• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 415
  • 136
  • 86
  • 60
  • 44
  • 30
  • 25
  • 11
  • 11
  • 11
  • 6
  • 4
  • 3
  • 3
  • 3
  • Tagged with
  • 1001
  • 145
  • 142
  • 140
  • 111
  • 98
  • 98
  • 84
  • 82
  • 78
  • 76
  • 69
  • 67
  • 62
  • 60
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Conception et prototypage d’un lidar pour la mesure du contenu en eau liquide dans le brouillard / Design and prototyping of a lidar for the measurement of liquid water content in fog

Klein, Céline 20 November 2013 (has links)
L’objet de cette thèse est la conception d’un lidar dédié à la mesure du profil du contenu en eau liquide dans le brouillard. Actuellement, ce paramètre n’est mesuré que sur des volumes restreints, au sol ou à des altitudes données. Or sa connaissance devrait apporter une meilleure compréhension de l’évolution du brouillard et devrait aussi constituer une information intéressante pour le contrôle des modèles de prévision du phénomène. Elle est en théorie possible, car il a été montré à la fin des années 70 qu’il existe une relation empirique linéaire entre le contenu en eau liquide et le coefficient d’extinction optique pour une longueur d’onde de 11 μm. J’ai confirmé l’existence de cette relation sur la base d’observations récentes plus précises que celle des années 70, et ai déterminé ses limites de validité. Les brouillards se développent du sol jusqu’à quelques centaines de mètres, c’est donc sur cette gamme d’altitude que nous devons pouvoir faire la mesure. J’ai analysé les choix de conception offerts par les lidars afin de déterminer le système le plus performant pour notre application. Tout d’abord, j’ai analysé les capacités de mesure des lidars à détection directe et hétérodyne et j’ai mis en évidence que la détection hétérodyne est la plus appropriée. J’ai ensuite comparé les performances des configurations monostatique, bistatique et bistatique désaxé, et j’ai trouvé que la configuration monostatique est la plus appropriée. J’ai ensuite codé un simulateur instrumental et mis en évidence un biais de mesure lors de la restitution du coefficient d’extinction sur les premières centaines de mètres. J’ai proposé et validé une correction de ce biais. J’ai ensuite établi deux approximations analytiques pour le biais et l’écart-type de l’estimateur du coefficient d’extinction. Avec le simulateur, elles ont permis de se faire une idée de la portée et de la précision qui pourront être obtenues. / The objective of the present study is the design of a lidar for the measurement of vertical profiles of liquid water content in fogs. Presently, there is no system able to measure such profile. The liquid water content is measured at ground or at limited number of altitudes. Yet, the information would improve our understanding of fog processes and provide valuable data for controlling fog forecast models. Its feasibility is theoretically possible because it was shown in the late 70s that the liquid water content and the optical extinction at 11μm are empirically linked by a linear relationship. My first objective was to test this relationship with rencent observations more precise than in the late 70s. The relationship is confirmed within limits that I tried to determine. The vertical extension of fogs is several hundreds of meters. We thus need a lidar with a maximum range of several meters. I compared the range of a direct versus a heterodyne lidar and I found the heterodyne lidar is more appropriate. I have developed analytic approximations of the heterodyne efficiency for several transmitter configurations - monostatic, bistatic with parallel or non-parallel axes - and found the monostatic configuration gives the best results at short range. I coded a simulator for the lidar and showed the retrieval of the extinction coefficient from lidar signals with the usual signal processing technique produces biases at short range. I proposed and validated a correction scheme. I derived two analytic approximations for the bias and the standard deviation of the estimations of the extinction coefficient. They were used to estimate the practical range and accuracy a lidar can achieve for the measurement of the liquid water content in fogs.
2

Mapping surface fuels using LIDAR and multispectral data fusion for fire behavior modeling

