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991.
992.
Markov-random-field-based super-resolution mapping for identification of urban trees in VHR images 总被引:1,自引:0,他引:1
Juan P. Ardila Valentyn A. Tolpekin Wietske Bijker Alfred Stein 《ISPRS Journal of Photogrammetry and Remote Sensing》2011,66(6):762-775
Identification of tree crowns from remote sensing requires detailed spectral information and submeter spatial resolution imagery. Traditional pixel-based classification techniques do not fully exploit the spatial and spectral characteristics of remote sensing datasets. We propose a contextual and probabilistic method for detection of tree crowns in urban areas using a Markov random field based super resolution mapping (SRM) approach in very high resolution images. Our method defines an objective energy function in terms of the conditional probabilities of panchromatic and multispectral images and it locally optimizes the labeling of tree crown pixels. Energy and model parameter values are estimated from multiple implementations of SRM in tuning areas and the method is applied in QuickBird images to produce a 0.6 m tree crown map in a city of The Netherlands. The SRM output shows an identification rate of 66% and commission and omission errors in small trees and shrub areas. The method outperforms tree crown identification results obtained with maximum likelihood, support vector machines and SRM at nominal resolution (2.4 m) approaches. 相似文献
993.
Accuracy assessment of the GPS-TEC calibration constants by means of a simulation technique 总被引:2,自引:2,他引:0
During the last 2 decades, Global Positioning System (GPS) measurements have become a very important data-source for ionospheric
studies. However, it is not a direct and easy task to obtain accurate ionospheric information from these measurements because
it is necessary to perform a careful estimation of the calibration constants affecting the GPS observations, the so-called
differential code biases (DCBs). In this paper, the most common approximations used in several GPS calibration methods, e.g.
the La Plata Ionospheric Model (LPIM), are applied to a set of specially computed synthetic slant Total Electron Content datasets
to assess the accuracy of the DCB estimation in a global scale scenario. These synthetic datasets were generated using a modified
version of the NeQuick model, and have two important features: they show a realistic temporal and spatial behavior and all
a-priori DCBs are set to zero by construction. Then, after the application of the calibration method the deviations from zero
of the estimated DCBs are direct indicators of the accuracy of the method. To evaluate the effect of the solar activity radiation
level the analysis was performed for years 2001 (high solar activity) and 2006 (low solar activity). To take into account
seasonal changes of the ionosphere behavior, the analysis was repeated for three consecutive days close to each equinox and
solstice of every year. Then, a data package comprising 24 days from approximately 200 IGS permanent stations was processed.
In order to avoid unwanted geomagnetic storms effects, the selected days correspond to periods of quiet geomagnetic conditions.
The most important results of this work are: i) the estimated DCBs can be affected by errors around ±8 TECu for high solar
activity and ±3 TECu for low solar activity; and ii) DCB errors present a systematic behavior depending on the modip coordinate,
that is more evident for the positive modip region. 相似文献
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Juan Carlos Linares Antonio Delgado-Huertas Jos�� Antonio Carreira 《Climatic change》2011,105(1-2):67-90
The projected temperature rise, rainfall decrease and concentration of rainfall in extreme events could induce growth decline and die-off on tree populations located at the geographical distribution limit of the species. Understanding of adaptive capacity and regional vulnerability to climate change in Mediterranean forests is not well developed and requires more focused research efforts. We studied the relationships between spatiotemporal patterns of temperature and precipitation along the southwestern edge of the Betic range (southern Spain) and measured basal area increment (BAI) and carbon isotope (??) in tree ring series of Abies pinsapo and Pinus halepensis, two Mediterranean conifer trees with contrasting drought adaptive capacity. Climatic information was obtained from a network covering a wide range of elevations and distances from the Atlantic and Mediterranean coasts. Temperature trends were tested by the Mann?CKendall test, and precipitation was thoroughly analyzed by quantile regression. Climatic data showed a warming trend, enhanced since the 1970s, while quantile regressions revealed that drought events worsened during the course of the twentieth century. Long-term decrease of A. pinsapo BAI was related to regional warming and changing precipitation patterns, suggesting increasing drought stress on this species. Both temperature and precipitation in the summer influenced wood ?? in P. halepensis, whereas negative correlation between wood ?? and current autumn temperature was yielded for A. pinsapo. Increased intrinsic water use efficiency was inferred from wood ?? in both species; however, A. pinsapo showed sudden growth reductions under drier conditions, while pine trees were able to maintain almost constant BAI values and lower water costs under increasing long-term water stress. 相似文献
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1000.
Precipitation change and human impacts on hydrologic variables in Zhengshui River Basin, China 总被引:4,自引:3,他引:1
Juan Du Fei He Zhao Zhang Peijun Shi 《Stochastic Environmental Research and Risk Assessment (SERRA)》2011,25(7):1013-1025
Climate change and anthropogenic impacts on hydrologic variables have received significant attention in recent years. We assessed
stream flow and water level in the Zhengshui River basin, China, in the period 1960 to 2003 in response to precipitation variation
and anthropogenic factors. Analyses of daily discharge and water level records, and derived annual, seasonal and monthly series,
showed that Zhengshui River flows had a significant increasing trend, with an abrupt change point in 1990. There was a significant
decreasing water level trend with an abrupt change point in 1995. Human activities and precipitation contributed 53 and 47%,
respectively, to increase in stream flow during 1991–2003. Anthropogenic activities such as sand dredging, dominated the decrease
in water level during 1994–2003. Human-induced land use change, soil erosion and sand excavation driven by rapid economic
development have played a more important role than precipitation variation in hydrological changes in the Zhengshui River
basin over the past 50 years. 相似文献