全文获取类型
收费全文 | 12817篇 |
免费 | 929篇 |
国内免费 | 974篇 |
专业分类
测绘学 | 3118篇 |
大气科学 | 919篇 |
地球物理 | 1068篇 |
地质学 | 2092篇 |
海洋学 | 972篇 |
天文学 | 413篇 |
综合类 | 1045篇 |
自然地理 | 5093篇 |
出版年
2024年 | 55篇 |
2023年 | 135篇 |
2022年 | 555篇 |
2021年 | 633篇 |
2020年 | 594篇 |
2019年 | 668篇 |
2018年 | 504篇 |
2017年 | 649篇 |
2016年 | 658篇 |
2015年 | 655篇 |
2014年 | 707篇 |
2013年 | 1004篇 |
2012年 | 768篇 |
2011年 | 753篇 |
2010年 | 566篇 |
2009年 | 663篇 |
2008年 | 626篇 |
2007年 | 714篇 |
2006年 | 659篇 |
2005年 | 530篇 |
2004年 | 509篇 |
2003年 | 386篇 |
2002年 | 346篇 |
2001年 | 243篇 |
2000年 | 205篇 |
1999年 | 164篇 |
1998年 | 127篇 |
1997年 | 103篇 |
1996年 | 90篇 |
1995年 | 69篇 |
1994年 | 68篇 |
1993年 | 55篇 |
1992年 | 66篇 |
1991年 | 29篇 |
1990年 | 32篇 |
1989年 | 29篇 |
1988年 | 21篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 22篇 |
1984年 | 12篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1971年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
This study aims at evaluating the global geoid model for a regional shoreline fitting using advanced soft computing techniques and global navigation satellite system/leveling measurements. Artificial neural networks, fuzzy logic, and least square support vector machine models are developed and used to fit the global geoid model for the north coastal Egyptian line. In addition, a novel estimation geoid model is designed and evaluated based on the latest global geoid models. The results of the three estimation models show that they can be used to correct the shoreline geoid model, in terms of root mean square error that ranges from 1.7 to 8.5?cm. Moreover, it is found that the least square vector machine model is a competitive approach with certain advantage in solving complex problems represented by missing data. 相似文献
63.
中国省域煤矿事故时空分异特征及影响因素研究 总被引:1,自引:0,他引:1
煤矿事故给国家和人民带来巨大的生命财产损失,科学把握煤矿事故发生的宏观规律及区域煤矿安全的影响因素有助于促进煤矿安全生产。本文尝试采用GIS空间分析方法开展区域煤矿事故的研究,基于空间自相关性、时空演化规律方法研究了2006-2015年中国省域煤矿事故的时空分异特点,并分析了省域煤矿安全的影响因素及相对风险度。主要结论:①中国各省区之间煤矿事故存在明显的空间自相关性,但随着各省区煤矿安全生产形势改善情况的不同,其自相关性在不断下降;②以空间重心转移曲线分析中国煤矿事故发生的时空演化规律,10年间煤矿事故空间重心向东北方向转移。③重特大事故频发的省区由于监管措施的有力执行往往具有较低的煤矿百万吨死亡率;平均煤矿生产能力低的省区具有较高的煤矿百万吨死亡率;④建立基于解释变量的贝叶斯空间模型,以煤炭百万吨死亡率为指标评估中国各省区煤矿事故相对风险度,中国煤矿安全生产相对危险度存在明显的空间分布规律且各省区差异较大,相对危险度较高的省区主要集中在南方地区,包括湖北、福建等,危险度较低的省区主要集中在中东部地区,包括山西、内蒙古等。 相似文献
64.
This paper proposes a scheme of axial triple-well optical dipole trap by employing a simple optical system composed of a circular cosine grating and a lens. Three optical wells separated averagely by ~37 μm were created when illuminating by a YAG laser with power 1 mW. These wells with average trapping depth ~0.5 μK and volume ~74 μm3 are suitable to trap and manipulate an atomic Bose--Einstein condensation (BEC). Due to a controllable grating implemented by a spatial light modulator, an evolution between a triple-well trap and a single-well one is achievable by adjusting the height of potential barrier between adjacent wells. Based on this novel triple-well potentials, the loading and splitting of BEC, as well as the interference between three freely expanding BECs, are also numerically stimulated within the framework of mean-field treatment. By fitting three cosine functions with three Gaussian envelopes to interference fringe, the information of relative phases among three condensates is extracted. 相似文献
65.