Mutlu, Muge 15 May 2009 (has links)
Fires have become intense and more frequent in the United States. Improving the accuracy of mapping fuel models is essential for fuel management decisions and explicit fire behavior prediction for real-time support of suppression tactics and logistics decisions. This study has two main objectives. The first objective is to develop the use of LIght Detection and Ranging (LIDAR) remote sensing to assess fuel models in East Texas accurately and effectively. More specific goals include: (1) developing LIDAR derived products and the methodology to use them for assessing fuel models; (2) investigating the use of several techniques for data fusion of LIDAR and multispectral imagery for assessing fuel models; (3) investigating the gain in fuels mapping accuracy with LIDAR as opposed to QuickBird imagery alone; and, (4) producing spatially explicit digital fuel maps. The second objective is to model fire behavior using FARSITE (Fire Area Simulator) and to investigate differences in modeling outputs using fuel model maps, which differ in accuracy, in east Texas. Estimates of fuel models were compared with in situ data collected over 62 plots. Supervised image classification methods provided better accuracy (90.10%) with the fusion of airborne LIDAR data and QuickBird data than with QuickBird imagery alone (76.52%). These two fuel model maps obtained from the first objective were used to see the differences in fire growth with fuel model maps of different accuracies. According to our results, LIDAR derived data provides accurate estimates of surface fuel parameters efficiently and accurately over extensive areas of forests. This study demonstrates the importance of using accurate maps of fuel models derived using new LIDAR remote sensing techniques.
3

Process analysis of rockfalls with stationary terrestrial LiDAR and RockFall Analyst

Russell, James A. Unknown Date
No description available.
4

Die eisenzeitliche Keramik von Lidar Höyük

Müller, Uwe. January 1900 (has links)
Heidelberg, Universiẗat, Diss., 1997. / Dateien im PDF-Format. - Erscheinungsjahr an der Haupttitelstelle: 1996.
5

Comparaison de modèles numériques d'altitude de zones forestières produits par altimétrie laser et interpolation de courbes de niveau /

Dionne, Pascal. January 2003 (has links)
Thèse (M.Sc)--Université Laval, 2003. / Bibliogr. Publié aussi en version électronique.
6

Modification of the Naval Postgraduate School Lidar System

Gunal, Murat 09 1900 (has links)
The views expressed in this thesis are those of the author and do not relfect the offical policy or position of the Department of Defense or the U.S. Government. / Previous modifications were made to the Naval Postgraduate School (NPS) Lidar System to enable comparison of lidar returns with radiosonde vertical profiles. Inaccuracies due to flexing of the lidar mounting structure limited observation of correlation between lidar and balloon data. Reconstruction with anew telescope and integral mounting now provides stable overlap of laser illumination and receiver field-of-view while maintaining eye safety. This revised system permits more measurement of profile correlation. Both day and night lidar measurement series have been compared with concurrent radiosonde launches in the Monterey Bay area. Maximum ranges from clouds greater than 3500 meters by day and greater than 5000 meters by night have been achieved. Within this range comparison can be made with moderate accuracy with the temperature and pressure profile boundary layer ceiling. Recommendations are made for future enhancement of sensitivity of correlation.
7

3D et géomorphologie karstique : La grotte Chauvet et les cavités des Gorges de l'Ardèche / 3D and Geomorphology : The Chauvet cave and the caves of the Ardèche canyon

