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281.
Quality Assessment of the French OpenStreetMap Dataset 总被引:9,自引:1,他引:8
The concept of Volunteered Geographic Information (VGI) has recently emerged from the new Web 2.0 technologies. The OpenStreetMap project is currently the most significant example of a system based on VGI. It aims at producing free vector geographic databases using contributions from Internet users. Spatial data quality becomes a key consideration in this context of freely downloadable geographic databases. This article studies the quality of French OpenStreetMap data. It extends the work of Haklay to France, provides a larger set of spatial data quality element assessments (i.e. geometric, attribute, semantic and temporal accuracy, logical consistency, completeness, lineage, and usage), and uses different methods of quality control. The outcome of the study raises questions such as the heterogeneity of processes, scales of production, and the compliance to standardized and accepted specifications. In order to improve data quality, a balance has to be struck between the contributors' freedom and their respect of specifications. The development of appropriate solutions to provide this balance is an important research issue in the domain of user‐generated content. 相似文献
282.
David V. Bekaert Guillaume Avice Bernard Marty 《Meteoritics & planetary science》2018,53(6):1238-1251
Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm-sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO2 laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic 21Ne, 38Ar, 83Kr, and 126Xe, respectively, produced over the basalt's 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for 3He and 21Ne, and to a lesser extent for 36Ar (Signer et al. 1977 ), also occurs to a negligible extent for the heaviest noble gases Kr and Xe. 相似文献
283.
Jean-Pierre Rivet Farrokh Vakili Olivier Lai David Vernet Mathilde Fouché William Guerin Guillaume Labeyrie Robin Kaiser 《Experimental Astronomy》2018,46(3):531-542
More than sixty years after the first intensity correlation experiments by Hanbury Brown and Twiss, there is renewed interest for intensity interferometry techniques for high angular resolution studies of celestial sources. We report on a successful attempt to measure the bunching peak in the intensity correlation function for bright stellar sources with 1 meter telescopes (I2C project). We propose further improvements of our preliminary experiments of spatial interferometry between two 1 m telescopes, and discuss the possibility to export our method to existing large arrays of telescopes. 相似文献
284.
Zadig Mouradian Guillaume Prevot Germaine Zlicaric Adriana Galvao Garcia 《Solar physics》1995,158(2):379-381
The synoptic charts of the filaments and active regions near the solar limb are given for the eclipse of November 3, 1994, along with the positions of the prominences. A table helps to associate coronal and chromospheric structures. 相似文献
285.
Guillaume Martelet Nicole Debéglia Catherine Truffert 《Comptes Rendus Geoscience》2002,334(7):449-454
For regional and national study purposes, there is a high need for updating the terrain corrections (TC) in the French gravity database. We have recomputed the TC for all the French gravity stations from 50 m out to a distance of 167 km. We compute the TC with a flat-top-prism algorithm and three DEM with grid spacing of 50, 250 and 1000 m, used in the zones 53 m/3 km, 3 km/10 km et 10 km/167 km, respectively. Analysing the DEM/station Δz and comparing our results to the ones previously obtained in the Alps area, we estimate the accuracy of our TC to be better than 1 mGal. To cite this article: G. Martelet et al., C. R. Geoscience 334 (2002) 449–454. 相似文献
286.
Jean-Pierre Girard 《Chemical Geology》2002,191(4):329-343
Oxygen isotopic compositions of weathering goethite, hematite, kaolinite and gibbsite in duricrust paleosurfaces of the Cayenne region, northeastern French Guiana, were investigated. Samples were microdrilled from different horizons and different pedogenetic microfacies of three well-characterized duricrust units of Eocene (unit 1) and Miocene (units 2 and 3) age. The results provided valuable information on duricrust formation processes and on climate change over French Guiana.The pristine original isotopic signal, suited for reconstruction of past climates, appears to be best preserved in the massive duricrust facies (middle part of the duricrust). In the overlying pisolotic duricrust and in the underlying mottled zone, the isotopic signal is affected by subsequent re-activation of weathering processes. Oxygen isotope data from massive duricrust indicate that Cayenne duricrusts formed from meteoric waters increasingly depleted in 18O with age. The youngest duricrust paleosurface (unit 3) formed under humid climate similar to present-day, while the two older paleosurfaces formed from meteoric paleo-water distinctly depleted in 18O, reflecting a “moonsonal” contrasted ancient climate for unit 2 added to the 18O-depleted nature of the pre-Miocene ocean for unit 1. The results further suggest that the major climatic change, from tropical wet-and-dry to present-day humid regime, recorded in Amazonia most likely occurred in the Late Miocene, and that Miocene laterization events may be responsible for the secondary bauxitization of the Eocene duricrust paleosurface.The oxygen isotopic composition of weathering minerals occurring in massive duricrust horizons, and in particular in hematite nodules and matrix, may be used as a tool to discriminate among the different duricrust paleosurfaces encountered in Amazonia and constrain the evolution of past climates. 相似文献
287.
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290.
Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years 总被引:1,自引:0,他引:1
Yannick Garcin David Williamson Laurent Bergonzini Olivier Radakovitch Annie Vincens Guillaume Buchet Joël Guiot Simon Brewer Pierre-Etienne Mathé Amos Majule 《Journal of Paleolimnology》2007,37(4):475-490
The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years.
Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic
carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming
that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response
varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the
clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble
the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation
supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact
is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this
remote basin during colonial and post-colonial times. 相似文献