全文获取类型
收费全文 | 729篇 |
免费 | 90篇 |
国内免费 | 70篇 |
专业分类
测绘学 | 245篇 |
大气科学 | 37篇 |
地球物理 | 184篇 |
地质学 | 129篇 |
海洋学 | 144篇 |
天文学 | 14篇 |
综合类 | 80篇 |
自然地理 | 56篇 |
出版年
2023年 | 2篇 |
2022年 | 13篇 |
2021年 | 18篇 |
2020年 | 18篇 |
2019年 | 34篇 |
2018年 | 21篇 |
2017年 | 36篇 |
2016年 | 19篇 |
2015年 | 31篇 |
2014年 | 36篇 |
2013年 | 42篇 |
2012年 | 61篇 |
2011年 | 46篇 |
2010年 | 37篇 |
2009年 | 39篇 |
2008年 | 47篇 |
2007年 | 43篇 |
2006年 | 37篇 |
2005年 | 41篇 |
2004年 | 30篇 |
2003年 | 26篇 |
2002年 | 17篇 |
2001年 | 16篇 |
2000年 | 26篇 |
1999年 | 12篇 |
1998年 | 22篇 |
1997年 | 11篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 13篇 |
1993年 | 20篇 |
1992年 | 13篇 |
1991年 | 2篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有889条查询结果,搜索用时 31 毫秒
91.
Summary. Evidence of a conductivity anomaly in the Rhine-Graben was first given about 15 years ago and consequently led to the definition of various models of induction in the region for periods ranging from a few minutes to a few hours. These models reflect two antagonistic ways of explaining the observed anomalous variations of the magnetic field: direct induction in a two-dimensional (2-D) structure or static distortion of telluric currents by the resistive crystalline Vosges (France) and Schwarzwalde (Germany) massifs. We discuss the two approaches using a simple formalism. In particular, we show that the self-induction related to the anomalous currents flowing in the Rhine-Graben is negligible for periods larger than 1000 s, and that, even though the static distortion of telluric currents does account for the observed anomaly, 2-D models can explain some of its features. We also show how the channelled currents are induced in the large sedimentary basins surrounding the area under study.
An experimental verification of this result is given. 相似文献
An experimental verification of this result is given. 相似文献
92.
Cluster analysis groups samples, but does not generally show gradations between clusters. To illustrate these relationships, principal-coordinate analysis and multidimensional scaling can be employed, but these methods may not be appropriate due to structural distortion. A minimum spanning tree (MST) computes a point-to-point path through the original matrix, and clusters are mapped on the MST diagram. The MST linkages, therefore, provide a natural continuum between the clusters, without distortion. Forty-six Silurian Tonoloway Limestone samples were analyzed for 16 sedimentary features indicative of subaerial exposure. Cluster analysis identified nine clusters which were reduced to five final, ordered groups by MST. 相似文献
93.
94.
95.
Turbulent Intensities and Velocity Spectra for Bare and Forested Gentle Hills: Flume Experiments 总被引:1,自引:1,他引:0
To investigate how velocity variances and spectra are modified by the simultaneous action of topography and canopy, two flume
experiments were carried out on a train of gentle cosine hills differing in surface cover. The first experiment was conducted
above a bare surface while the second experiment was conducted within and above a densely arrayed rod canopy. The velocity
variances and spectra from these two experiments were compared in the middle, inner, and near-surface layers. In the middle
layer, and for the canopy surface, longitudinal and vertical velocity variances () were in phase with the hill-induced spatial mean velocity perturbation (Δu) around the so-called background state (taken here as the longitudinal mean at a given height) as predicted by rapid distortion
theory (RDT). However, for the bare surface case, and remained out of phase with Δu by about L/2, where L is the hill half-length. In the canopy layer, wake production was a significant source of turbulent energy for , and its action was to re-align velocity variances with Δu in those layers, a mechanism completely absent for the bare surface case. Such a lower ‘boundary condition’ resulted in longitudinal
variations of to be nearly in phase with Δu above the canopy surface. In the inner and middle layers, the spectral distortions by the hill remained significant for the
background state of the bare surface case but not for the canopy surface case. In particular, in the inner and middle layers
of the bare surface case, the effective exponents derived from the locally measured power spectra diverged from their expected
− 5/3 value for inertial subrange scales. These departures spatially correlated with the hill surface. However, for the canopy
surface case, the spectral exponents were near − 5/3 above the canopy though the minor differences from − 5/3 were also
correlated with the hill surface. Inside the canopy, wake production and energy short-circuiting resulted in significant departures
from − 5/3. These departures from − 5/3 also appeared correlated with the hill surface through the wake production contribution
and its alignment with Δu. Moreover, scales commensurate with Von Karman street vorticies well described wake production scales inside the canopy,
confirming the important role of the mean flow in producing wakes. The spectra inside the canopy on the lee side of the hill,
where a negative mean flow delineated a recirculation zone, suggested that the wake production scales there were ‘broader’
when compared to their counterpart outside the recirculation zone. Inside the recirculation zone, there was significantly
more energy at higher frequencies when compared to regions outside the recirculation zone. 相似文献
96.
江淮梅雨的时空变化特征 总被引:11,自引:1,他引:10
利用江淮地区37站1954--2001年48a梅雨特征量资料,采用谐波分析、EOF和最大熵谱分析等方法讨论了江淮梅雨的时空变化特征。结果表明:江淮梅雨时空分布不均,梅雨特征量存在显著的年际一年代际变化特征,梅雨特征量年代际变化之间存在明显的负相关或正相关关系;梅雨特征量存在显著不同的多时间尺度振荡周期和长期演变趋势且江淮地区雨季也呈不同的年际和年代际变化特征。 相似文献
97.
Based on the precipitation data of Meiyu at 37 stations in the valleys of Yangtze and Huaihe
Rivers from 1954 to 2001, the temporal-spatial characteristics of Meiyu precipitation and their
relationships with the sea surface temperature in northern Pacific are investigated using such methods as
harmonic analysis, empirical orthogonal function (EOF), composite analysis and singular value
decomposition (SVD). The results show that the temporal evolution and spatial distribution of Meiyu
precipitation are not homogeneous in the Yangtze-Huaihe Rivers basins but with prominent inter-annual
and inter-decadal variabilities. The key region between the anomalies of Meiyu precipitation and the
monthly sea surface temperature anomalies (SSTA) lies in the west wind drift of North Pacific, which
influences the precipitation anomaly of Meiyu precipitation over a key period of time from January to
March in the same year. When the SST in the North Pacific west wind drift is warmer (colder) than
average during these months, Meiyu precipitation anomalously increases (decreases) in the concurrent year.
Results of SVD are consistent with those of composite analysis which pass the significance test of
Monte-Carlo at 0.05. 相似文献
98.
针对海拔较高的城市建立或改造独立坐标系,存在着如何选择模型及各模型对长度变形的影响等问题,通过算例对常见椭球变换和比例缩放模型进行较为全面系统的分析,归纳出常见模型在不同投影带区域、不同投影面高度对长度变形的影响差异,并得出有益的结论。 相似文献
99.
100.
李玉平 《测绘与空间地理信息》2017,(2):220-221
为了满足城市建设对测绘基准的要求,需建立相对独立的平面坐标系,我国测绘法等相关法律法规对独立平面坐标系的建立及管理有着明确的要求,本文结合新疆某县城相对独立平面坐标系的建立技术方案、申报材料准备、申报组织以及发布等方面进行了详细论述,希望能为测绘同行和测绘管理单位建立城市相对独立平面坐标系起到一定的参考作用。 相似文献