全文获取类型
收费全文 | 3822篇 |
免费 | 322篇 |
国内免费 | 471篇 |
专业分类
测绘学 | 1350篇 |
大气科学 | 292篇 |
地球物理 | 282篇 |
地质学 | 1081篇 |
海洋学 | 271篇 |
天文学 | 449篇 |
综合类 | 325篇 |
自然地理 | 565篇 |
出版年
2024年 | 17篇 |
2023年 | 46篇 |
2022年 | 122篇 |
2021年 | 148篇 |
2020年 | 189篇 |
2019年 | 190篇 |
2018年 | 120篇 |
2017年 | 194篇 |
2016年 | 175篇 |
2015年 | 176篇 |
2014年 | 169篇 |
2013年 | 255篇 |
2012年 | 186篇 |
2011年 | 206篇 |
2010年 | 155篇 |
2009年 | 226篇 |
2008年 | 232篇 |
2007年 | 263篇 |
2006年 | 206篇 |
2005年 | 159篇 |
2004年 | 148篇 |
2003年 | 150篇 |
2002年 | 119篇 |
2001年 | 98篇 |
2000年 | 95篇 |
1999年 | 93篇 |
1998年 | 77篇 |
1997年 | 60篇 |
1996年 | 55篇 |
1995年 | 43篇 |
1994年 | 52篇 |
1993年 | 39篇 |
1992年 | 30篇 |
1991年 | 16篇 |
1990年 | 23篇 |
1989年 | 17篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1975年 | 1篇 |
1973年 | 7篇 |
1972年 | 1篇 |
1971年 | 3篇 |
排序方式: 共有4615条查询结果,搜索用时 15 毫秒
1.
New Earth observation missions and technologies are delivering large amounts of data. Processing this data requires developing and evaluating novel dimensionality reduction approaches to identify the most informative features for classification and regression tasks. Here we present an exhaustive evaluation of Guided Regularized Random Forest (GRRF), a feature selection method based on Random Forest. GRRF does not require fixing a priori the number of features to be selected or setting a threshold of the feature importance. Moreover, the use of regularization ensures that features selected by GRRF are non-redundant and representative. Our experiments based on various kinds of remote sensing images, show that GRRF selected features provides similar results to those obtained when using all the available features. However, the comparison between GRRF and standard random forest features shows substantial differences: in classification, the mean overall accuracy increases by almost 6% and, in regression, the decrease in RMSE almost reaches 2%. These results demonstrate the potential of GRRF for remote sensing image classification and regression. Especially in the context of increasingly large geodatabases that challenge the application of traditional methods. 相似文献
2.
本文综述了ASCA ,RXTE ,BeppoSAX和XMM -NEWTON对TeV耀变体 (Blazar)X射线研究的最新进展。主要集中在X射线资料的时间和光谱分析的最重要的方面 ,包括功率谱密度 ,结构函数 ,带间时间延迟 ,同步峰值能量的漂移和谱演化。对这些新结果的物理意义也进行了简要讨论。 相似文献
3.
Using statistical orbital ranging, we systematically study the orbit computation problem for transneptunian objects (TNOs). We have automated orbit computation for large numbers of objects, and, more importantly, we are able to obtain orbits even for the most sparsely observed objects (observational arcs of a few days). For such objects, the resulting orbit distributions include a large number of high-eccentricity orbits, in which TNOs can be perturbed by close encounters with Neptune. The stability of bodies on the computed orbits has therefore been ascertained by performing a study of close encounters with the major planets. We classify TNO orbit distributions statistically, and we study the evolution of their ephemeris uncertainties. We find that the orbital element distributions for the most numerous single-apparition TNOs do not support the existence of a postulated sharp edge to the belt beyond 50 AU. The technique of statistical ranging provides ephemeris predictions more generally than previously possible also for poorly observed TNOs. 相似文献
4.
We examine possible locations for the primordial disk of the Edgeworth-Kuiper Belt (EKB), using several subsets of the known objects as markers of the total mass distribution. Using a secular perturbation theory, we find that the primordial plane of the EKB could have remained thin enough to escape detection only if it is clustered very closely about the invariable plane of the Solar System. 相似文献
5.
6.
L. K. Erastova 《Astrophysics》2004,47(4):547-554
Some features of a sample of galaxies from the Second Byurakan Survey are discussed. Most of them are small galaxies with star formation. It is shown that different types of galaxies are in the Survey, with high, medium, and low levels of excitation and continua ranging from blue to relatively red. Many of the galaxies have absorption lines in their spectra; their continua are formed by stars in later spectral classes. 相似文献
7.
Mark D. Schwartz 《The Professional geographer》1991,43(1):77-91
Both subjective and objective schemes for air mass classification offer advantages to climatological studies. An integrated approach is developed and applied to 850 mb level data in the north central United States. Each air mass type was initially delimited by a subjective analysis. Final limits were then determined with an objective technique that separates temperature curves into normal components based on interpretation from the subjective method. Continental air was most frequent in all seasons except summer. The methodology also allows estimation of classification error. 相似文献
8.
Dan Gill 《Mathematical Geology》1993,25(4):471-482
The ability of association analysis to discriminate sedimentary facies was tested on Purdy's modal analyses of modern sediments of the Great Bahama Bank. Purdy's data set has served in the past as a standard reference for evaluating various multivariate classification algorithms. In order to adapt Purdy's data to association analysis, the percent abundance of the 12 constituents was converted to binary form by dichotomizing each variable on its mean value. The results obtained by association analysis are virtually identical to those obtained by Purdy and other authors. The same four main sedimentary facies were discriminated; 86% of the samples were identically classified (97% when misclassified borderline cases are counted as matches); the total partition variance of the classification is only negligibly greater (4%); and the grouping of the variables yielded the same four groups. The rank order of the three division-attributes responsible for the sample classification is fines, oolites, and corals. Association analysis has been employed by other authors to differentiate meaningful facies groups in studies of ancient reef carbonates, modern reef sediments, and heavy minerals in stream sediments. In all these studies, the results were found to be compatible with those obtained by using the continuous quantitative measurements, indicating that qualitative binary data may often be sufficient for the purpose of facies discrimination in many branches of geology and that association analysis is an effective method for this purpose. 相似文献
9.
Wing-Huen Ip 《Celestial Mechanics and Dynamical Astronomy》2003,87(1-2):197-202
In the context of the survival of periodic comets of different origins, rotational breakup and tidal disruption could be important, especially of the short period comets injected from the Kuiper belt. This is because long-period comets from the distant Oort cloud tend to be subject to thermal stress and volatile 'explosion' far more severely. A simple calculation using the Öpik method of random planetary close encounters was performed to estimate the probability of tidal disruption of comets and scattered Kuiper belt objects (SKBOs) during their orbital migration. It was found that a large fraction of the short period comets and SKBOs might have been internally fragmented by single or multiple close encounters with the outer planets. 相似文献
10.
S. Viti 《Astrophysics and Space Science》2003,285(3-4):791-800
In this article, some aspects of the clumpy nature of molecular clouds are reviewed. In particular the observational evidence
for small-scale structures both in low and high mass star forming regions will be discussed. I will review some examples of
`clumpiness' such as: i) the molecular clumps ahead of HH objects and how the study of the physical and chemical nature of
these clumps is important for the understanding of the clumpiness of the Interstellar Medium; and ii)hot cores and their use
as a tool to study the early phases of massive star formation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献