Sadier, Benjamin 13 December 2013 (has links)
Cette thèse CIFRE a pour objet d'explorer, de développer et d'exploiter le potentiel des outils de représentation et de modélisation 3D dans l'étude géomorphologique du karst dont la structure géométrique est intrinséquement en trois dimensions. Ce travail a pour support d'étude les gorges de l'Ardèche dont l'évolution géomorphologique et paléogéographique, tant aux temps géologiques qu'aux temps des hommes, est extrêmement riche. Ce territoire propose en outre des défis scientifiques mais aussi sociétaux depuis la découverte de la désormais célèbre grotte Chauvet et la mise en place de l'ERGC (Espace de Restitution de la Grotte Chauvet). C'est dans ce cadre qu'une méthodologie de recherche appliqué au domaine souterrain a été élaborée et appliquée à l'étude de la grotte Chauvet et des cavités des gorges de l'Ardèche pour (i) l'étude de la spéléogenèse et de la karstogenèse, (ii) l'évolution géomorphologique d'une grotte ornée (grotte Chauvet) et (iii) la réalisation d'un projet culturel : l'ERGC. La première partie (chapitre 1, 2 et 3) présente les verrous actuels en géomorphologie karstique, la méthodologie de recherche développée et les apports de la connaissance géomorphologique de la grotte Chauvet et de l'utilisation de modèles 3D dans la construction d'un fac-similé de grande envergure. La deuxième partie (chapitre 4, 5 et 6) présente les problématiques et les résultats de l'étude géomorphologique 3D des gorges de l'Ardèche et des cavités environnantes. Les principaux apports se situent dans la connaissance des formes élémentaires du karst et la géométrie des réseaux karstiques. Ceci a permis d'élaborer un scénario global d'évolution paléogéographique de ce secteur depuis le Néogène jusqu'à l'Actuel. Enfin la troisième partie (chapitre 7, 8 et 9) aborde les problématiques d'étude et de recherches spécifiques des grottes ornées et de leur fermeture. La grotte Chauvet et sa zone d'entrée font l'objet d'une analyse détaillée permettant de répondre aux questionnements pluridisciplinaires (archéologie, préhistoire, conservation, valorisation …). L'approche géomorphologique 3D développée pour répondre aux différentes problématiques des sciences de la Nature et des Hommes est ici posée et discutée. / This doctoral thesis aims to explore, develop and exploit the potential of 3D visualization tools and 3D modelling for research on karst morphology and geometry, intrinsically three-dimensional landscape features. This work is based on research in the gorges of the Ardeche region where geomorphological formations and paleogeographic changes are rich and varied, over both geological and human time frames. Since Chauvet Cave's discovery and the establishment of the ERGC (‘Espace de Restitution de la Grotte Chauvet'), this region has presented scientific as well as societal challenges. It is in this context, and in accordance with specificities of the subterranean condition of this karst environment, that 3D visualization and mapping tools were developed and applied at Chauvet Cave and at several other nearby Ardèche canyon caves. The aims of this research were to: (i) Study the speleogenesis and karstogenesis of the cave systems. (ii) Study the geomorphological evolution of a decorated cave (Chauvet Cave). (iii) Advise construction of the facsimile of Chauvet Cave (ERGC project). The first part of this thesis (Chapters 1, 2 and 3) discusses the challenges associated with research on karst geomorphology; the methodology employed to address those challenges; and the contribution of geomorphological knowledge and the usefulness of 3D modelling towards the construction of a facsimile of Chauvet Cave. The second part (Chapters 4, 5 and 6) presents the results of the 3D geomorphological study of the cave systems in the vicinity of the Ardèche canyon. These major results bring new knowledge of karst morphologies and geometric networks that enable a new palaeo-geographic model of landscape evolution since the Neogene to the present. The third and final part of the thesis (Chapters 7, 8 and 9) focuses on research in the prehistoric cave of Chauvet (including its entrance area). This site has been the subject of detailed multidisciplinary investigations (incorporating archaeology, conservation and heritage assessment) on questions relating to the morphology of the palaeo-entrance and the age of the cave's closure through rock collapse.
8

Turbine-Mounted Lidar:The pulsed lidar as a reliable alternative.

Braña, Isaac January 2011 (has links)
Expectations for turbine-mounted lidar are increasing. The installation of lidars in wind turbine nacelles for measuring incoming winds, preventing wind gusts and increasing energy productions is after recently studies, technically and economically feasible. Among available lidar types, the most studied were continuous wave lidars because they were the most reliable apparatus when this initiative began. However, after studying technical considerations and checking commercial lidars, it was found that pulsed lidarslead this technology due to their promising results. The purpose of this report is to fill the gap between the interest in this technology and the absence of any academic papers that analyzes continuous-wave and pulsed lidars forthe mounted lidar concept. Hence, this report discusses the importance of turbine mounted lidars for wind power industry, different possible configurations and explains why specifically pulsed lidars are becoming more important for the mounted lidarmarket.
9

Development and Progression of Aeolian Blowouts in Padre Island National Seashore

