Rendu en Portion de Ciel Visible de Gros Nuages de Points 3D

International audienceNous nous intéressons au traitement de très gros nuages de points (plusieurs dizaines de millions). Le rendu de tels nuages s'effectue, de manière rapide, à l'aide de modèles d'éclairage locaux. Mais le réalisme est moindre que celui obtenu avec des modèles d'éclairage globaux. Ces derniers requièrent quant à eux la définition d'une surface à partir du nuage de points. Nous utilisons un modèle d'éclairage intermédiaire, la portion de ciel visible. Nous déclinons ce modèle en deux versions : une version 2D1/2 et une version 3D. La première peut être appliquée à des terrains ou assimilés ; la seconde peut opérer sur des nuages de points 3D issus de mesures, discrétisées dans un arbre octaire. Les résultats de ces méthodes ont été utilisés par des archéologues pour guider les travaux d'accostage et d'assemblage de fragments d'une grande colonne grecque. La version 2D1/2 sera utilisée à des fins d'illustration dans des publications archéologiques

Flexible decoupled camera and projector fringe projection system using inertial sensors

Measurement of objects with complex geometry and many self occlusions is increasingly important in many fields, including additive manufacturing. In a fringe projection system, the camera and the projector cannot move independently with respect to each other, which limits the ability of the system to overcome object self-occlusions. We demonstrate a fringe projection setup where the camera can move independently with respect to the projector, thus minimizing the effects of self-occlusion. The angular motion of the camera is tracked and recalibrated using an on-board inertial angular sensor, which can additionally perform automated point cloud registration

Small unmanned aerial model accuracy for photogrammetrical fluvial bathymetric survey

Fluvial systems offer a challenging and varied environment for topographic survey, displaying a rapidly varying morphology, vegetation assemblage and degree of submergence. Traditionally theodolite or GPS based systems have been used to capture cross-section and breakline based topographic data which has subsequently been interpolated. Advances in survey technology has resulted in an improved ability to capture larger volumes of information with infrared terrestrial and aerial LiDAR systems capturing high density (<0.02 m) point data across terrestrial surfaces. The rise of Structure from Motion (SfM) photogrammetry, coupled with small unmanned aerial vehicles (sUAV), has potential to record elevation data at reach scale sub decimetre density. The approach has the additional advantage over LiDAR of seeing through clear water to capture bed detail, whilst also generating ortho-rectified photographic mosaics of the survey reach. However, data accuracy has yet to be comprehensively assessed. Here we present a survey protocol for sUAV deployment and provide a reach scale comparison between a theodolite and SfM sUAV survey on the River Sprint, Kendal, the River Ehen at Egremont, England and the Afon Elwy, at Llanfair Talhaiarn, Wales. Comparative analysis between theodolite survey and SfM suggest similar accuracy and precision across terrestrial surfaces with error lowest over solid surfaces, increasing with vegetation complexity. Submerged SfM data, captured bed levels generally to within ±0.25 m with only a weak relationship recorded between error and flow depth. Significantly, associated error when linked to channel D50 highlights the ability of unmanned aerial vehicles to capture accurate fluvial data across a range of river biotopes and depths to 2.4 m

A Point-Based Approach for Capture, Display and Illustration of Very Complex Archeological Artefacts

no noteInternational audienceIn this paper we present a complete point-based pipeline for the capture, display and illustration of very large scans of archeological artifacts. This approach was developed as part of a project involving archeologists and computer graphics researchers, working on the Delphi "Dancers Column". We first determined the archeologists' requirements for interactive viewing and documentary illustration. To satisfy these needs we use a compact pointbased structure of the very large data, permitting interactive viewing in 3D. This helps the archeologists to examine and position the fragments. We introduce efficient construction algorithms for this structure, allowing it to be built on limited-memory platforms, such as those available on the field. We also propose a new stylized rendering approach based on an inverse cylindrical projection and 2D skydome rendering. This illustrative style has been used as a planning tool for fragment docking and as a substitute for traditional illustration in an archeological publication. Other uses of these tools are currently under way in the context of this project

Structure from Motion (SfM) Photogrammetry with Drone Data: A Low Cost Method for Monitoring Greenhouse Gas Emissions from Forests in Developing Countries

