全文获取类型
收费全文 | 82388篇 |
免费 | 2087篇 |
国内免费 | 2958篇 |
专业分类
测绘学 | 2988篇 |
大气科学 | 6975篇 |
地球物理 | 15814篇 |
地质学 | 32839篇 |
海洋学 | 6840篇 |
天文学 | 14539篇 |
综合类 | 3000篇 |
自然地理 | 4438篇 |
出版年
2022年 | 671篇 |
2021年 | 971篇 |
2020年 | 978篇 |
2019年 | 991篇 |
2018年 | 6310篇 |
2017年 | 5508篇 |
2016年 | 4627篇 |
2015年 | 1490篇 |
2014年 | 2195篇 |
2013年 | 3234篇 |
2012年 | 3073篇 |
2011年 | 5217篇 |
2010年 | 4321篇 |
2009年 | 5119篇 |
2008年 | 4317篇 |
2007年 | 4809篇 |
2006年 | 2488篇 |
2005年 | 2004篇 |
2004年 | 2196篇 |
2003年 | 2116篇 |
2002年 | 1899篇 |
2001年 | 1520篇 |
2000年 | 1468篇 |
1999年 | 1315篇 |
1998年 | 1342篇 |
1997年 | 1222篇 |
1996年 | 1045篇 |
1995年 | 1018篇 |
1994年 | 896篇 |
1993年 | 774篇 |
1992年 | 743篇 |
1991年 | 684篇 |
1990年 | 713篇 |
1989年 | 563篇 |
1988年 | 554篇 |
1987年 | 564篇 |
1986年 | 509篇 |
1985年 | 631篇 |
1984年 | 698篇 |
1983年 | 605篇 |
1982年 | 579篇 |
1981年 | 535篇 |
1980年 | 498篇 |
1979年 | 488篇 |
1978年 | 461篇 |
1977年 | 371篇 |
1976年 | 353篇 |
1975年 | 359篇 |
1974年 | 308篇 |
1973年 | 343篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
311.
P.V. Radhadevi S.S. Solanki M.V. Jyothi V. Nagasubramanian Geeta Varadan 《ISPRS Journal of Photogrammetry and Remote Sensing》2009,64(1):17-26
Three multi-spectral bands of the Liss-4 camera of IRS-P6 satellite are physically separated in the focal plane in the along-track direction. The time separation of 2.1 s between the acquisition of first and last bands causes scan lines acquired by different bands to lie along different lines on the ground which are not parallel. Therefore, the raw images of multi-spectral bands need to be registered prior to any simple application like data visualization. This paper describes a method for co-registration of multiple bands of Liss-4 camera through photogrammetric means using the collinearity equations. A trajectory fit using the given ephemeris and attitude data, followed by direct georeferencing is being employed in this model. It is also augmented with a public domain DEM for the terrain dependent input to the model. Finer offsets after the application of this parametric technique are addressed by matching a small subsection of the bands (100×100 pixels) using an image-based method. Resampling is done by going back to original raw data when creating the product after refining image coordinates with the offsets. Two types of aligned products are defined in this paper and their operational flow is described. Datasets covering different types of terrain and also viewed with different geometries are studied with extensive number of points. The band-to-band registration (BBR) accuracies are reported. The algorithm described in this paper for co-registration of Liss-4 bands is an integral part of the software package Value Added Products generation System (VAPS) for operational generation of IRS-P6 data products. 相似文献
312.
Recent data on the Tully–Fisher relation for spiral galaxies are compatible with the traditional correlation for astrophysical
systems, where the angular momentum varies as the square of the mass. Such a correlation is consistent with standard gravitational
theory, but is not explained by it. We here show that the noted relation follows from currently popular accounts of extended
or higher-dimensional gravitational theory. The latter also predicts that the spins of spirals should decay as the universe
expands, which can be tested by extending the Tully–Fisher data to higher redshifts. 相似文献
313.
大都市区的形成机制及其定界:以北京为例 总被引:49,自引:1,他引:49
源于欧美的大都市区,在中国具有特殊的形成机制和联系形式。对北京的研究表明,其形成主要由中心城市工业化的扩散和农村非农业化两种过程所决定,并据此提出了中国大都市区定界的初步方法。进一步研究还发现中国已经出现了都市连绵带。 相似文献
314.
315.
