the consequent R-SLAM trajectory and radar map. The a priori knowledge of the area topography is provided by a digital elevation model. By applying the method to the data obtained from a path with large variations in altitude it is shown that the corrected panoramic radar images are contracted by the orthorectification process. The efficiency of the orthorectification process is assessed firstly by comparing R-SLAM trajectories to a GPS trajectory and secondly by comparing the position of Ground Control Points on the radar map with their GPS position. The RMS positioning error moves from 5.56 m for the raw radar map to 0.75 m for the orthorectified radar map

DRELIO : Un drone hélicoptère pour le suivi des zones littorales

International audienceLa connaissance du littoral, sa protection, son aménagement nécessitent un suivi des changements qui s'y produisent. Toutefois, en France, devant l'étendue des façades maritimes à couvrir, les techniques de mesures in situ ne peuvent être appliquées de façon systématique. Dans ce cas, les techniques de télédétection spatiale ou aérienne sont des approches complémentaires. A partir de ces plates-formes, des images stéréoscopiques multi-temporelles sont, après une série de traitements photogrammétriques appropriés, directement exploitables sous formes d'orthophotographies et de Modèles Numériques de Terrain (MNT). Ce type de produits permet notamment la quantification des changements morphosédimentaires à l'interface Terre-Mer (transport transversal et longitudinal de sédiments, érosion,...). Actuellement, la résolution spatiale des MNT générés à partir d'images aériennes ou satellitaires (<50 cm) est insuffisante pour la plupart des applications sur la frange littorale. En outre, dans le cas d'interventions consécutives à un événement extrême (tempête, raz de marée, pollution...), ces systèmes manquent de souplesse (trajectoires prédéfinies, contraintes d'altitude, de vitesse, coût de mise en oeuvre...) L'utilisation de drones constitue donc une alternative intéressante pour des suivis de précision ou des interventions rapides

Assessing the Accuracy of High Resolution Digital Surface Models Computed by PhotoScan® and MicMac® in Sub-Optimal Survey Conditions

For monitoring purposes and in the context of geomorphological research, Unmanned Aerial Vehicles (UAV) appear to be a promising solution to provide multi-temporal Digital Surface Models (DSMs) and orthophotographs. There are a variety of photogrammetric software tools available for UAV-based data. The objective of this study is to investigate the level of accuracy that can be achieved using two of these software tools: Agisoft PhotoScan® Pro and an open-source alternative, IGN© MicMac®, in sub-optimal survey conditions (rugged terrain, with a large variety of morphological features covering a range of roughness sizes, poor GPS reception). A set of UAV images has been taken by a hexacopter drone above the Rivière des Remparts, a river on Reunion Island. This site was chosen for its challenging survey conditions: the topography of the study area (i) involved constraints on the flight plan; (ii) implied errors on some GPS measurements; (iii) prevented an optimal distribution of the Ground Control Points (GCPs) and; (iv) was very complex to reconstruct. Several image processing tests are performed with different scenarios in order to analyze the sensitivity of each software package to different parameters (image quality, numbers of GCPs, etc.). When computing the horizontal and vertical errors within a control region on a set of ground reference targets, both methods provide rather similar results. A precision up to 3–4 cm is achievable with these software packages. The DSM quality is also assessed over the entire study area comparing PhotoScan DSM and MicMac DSM with a Terrestrial Laser Scanner (TLS) point cloud. PhotoScan and MicMac DSM are also compared at the scale of particular features. Both software packages provide satisfying results: PhotoScan is more straightforward to use but its source code is not open; MicMac is recommended for experimented users as it is more flexible

Techniques d'observation et de mesure haute résolution des transferts sédimentaires dans la frange littorale