In most fisheries catch per unit effort shows significant seasonal and spatial variation. Traditionally, i.e. when vessels are free to choose their harvesting strategy, 80% of the Norwegian cod is landed during the winter in a limited geographical area. To alter the seasonal and spatial supply of cod to the fish processing industry, rural community quotas were introduced. In this paper, we develop a model to predict how different vessels will adapt to the quotas introduced. The hypotheses developed are mostly confirmed in an empirical study. Theoretical, methodological, and managerial implications of the findings are discussed. 相似文献
66.
Melissa M. Foley Benjamin S. Halpern Fiorenza Micheli Matthew H. Armsby Margaret R. Caldwell Caitlin M. Crain Erin Prahler Nicole Rohr Deborah Sivas Michael W. Beck Mark H. Carr Larry B. Crowder J. Emmett Duffy Sally D. Hacker Karen L. McLeod Stephen R. Palumbi Charles H. Peterson Helen M. Regan Mary H. Ruckelshaus Paul A. Sandifer Robert S. Steneck 《Marine Policy》2010
The declining health of marine ecosystems around the world is evidence that current piecemeal governance is inadequate to successfully support healthy coastal and ocean ecosystems and sustain human uses of the ocean. One proposed solution to this problem is ecosystem-based marine spatial planning (MSP), which is a process that informs the spatial distribution of activities in the ocean so that existing and emerging uses can be maintained, use conflicts reduced, and ecosystem health and services protected and sustained for future generations. Because a key goal of ecosystem-based MSP is to maintain the delivery of ecosystem services that humans want and need, it must be based on ecological principles that articulate the scientifically recognized attributes of healthy, functioning ecosystems. These principles should be incorporated into a decision-making framework with clearly defined targets for these ecological attributes. This paper identifies ecological principles for MSP based on a synthesis of previously suggested and/or operationalized principles, along with recommendations generated by a group of twenty ecologists and marine scientists with diverse backgrounds and perspectives on MSP. The proposed four main ecological principles to guide MSP—maintaining or restoring: native species diversity, habitat diversity and heterogeneity, key species, and connectivity—and two additional guidelines, the need to account for context and uncertainty, must be explicitly taken into account in the planning process. When applied in concert with social, economic, and governance principles, these ecological principles can inform the designation and siting of ocean uses and the management of activities in the ocean to maintain or restore healthy ecosystems, allow delivery of marine ecosystem services, and ensure sustainable economic and social benefits. 相似文献
67.
Map sheets have been often used as a basic spatial unit for managing spatial data produced from paper maps. This often results
in incompatibility between adjacent map sheets, because spatial objects do not cross the boundaries smoothly and even the
boundaries themselves do not match their neighbors exactly. To solve the problem this paper proposes a computational procedure
for joining separate map sheets to obtain seamless spatial data. Line objects digitized separately in different map sheets
are considered, which are frequently used to represent road networks, gas pipelines, and boundaries of polygon objects. The
procedure consists of three steps: (1) extraction of end nodes, (2) detection of matching nodes, and (3) transformation of
the map sheet. Each step goes interactively so that unexpected errors can be avoided by human observation. To test the validity
of the procedure, map sheets are combined containing the road network data of Tokyo 23-ku area, Japan.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
68.
69.
70.
Marc G. Genton 《Mathematical Geology》1998,30(7):853-871
In the context of robust statistics, the breakdown point of an estimator is an important feature of reliability. It measures the highest fraction of contamination in the data that an estimator can support before being destroyed. In geostatistics, variogram estimators are based on measurements taken at various spatial locations. The classical notion of breakdown point needs to be extended to a spatial one, depending on the construction of most unfavorable configurations of perturbation. Explicit upper and lower bounds are available for the spatial breakdown point in the regular unidimensional case. The difficulties arising in the multidimensional case are presented on an easy example in IR2
, as well as some simulations on irregular grids. In order to study the global effects of perturbations on variogram estimators, further simulations are carried out on data located on a regular or irregular bidimensional grid. Results show that if variogram estimation is performed with a 50% classical breakdown point scale estimator, the number of initial data likely to be contaminated before destruction of the estimator is roughly 30% on average. Theoretical results confirm the previous statement on data in IRd
, d 1. 相似文献