Jewell, Mallorie E 16 December 2013 (has links)
This study characterizes the development and migration of blowouts within Padre Island National Seashore (PAIS). A combination of aerial photographs and Light Detection and Ranging (LIDAR) are used to track the migration of eighteen blowouts, while Ground-penetrating Radar (GPR) is used to investigate the subsurface at two smaller sites in the study area. This data, coupled with beach morphology and changing anthropogenic factors, helps understand why the dune blowouts develop and are restricted to a particular section of the National Seashore. Aerial Photographs taken at least twice a decade since 1969 were used to track blowouts. Each blowout was digitized in order to understand its morphometric characteristics by studying its length, width, area, segmentation, perimeter, and the width of the neck, when present, through the foredune. The velocity and direction of movement were also calculated. Cluster analysis was used to analyze the blowouts using these morphological variables. Based on this data, blows appear to group into two morphologically different clusters. Blowouts grouped into Cluster 1 are longer, thinner, have smaller perimeters and areas, smaller throat widths, and are furthest from the beach access road. A lower dune elevation leads to a larger wave runup to crest height ratio. A larger ratio suggests that the dunes are more easily overtopped during large storms, thus scarping, a precursor to blowout development, is increased. Cluster 2 blowouts tend to be longer, wider, and stabilized faster leading to a more undulated perimeter in addition to a smaller wave runup potential due to a higher dune elevation. Historically blowouts covered the entire northern portion of PAIS. In the 1970’s the portion of the beach north of Park Road 22 was designated as non-driving. Since then all blowouts in this section have revegetated, while, blowouts in the driving section are still active. Beach driving pulverizes seaweed leading to less deposition along the dune toe and therefore a lower elevation of the backshore. As a result there is a greater wave runup in storms leading to an increase in susceptibility to scarping, and therefore, blowouts. Despite the fact that storms are the primary mechanism for blow development, anthropogenic effects, such as vehicle traffic, flatten the beach profile allowing for lower areas to become inundated during storms. This, along with decreased sediment budget and increased storm frequency increases the potential for blowouts to form events and leave the island vulnerable to an increased rate of sea level rise. GPR surveys were completed at two sites; an active blowout with a foredune that is not completely reestablished (Site 1) and a blowout that is stabilized by vegetation (Site 2). Six GPR surveys were completed at Site 1 and four surveys were completed at Site 2 that show the preservation of historic phases, surfaces, and facies used to interpret sequences and compare to aerial photography and LiDAR data. Site 1 moves through five phases that begin in 1969 and end at the present location, while Site 2 moves through three active phases and then ends in a fourth phase by becoming completely stabilized with vegetation in 2010.
10

Studying Clouds and Aerosols with Lidar Depolarization Ratio and Backscatter Relationships

Cho, Hyoun-Myoung 2011 December 1900 (has links)
This dissertation consists of three parts, each devoted to a particular issue of significant importance for CALIPSO lidar observation of depolarization ratio (delta) and backscatter (gamma?) to improve current understanding of the microphysical properties of clouds and aerosols. The relationships between depolarization ratio and backscatter allow us to retrieve particle thermodynamic phase and shape and/or orientation of aerosols and clouds. The first part is devoted to the investigation of the relationships between lidar backscatter and the corresponding depolarization ratio for different cloud classifications and aerosol types. For each cloud and aerosol types, layer-averaged backscatter and backscattering depolarization ratio from the CALIPSO measurements are discussed. The present results demonstrate the unique capabilities of the CALIPSO lidar instrument for determining cloud phase and aerosols subtypes. In the second part, we evaluate the MODIS IR cloud phase with the CALIPSO cloud products. The three possible misclassifications of MODIS IR cloud phasealgorithm, which are studied by Nasiri and Kahn (2008) with radiative transfer modeling, are tested by comparing between MODIS IR phase and CALIOP observations. The current results support their hypotheses, which is that the MODIS phase algorithm may tend to classify thin cirrus clouds as water clouds or mixed phase clouds or unknown, and classify midlevel and/or mid-temperature clouds as mixed or unknown phase. In the third part, we present a comparison of mineral dust aerosol retrievals from two instruments, MODIS and CALIPSO lidar. And, we implement and evaluate a new mineral dust detection algorithm based on the analysis of thin dust radiative signature. In comparison, three commonly used visible and IR mineral dust detection algorithms, including BTD procedure, D parameter method, and multi-channel image algorithm, are evaluated with CALIPSO aerosol classification. The comparison reveals that those dust detection algorithms are not effective for optically thin dust layers, but for thick dust storm. The new algorithm using discriminant analysis with CALIPSO observation is much better in detecting thin dust layer of optical thickness between 0.1 and 2.

Page generated in 0.0522 seconds