With an increasing population in the littoral fringe and growing coastal risks, there is a pressing need for the development of effective coastal integrated monitoring strategies. This monitoring is underpinned by recurrent surveys, with techniques suited to such environment, measuring changes and allowing a better understanding of the underlying processes. To address this issue, this thesis focuses on the development of methods of survey and of quantification of morphodynamic evolution. It is centred around a very-high resolving 3D monitoring of the land-sea interface. Specifically, this research has concentrated on remote-sensing techniques : stereo-photogrammetry with a drone, terrestrial laser scanning and shallow-water multi-beam echo-sounding above the subtidal beach. For each technique, specific protocols of data gathering and data processing have been developed to compute Digital Elevation Model (DEM) and then differential DEM. This multi-sensor strategy offers a synoptic view of transfers in the sedimentary cell. The monitoring methods were applied to the sandy beach of Porsmilin (Mer d'Iroise). The results of the recurring surveys were explored to analyse the morpho-sedimentary variability at different spatial and temporal scales. The monitoring of the sedimentary cell results in the determination of closure depth (at annual scale) and in the identification of seasonal tendencies. Considering greater spatio-temporal scales, sedimentary features such as berms or beach cusps have been studied. As far as possible, each observation of topographic variations is linked to the underlying hydrodynamic forcing.Dans un contexte d'anthropisation croissante de la frange littorale, et d'intensification des risques côtiers, la mise en œuvre de politiques de Gestion Intégrée des Zones Côtières (GIZC) s'impose. Cette gestion se concrétise notamment par un suivi de l'évolution littorale, nécessitant des techniques adaptées à ce type d'environnement et permettant de mesurer les variations morphologiques mais également de mieux comprendre les processus mis en jeu. Cette thèse s'inscrit dans la problématique de développement de méthodes d'observation et de quantification de l'évolution morphodynamique. Elle se focalise sur un suivi 3D à très haute résolution spatiale de l'interface Terre-Mer au niveau d'une plage sableuse, de l'échelle journalière à pluri-annuelle. Les méthodes proposées s'appuient sur des techniques de télédétection : stéréo-photogrammétrie depuis un drone hélicoptère, acquisitions au Scanner Laser Terrestre (TLS) et acquisitions au Sondeur Multi-Faisceaux (SMF) petits fonds sur les parties immergées de la plage. Pour chacune de ces techniques, nous avons élaboré un protocole spécifique d'acquisition et de traitement des données aboutissant à la génération de Modèles Numériques de Terrain (MNT), puis de MNT différentiels diachroniques. Cette approche multi-source permet de fusionner les données terre-mer (sur une frange s'étendant du haut de plage à 15 m de profondeur) et offre ainsi une vision synoptique des transferts de matière au sein du compartiment sédimentaire. Les techniques présentées ont été mises en oeuvre à la Plage de Porsmilin (Mer d'Iroise). Les résultats des levés récurrents ont fait l'objet d'une analyse de la variabilité morpho-sédimentaire à différentes échelles spatio-temporelles. Le suivi de la cellule sédimentaire a permis de déterminer la profondeur de fermeture à l'échelle annuelle et d'identifier les tendances saisonnières. Aux échelles spatio-temporelles plus fines, nous avons pu étudier l'évolution de figures sédimentaires telles que des bermes ou des croissants de plages. Autant que possible, chaque observation s'accompagne d'une identification du (des) forçage(s) hydrodynamique(s) sous-jacent(s)

Easily Implemented Methods of Radiometric Corrections for Hyperspectral&ndash;UAV&mdash;Application to Guianese Equatorial Mudbanks Colonized by Pioneer Mangroves

Hyper-DRELIO (Hyperspectral DRone for Environmental and LIttoral Observations) is a custom, mini-UAV (unmanned aerial vehicle) platform (&lt;20 kg), equipped with a light push broom hyperspectral sensor combined with a navigation module measuring position and orientation. Because of the particularities of UAV surveys (low flight altitude, small spatial scale, and high resolution), dedicated pre-processing methods have to be developed when reconstructing hyperspectral imagery. This article presents light, easy-implementation, in situ methods, using only two Spectralon&reg; and a field spectrometer, allowing performance of an initial calibration of the sensor in order to correct &ldquo;vignetting effects&rdquo; and a field standardization to convert digital numbers (DN) collected by the hyperspectral camera to reflectance, taking into account the time-varying illumination conditions. Radiometric corrections are applied to a subset of a dataset collected above mudflats colonized by pioneer mangroves in French Guiana. The efficiency of the radiometric corrections is assessed by comparing spectra from Hyper-DRELIO imagery to in situ spectrometer measurements above the intertidal benthic biofilm and mangroves. The shapes of the spectra were consistent, and the spectral angle mapper (SAM) distance was 0.039 above the benthic biofilm and 0.159 above the mangroves. These preliminary results provide new perspectives for quantifying and mapping the benthic biofilm and mangroves at the scale of the Guianese intertidal mudbanks system, given their importance in the coastal food webs, biogeochemical cycles, and the sediment stabilization

