全文获取类型
收费全文 | 2141篇 |
免费 | 215篇 |
国内免费 | 295篇 |
专业分类
测绘学 | 529篇 |
大气科学 | 348篇 |
地球物理 | 380篇 |
地质学 | 454篇 |
海洋学 | 248篇 |
天文学 | 117篇 |
综合类 | 171篇 |
自然地理 | 404篇 |
出版年
2024年 | 2篇 |
2023年 | 28篇 |
2022年 | 66篇 |
2021年 | 97篇 |
2020年 | 97篇 |
2019年 | 123篇 |
2018年 | 84篇 |
2017年 | 112篇 |
2016年 | 93篇 |
2015年 | 104篇 |
2014年 | 114篇 |
2013年 | 171篇 |
2012年 | 137篇 |
2011年 | 106篇 |
2010年 | 98篇 |
2009年 | 131篇 |
2008年 | 103篇 |
2007年 | 123篇 |
2006年 | 142篇 |
2005年 | 106篇 |
2004年 | 101篇 |
2003年 | 66篇 |
2002年 | 73篇 |
2001年 | 60篇 |
2000年 | 71篇 |
1999年 | 33篇 |
1998年 | 31篇 |
1997年 | 23篇 |
1996年 | 26篇 |
1995年 | 21篇 |
1994年 | 24篇 |
1993年 | 22篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 9篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有2651条查询结果,搜索用时 15 毫秒
41.
By means of barotropic model, the characteristic and initial value problems are investigated to reveal the local two-dimensional
barotropic instability of the nonuniform current to the dynamic mechanism of the formation of the Yangtze-Huaihe River severe
storm in July 1991. Analytical theory and numerical experiment show that (i) the unstable developing modes are chiefly the
two periods of about 44 d and 10 d, which are fundamentally consistent with that of the precipitation change of the Yangtze-Huaihe
River. (ii) The growth rate of the local perturbation is dominated by the meridional wave numbern = 1–5 and zonal wave numberk = 1–12, i.e. the severe storm over the Yangtze-Huaihe River results from the interaction of the systems at different latitudes
and waves of different scales, (iii) The perturbation over the Yangtze-Huaihe River possesses the property of local intensification,
which slowly migrates westward over the lower and middle reaches of the Yangtze-Huaihe River. (iv) The growth rate of the
instability, especially the propagation velocity of the perturbation, is sensitive to the external parameters ū and α.
Project supported by the National Natural Science Foundation of China. 相似文献
42.
43.
Abstract To determine how local geological events contributed to the evolution of accretionary complexes and eventual exposure of rocks with different structural levels, geochronological mapping was carried out using fission track (FT) analysis at the Kii Peninsula, southwest Japan. At this site, the original zonal structure of Cretaceous accretionary complexes parallel to the subduction zone is disturbed by the northward projection of the Shimanto accretionary complex. Twenty-six zircon FT ages were obtained from an area of ∼12 km in an east–west direction and ∼15 km in a north–south direction, and classified into three groups: (i) ages ∼15 Ma (range ∼10–20 Ma), which are distributed along the northwest–southeast valley; (ii) ages of ∼50 Ma in the northwest of the study area; and (iii) ages older than those in Groups 1 and 2. Based on results from eight zircon FT length distributions, the Miocene ages appear to be the result of spatial variations in heat influx and cooling after the regional exhumation of the area, as recorded by FT ages of ∼50 Ma. 相似文献
44.
GPS水汽遥感中的大气干延迟局地订正模型研究 总被引:9,自引:2,他引:9
在GPS遥感水汽过程中,大气干延迟模型的精度直接影响水汽遥感的精度。根据广东清远站1995~2001年的气象探空资料,计算了GPS水汽测量中的实际大气干延迟。在此资料基础上,利用地面气象要素建立了大气干延迟的年和分月局地订正模型。分析结果表明,年模型的精度优于目前广泛使用的普适模型;月模型与年模型相比,效果不是很明显,建立局地分月订正模型意义不大;在对高度角的敏感程度上,局地模型略大些;当高度角小于75时,大气干延迟弯曲路径与直线路径之差ΔS随天顶角增大而迅速增大。 相似文献
45.
Researching the South African tourism and development nexus 总被引:1,自引:0,他引:1
Under apartheid, tourism was essentially anti-developmental in focus and tourism research largely centred on the recreation
of white South Africans. Since the 1994 democratic transition tourism has become recognised as an increasingly important sector
for South Africa's economy and for achieving the government's goals for reconstruction and development. In this paper a review
and critical analysis is presented of the changing directions and foci of tourism scholarship in South Africa. It is shown
that South African scholars are beginning to respond to the challenges posed by tourism as a developmental focus.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
46.
自适应空间信息移动服务 总被引:4,自引:0,他引:4
空间信息移动服务系统是空间信息系统继由传统PC计算环境向有线Web分布式计算环境扩展后,向移动计算环境的新发展。文中在分析移动计算环境概念和特点的基础上,讨论了空间信息移动服务的若干特点及关键技术,研究了自适应空间信息移动服务方案。 相似文献
47.
48.
基于方差分量估计的自适应融合导航 总被引:23,自引:4,他引:23
多源传感器观测信息的精度一般不一致,利用方差分量估计理论可以合理地顾及各传感器观测信息在融合导航解中的权比.首先给出基于多源观测信息的融合导航解算模式,进而基于方差分量估计讨论了各传感器观测信息在融合导航解中的合理平衡问题,然后应用自适应因子顾及动力学模型预报信息的贡献,给出了计算过程,并利用模拟数据进行了试算与比较. 相似文献
49.
50.
Local geography and gender are two major factors determining which crop varieties are cultivated in a case study of two rural villages in Bangladesh. This paper explores the interrelationships between gender, agrobiodiversity, and the use of, and preferences for, improved and local crop varieties. These are examined in relation to rice, minor field crops, and home garden fruits and vegetables. Reasons for both the displacement and the persistence of local varieties (LVs) are analysed in comparison to improved variety (IV) diffusion. The research evaluates agrobiodiversity through the number, types, and varieties of crops grown in fields and home gardens. The desired agroecological, economic, and cultural characteristics of crops grown document how respondents rank their variety preferences. Variety preferences and the perceived importance of LV preservation are compared with what is actually grown. The study indicates that there was little variation between villages in their approach towards the use of IV and LV rice; IVs were cultivated for their high yields and LV rice for taste and culinary uses. However, there were significant differences in relative agricultural dependence between the two villages which led to unique variety preferences. In both villages, women's preferences for IVs or LVs play a major role in crop choices, particularly as they manifest themselves in gendered domains of authority. 相似文献