ArticleThis is the final version of the article. Available from MDPI via the DOI in this record.Structure from Motion (SfM) photogrammetry applied to photographs captured from Unmanned Aerial Vehicle (UAV) platforms is increasingly being utilised for a wide range of applications including structural characterisation of forests. The aim of this study was to undertake a first evaluation of whether SfM from UAVs has potential as a low cost method for forest monitoring within developing countries in the context of Reducing Emissions from Deforestation and forest Degradation (REDD+). The project evaluated SfM horizontal and vertical accuracy for measuring the height of individual trees. Aerial image data were collected for two test sites; Meshaw (Devon, UK) and Dryden (Scotland, UK) using a Quest QPOD fixed wing UAV and DJI Phantom 2 quadcopter UAV, respectively. Comparisons were made between SfM and airborne LiDAR point clouds and surface models at the Meshaw site, while at Dryden, SfM tree heights were compared to ground measured tree heights. Results obtained showed a strong correlation between SfM and LiDAR digital surface models (R2 = 0.89) and canopy height models (R2 = 0.75). However, at Dryden, a poor correlation was observed between SfM tree heights and ground measured heights (R2 = 0.19). The poor results at Dryden were explained by the fact that the forest plot had a closed canopy structure such that SfM failed to generate enough below-canopy ground points. Finally, an evaluation of UAV surveying methods was also undertaken to determine their usefulness and cost-effectiveness for plot-level forest monitoring. The study concluded that although SfM from UAVs performs poorly in closed canopies, it can still provide a low cost solution in those developing countries where forests have sparse canopy cover (<50%) with individual tree crowns and ground surfaces well-captured by SfM photogrammetry. Since more than half of the forest covered areas of the world have canopy cover <50%, we can conclude that SfM has enormous potential for forest mapping in developing countries.Leon DeBell flew the QPOD Quest UAV at Meshaw as part of flight testing for the QuestEarthWater project, which was funded by the UK Technology Strategy Board and NERC, and we are also grateful for the field assistance of Naomi Gatis and David Luscombe at this site. The NERC Tellus SouthWest project is acknowledged for providing the LiDAR data used at Meshaw. The authors would also like to thank Mark Buie, Bruce Gittings and Alasdair Mac Arthur for helping out with the UAV fieldwork at Dryden Farm

Electron-lattice kinetics of metals heated by ultrashort laser pulses

We propose a kinetic model of transient nonequilibrium phenomena in metals exposed to ultrashort laser pulses when heated electrons affect the lattice through direct electron-phonon interaction. This model describes the destruction of a metal under intense laser pumping. We derive the system of equations for the metal, which consists of hot electrons and a cold lattice. Hot electrons are described with the help of the Boltzmann equation and equation of thermoconductivity. We use the equations of motion for lattice displacements with the electron force included. The lattice deformation is estimated immediately after the laser pulse up to the time of electron temperature relaxation. An estimate shows that the ablation regime can be achieved.Comment: 7 pages; Revtex. to appear in JETP 88, #1 (1999

A review of the use of terrestrial laser scanning application for change detection and deformation monitoring of structures

Change detection and deformation monitoring is an active area of research within the field of engineering surveying as well as overlapping areas such as structural and civil engineering. The application of Terrestrial Laser Scanning (TLS) techniques for change detection and deformation monitoring of concrete structures has increased over the years as illustrated in the past studies. This paper presents a review of literature on TLS application in the monitoring of structures and discusses registration and georeferencing of TLS point cloud data as a critical issue in the process chain of accurate deformation analysis. Past TLS research work has shown some trends in addressing issues such as accurate registration and georeferencing of the scans and the need of a stable reference frame, TLS error modelling and reduction, point cloud processing techniques for deformation analysis, scanner calibration issues and assessing the potential of TLS in detecting sub-centimetre and millimetre deformations. However, several issues are still open to investigation as far as TLS is concerned in change detection and deformation monitoring studies such as rigorous and efficient workflow methodology of point cloud processing for change detection and deformation analysis, incorporation of measurement geometry in deformation measurements of high-rise structures, design of data acquisition and quality assessment for precise measurements and modelling the environmental effects on the performance of laser scanning. Even though some studies have attempted to address these issues, some gaps exist as information is still limited. Some methods reviewed in the case studies have been applied in landslide monitoring and they seem promising to be applied in engineering surveying to monitor structures. Hence the proposal of a three-stage process model for deformation analysis is presented. Furthermore, with technological advancements new TLS instruments with better accuracy are being developed necessitating more research for precise measurements in the monitoring of structures