Assessing DEM quality and minimizing registration error in repeated geomorphic surveys with multi-temporal ground truths of invariant features: Application to a long-term dataset of beach topography and nearshore bathymetry

Remotely-sensed digital elevation models (DEMs) and uncertainty-based geomorphic change detection (GCD) have become very practical tools for geoscientists, including for coastal research. Through the analysis of DEMs of differences (DoDs) and the provision of DEM quality, it allows monitoring complex landforms and confidently relating the changes observed to environmental forcing conditions. With continuing remote sensing advances, and as some monitoring programs are reaching several decades of repeated data collection, it is timely to consider approaches that enable the reconciliation of DEMs of variable and potentially unknown quality before their subsequent geomorphic analyses. In this paper, we present an original workflow whereby composite data formed by fusing available measurements over invariant features serve as multi-temporal ground truths for assessing repeated DEMs. Results of the evaluation enable identification of DEMs of lower quality (bias and precision) and correction of registration error (horizontal and vertical bias), and thus offer the in-built capacity for estimating and improving change detection levels afforded by the data. The workflow was applied to a freely-accessible multi-sensor: RTK-GNSS, terrestrial laser-scanning, drone photogrammetry and multibeam echo-sounding dataset of high-resolution topographic and nearshore bathymetric DEMs collected at the macrotidal pocket beach of Porsmilin (France) over the period 2003-2019. Our results show that consistently high DEM precision can be achieved in a long-term multi-sensor dataset, but registration errors may be present and can be minimised through co-registration with the purpose-built ground truths. Although the study focuses primarily on measuring height discrepancies, which is directly relevant for DoD analysis, we show that the methods can be used also for dealing with horizontal error when high-resolution imagery is available. Finally, the detailed DEM evaluations presented, in application to a rare dataset documenting beach and shoreface change for nearly two decades, provide original insights on the performance of usual topo-bathymetric surveying techniques

Techniques d'observation et de mesure haute résolution des transferts sédimentaires dans la frange littorale

Cette thèse s'inscrit dans la problématique de développement de méthodes d'observation et de quantification de l évolution morphodynamique. Elle se focalise sur un suivi 3D à très haute résolution spatiale de l interface terre-mer au niveau d une plage sableuse, de l échelle journalière à pluri-annuelle. Les méthodes proposées s appuient sur des techniques de télédétection : stéréo-photogrammétrie depuis un drone hélicoptère, acquisitions au scanner laser terrestre (TLS) et acquisitions au Sondeur Multi-Faisceaux (SMF) petits fonds sur les parties immergées de la plage. Pour chacune de ces techniques, nous avons élaboré un protocole spécifique d acquisition et de traitement des données aboutissant à la génération de Modèles Numériques de Terrain (MNT), puis de MNT différentiels diachroniques. Cette approche multi-source permet de fusionner les données terre-mer et offre ainsi une vision synoptique des transferts de matière au sein du compartiment sédimentaire. Les techniques présentées ont été mises en oeuvre à la plage de Porsmilin (mer d Iroise). Les résultats des levés récurrents ont fait l objet d une analyse de la variabilité morpho-sédimentaire à différentes échelles spatio-temporelles. Le suivi de la cellule sédimentaire a permis de déterminer la profondeur de fermeture à l échelle annuelle et d'identifier les tendances saisonnières. Aux échelles spatio-temporelles plus fines, nous avons pu étudier l'évolution de figures sédimentaires telles que de bermes ou des croissants de plages. Autant que possible, chaque observation s'accompagne d une identification du (des) forçage(s) hydrodynamique(s) sous-jacent(s).With an increasing population in the littoral fringe and growing coastal risks, there is a pressing need for the development of effective coastal integrated monitoring strategies. This monitoring is underpinned by recurrent surveys, with techniques suited to such environment, measuring changes an allowing a setter understanding of the underlying processes. To address this issue, this thesis focuses on the development of methods of survey and of quantification of morphodynamic evolution. It is centred around a very-high resolving 3D monitoring of the land-sea interface. Specifically, this research has concentrated on remote-sensing techniques: stereo-photogrammetry with a drone, terrestrial laser scanning and shallow-water multi-beam echo-sounding above the subtidal beach. For each technique, specific protocols of data gathering and data processing have been developed to compute Digital Elevation Model (DEM) and then differential DEM this multi-sensor strategy offers a synoptic view of transfers in the sedimentary cell. The monitoring methods were applied to the sandy beach of Porsmilin (mer d Iroise). The results of the recurring surveys were explored to analyse the morpho-sed1mentary variability at different spatial and temporal scales. The monitoring of the sedimentary cell results in the determination of closure depth (at annual scale) and in the identification of seasonal tendencies. Considering greater spatio-temporal scales, sedimentary features such as berms or beach cusps have been studied. As far a possible, each observation of topographic variations is linked to the underlying hydrodynamic forcing.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Potential of Smartphone SfM Photogrammetry to Measure Coastal Morphodynamics