Decorrelated GRACE time-variable gravity solutions by GFZ,and their validation using a hydrological model 总被引:8,自引:6,他引:8
We have analyzed recent gravity recovery and climate experiment (GRACE) RL04 monthly gravity solutions, using a new decorrelating
post-processing approach. We find very good agreement with mass anomalies derived from a global hydrological model. The post-processed
GRACE solutions exhibit only little amplitude damping and an almost negligible phase shift and period distortion for relevant
hydrological basins. Furthermore, these post-processed GRACE solutions have been inspected in terms of data fit with respect
to the original inter-satellite ranging and to SLR and GPS observations. This kind of comparison is new. We find variations
of the data fit due to solution post-processing only within very narrow limits. This confirms our suspicion that GRACE data
do not firmly ‘pinpoint’ the standard unconstrained solutions. Regarding the original Kusche (J Geod 81:733–749, 2007) decorrelation
and smoothing method, a simplified (order-convolution) approach has been developed. This simplified approach allows to realize
a higher resolution—as necessary, e.g., for generating computed GRACE observations—and needs far less coefficients to be stored. 相似文献
316.
Soil sediment samples of 10 layers with a spacing of 10 cm each were collected in different floodplain zones adjacent to Huolin
River in the Xianghai Nature Reserve, and contents of total N, total P and organic matters were analyzed. The results showed
that contents of total N, total P and organic matters were generally decreasing with the increase of distance from sample
locations to the river channel, and contents of the three items were generally higher in the upper soil layer than that in
the lower soil layer. The content variations displayed how flooding functions influenced nutrient matter content variations
in floodplain soils since the flood inundation frequencies of the sample locations varied. The correlation analysis displayed
that there were remarkable relativities between total N, total P and organic matters within definite spatial distance from
the Huolin River channel. 相似文献
317.
Potential promoted productivity and spatial patterns of medium- and low-yield cropland land in China 总被引:1,自引:1,他引:1
With a continuously increasing population and better food consumption levels, improving the efficiency of arable land use and increasing its productivity have become fundamental strategies to meet the growing food security needs in China. A spatial distribution map of medium- and low-yield cropland is necessary to implement plans for cropland improvement. In this study, we developed a new method to identify high-, medium-, and low-yield cropland from Moderate Resolution Imaging Spectroradiometer (MODIS) data at a spatial resolution of 500 m. The method could be used to reflect the regional heterogeneity of cropland productivity because the classification standard was based on the regionalization of cropping systems in China. The results showed that the proportion of high-, medium-, and low-yield cropland in China was 21%, 39%, and 40%, respectively. About 75% of the low-yield cropland was located in hilly and mountainous areas, and about 53% of the high-yield cropland was located in plain areas. The five provinces with the largest area of high-yield cropland were all located in the Huang-Huai-Hai region, and the area amounted to 42% of the national high-yield cropland area. Meanwhile, the proportion of high-yield cropland was lower than 15% in Heilongjiang, Sichuan, and Inner Mongolia, which had the largest area allocated to cropland in China. If all the medium-yield cropland could be improved to the productive level of high-yield cropland and the low-yield cropland could be improved to the level of medium-yield cropland, the total productivity of the land would increase 19% and 24%, respectively. 相似文献
318.
The accumulation of sediment in river channels is a phenomenon that is not only influenced by the channel morphology, but also by the physical and geographical characteristics and the endogenous and exogenous processes taking place in the catchment. This paper presents an analysis of the impact the changes in lithological conditions have on the morphological and morphometric parameters of the Udava River channel and their relation to the channel accumulation forms representative of the river’s longitudinal profile as well as of its planform. Results document when accumulation forms occur and what is their spatial distribution within the longitudinal and cross-sectional river profiles. More resistant structures created sections with a lower degree of sedimentation, while in depression segments the degree of sedimentation was higher. With the increase in longitudinal slope, the impact of channel width on the average channel bar size increased. Also a difference in the accumulation was observed between the left and right bank which could be possibly explained by the impact of Coriolis force. 相似文献
319.
This work is basically concerned with grain nucleation occurring in protostellar envelopes. On the basis of the dissociation equilibrium theory, molecular and atomic abundances are obtained for massive protostar envelopes. Application of time-independent homogeneous nucleation theory results in the possibility of atomic Fe condensation. 相似文献
320.
In order to thoroughly investigate the diversity of glacier microorganisms, four DNA extraction methods with different lysis patterns
were tested and two screened methods (the Bosshard-Bano method and the Zhou method) were optimized for the most effective
form of the filter membrane (cut vs. uncut), the DNA extraction method, and the precipitation method. The two optimized
methods were then compared with the commercial Mo-Bio DNA extraction kit, and the results showed that the kit was generally
suitable for extraction of microorganism DNA from glacier surface snow. Procedurally, it was found that a modified Bosshard-
Bano method (i.e., cutting the filter membrane into pieces, using a specific lysis pattern [lysozyme (5 mg/mL)-protease K
(1 mg/mL)-CTAB (1%)-SDS (1%)], performing the extraction only once by chloroform-isoamyl alcohol (24:1), and conducting
DNA precipitation by pure ethanol) was also an effective and less expensive method for extraction of microorganism DNA from
glacier surface snow. 相似文献