Coincident beach surveys using UAS, vehicle mounted and airborne laser scanner: point cloud inter-comparison and effects of surface type heterogeneity on elevation accuracies

Reliable and consistent topographic data is key to a multitude of environmental manangement and research applications. Unmanned Aerial Systems (UAS) are fast establishing themselves as a promising additional remote sensing platform that provides high spatial resolution not only of topography but also surface types and coastal features together with comparatively low costs and high deployment flexibility. However, comprehensive information on the accuracy of UAS-based elevation models in comparison to other available surveying methodology is regulary limited to be referenced to individual methods. This paper addresses this shortcoming by comparing coincident beach surveys of three different point cloud generating methods: ATV mounted mobile laser scan (MLS), airborne LiDAR (ALS), and UAS. This was complemented by two RTK-GPS surveys on a pole with wheel attachment and mounted on an ATV. We present results in relation to elevation accuracies on a concrete control surface, the entire beach and for six different beach surface types together with how differences between point clouds propagate during the construction of gridded elevation models. Overall, UAS point cloud elevations were consistently higher than those of ALS (+0.063 m) and MLS (+0.087 m). However, these results for the entire beach mask larger and smaller differences related to the individual surface characteristics. For all surface types, UAS records higher (from 0.006 m for wet sand to 0.118 m for cobbles, average of 0.063 m) elevations than ALS. The MLS on the other hand, records predominantly lower elevation than ALS (-0.005 m for beach gravel to -0.089 m for soft mud, average of -0.025 m) except for cobbles, where elevations are 0.056 m higher. The comparison shows that all point cloud methods produce elevations that are suitable for monitoring changes in beach topography in the context of operational coastal management applications. However, due to the systematic differences between respective monitoring approaches, care needs to be taken when analysing beach topographies for the same area based on different methods. The eventual choice of monitoring method is therefore guided by a range of practical factors, including capital cost of the system and operating costs per survey area, conditions under which the system can operate, data processing time, and legal restrictions in the use of the system such as air safety regulations or limitation of ground access to areas with environmental protection

Rainfall simulation and Structure-from-Motion photogrammetry for the analysis of soil water erosion in Mediterranean vineyards

Soilwater erosion is a serious problem, especially in agricultural lands. Among these, vineyards deserve attention, because they constitute for the Mediterranean areas a type of land use affected by high soil losses. A significant problem related to the study of soil water erosion in these areas consists in the lack of a standardized procedure of collecting data and reporting results, mainly due to a variability among the measurement methods applied. Given this issue and the seriousness of soilwater erosion inMediterranean vineyards, this works aims to quantify the soil losses caused by simulated rainstorms, and compare them with each other depending on two different methodologies: (i) rainfall simulation and (ii) surface elevation change-based, relying on high-resolution Digital Elevation Models (DEMs) derived from a photogrammetric technique (Structure-from-Motion or SfM). The experiments were carried out in a typical Mediterranean vineyard, located in eastern Spain, at very fine scales. SfMdatawere obtained fromone reflex camera and a smartphone built-in camera. An index of sediment connectivity was also applied to evaluate the potential effect of connectivity within the plots. DEMs derived from the smartphone and the reflex camera were comparable with each other in terms of accuracy and capability of estimating soil loss. Furthermore, soil loss estimatedwith the surface elevation change-basedmethod resulted to be of the same order ofmagnitude of that one obtained with rainfall simulation, as long as the sediment connectivity within the plotwas considered.High-resolution topography derived fromSfMrevealed to be essential in the sediment connectivity analysis and, therefore, in the estimation of eroded materials, when comparing themto those derived from the rainfall simulation methodology. The fact that smartphones built-in cameras could produce as much satisfying results as those derived from reflex cameras is a high value added for using SfM