With recent advances in photogrammetric processing methods and sensor technologies, smartphones represent a new opportunity of mainstream, low-cost sensor, with a great potential for Structure-from-Motion (SfM) photogrammetry, and in particular for participatory science programs or citizen observatories. Keeping in mind the application in citizen observatories, three smartphone models (Galaxy S7 (R), Lumia 930 (R) and iPhone 8 (R)) and a bridge camera were compared (separately and in combination) for coastal applications: A coastal cliff and a sandy beach. Various acquisition protocols, at different distances from a cliff face and using "linear" or "fan-shaped" capture mode, were also assessed in their efficiency. A simultaneous Terrestrial Laser Scanner (TLS) survey provided a reference dataset to assess the quality of the SfM reconstructions. Satisfactory reconstructions (mean error < 5 cm) of the cliff face were obtained using all smartphone models tested. To measure the cliff face, fan-shaped capturing mode allowed a quicker image acquisition on site and better results (mean error of 1.3 cm with a standard deviation of 0.1 cm at 20 m from the cliff face) than linear capturing mode (mean error of 2.5 cm with a standard deviation of 21.8 cm), provided that the distance to the cliff face is sufficient to ensure a good image overlap. To obtain satisfactory results over beaches, we show that it is preferable to have high-angle shots of the study area, which may limit the applicability of the method for certain sites

Towards LIDAR-RADAR based Terrain Mapping for Traversability Analysis

International audienceThis paper addresses the problem of perception for autonomous vehicle navigation in real environments. Integrity safe navigation of autonomous vehicles in unknown environments poses a traversability problem. We are interested in the integrity-safe navigation in unknown environments. Safe navigation is a task that depends on the knowledge of the surrounding environment and the vehicle dynamics. Classical navigation approach focus on obstacle avoidance often based on occupancy and elevation maps. We propose bi-modal perception system composed of two sensors: lidar and radar. We combine an optical sensor and an electromagnetic sensor to build a richer map of the environment which will be used for traversability analysis and path planning. The proposed lidar-radar map encodes the geometry of the environment such that traversability analysis and trajectory planning guarantee the robot’s integrity in a stability sense. The algorithm builds a map from lidar, radar, IMU and GPS sensors. Results have been validated experimentally

Conception de radar MMW aéroporté léger pour la génération DEM. Résultats de la simulation

International audienceThere is a growing need for lightweight airborne platforms that could provide precise information about the environment (topography, presence of obstacles, etc.) filling the data gap between aerial/satellite remote sensing and terrestrial systems. A major limitation of classical sensors such as vision or laser is that they are ineffective in degraded visual conditions. Millimeter-wave radar provides an alternative solution to overcome the shortcomings of optical solutions, because in the microwave range, data can be acquired independently of atmospheric conditions and time of the day. The intended application of a new radar sensor is the construction of digital elevation models of the overflown environments. As the design of new radar sensors for light airborne platforms is subject to specific technological constraints, a simulator of airborne radar surveys is developed. The objective of the simulator is to help the designer in defining the main parameters of the future airborne radar, and in developing radar signal processing algorithms