全文获取类型
收费全文 | 6438篇 |
免费 | 459篇 |
国内免费 | 421篇 |
专业分类
测绘学 | 3927篇 |
大气科学 | 296篇 |
地球物理 | 490篇 |
地质学 | 882篇 |
海洋学 | 552篇 |
天文学 | 287篇 |
综合类 | 481篇 |
自然地理 | 403篇 |
出版年
2024年 | 9篇 |
2023年 | 39篇 |
2022年 | 180篇 |
2021年 | 231篇 |
2020年 | 214篇 |
2019年 | 314篇 |
2018年 | 145篇 |
2017年 | 340篇 |
2016年 | 317篇 |
2015年 | 340篇 |
2014年 | 330篇 |
2013年 | 390篇 |
2012年 | 418篇 |
2011年 | 403篇 |
2010年 | 300篇 |
2009年 | 296篇 |
2008年 | 385篇 |
2007年 | 412篇 |
2006年 | 379篇 |
2005年 | 333篇 |
2004年 | 276篇 |
2003年 | 218篇 |
2002年 | 162篇 |
2001年 | 170篇 |
2000年 | 126篇 |
1999年 | 81篇 |
1998年 | 105篇 |
1997年 | 80篇 |
1996年 | 65篇 |
1995年 | 54篇 |
1994年 | 37篇 |
1993年 | 42篇 |
1992年 | 32篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 19篇 |
1988年 | 16篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1982年 | 3篇 |
1954年 | 2篇 |
排序方式: 共有7318条查询结果,搜索用时 93 毫秒
21.
K. G. Robertson 《Marine Geophysical Researches》1990,12(1-2):3-8
Accurate navigation forms an essential part of all research at sea and the deep ocean imposes it's own unique problems. This chapter discusses several of the techniques in current use on the research vessels of the Natural Environment Research Council (NERC), concentrating on those systems which provide global navigation facilities, as opposed to the more localised, coastal aids. Whilst most of the systems rely on surface propagation of radio waves, the use of acoustics and sea-bed mapping instruments constitute accurate alternatives for some sub-sea applications. 相似文献
22.
Scientific sea-floor dredging is currently used in marine geology primarily by the hard-rock community interested in the recovery of basement rock samples from the unsedimented deep ocean floor. The technique has generally been eclipsed by ocean drilling for recovery of sedimentary rocks, because of perceived uncertainties in the location of sampling and in the representativeness of recovered material. This contribution reviews dredging equipment currently in use by marine geological institutions and refers to pinger attachments that allow precise information on the behaviour of the dredge to be telemetered back to the ship. We argue that improvements in ship navigation and transponder navigation at the seafloor, when used in conjunction with surface and/or deeply towed sidescan and swathemapping surveys, now allow for considerably less uncertainty on the location of dredge sampling. Refined sorting criteria for dredge hauls are now also available. Recent comparisons of regional sample recovery by ocean drilling and by dredge sampling indicate that the dredge hauls can usefully supplement the drilling data in the construction of sedimentary and tectonic histories of seafloor areas. 相似文献
23.
Liu Jiaju Zhang Jingchao Prof. Senior Engineer River Harbour Department Nanjing Hydraulic Research Institute Nanjing Senior Engineer River Harbour Department Nanjing Hydralulic Research Institute Nanjing 《中国海洋工程》1992,(2)
- This paper, after briefly reviewing the experimental research on sediment transport on muddy beach since the 1950s, improves and perfects the method for forecasting siltation in navigation channels and harbour basins which was first put forward in China by the authors. In consideration of silty sediment and sand, some factors in forecasting methods have been changed and modified. Consequently, the modified methods can be used either to compute siltation in navigation channels and harbour basins on muddy beach or to compute siltation and scouring in navigation channels and harbour basins on both silty beach and sandy beach. The verification of field data from eleven large, medium and small natural harbours shows a good agreement between the forecasting by the modified method and the natural conditions. Finally, the paper deals with the rational utilization of water area after the construction of the West Dyke in Lianyungang, the maintenance of water depth of the navigation channel at the entrance, siltation distribution, siltation in the navigation channel and harbour basin for ships of 100 thousand tonnnage. Results once again prove that the prospect of constructing Lianyungang Harbour into a deepwater harbour is bright. 相似文献
24.
根据国际形象设计发展趋势,提出城市旅游形象设计应有3个层次:一是城市旅游整体形象;二是城市旅游吸引物的特色形象;三是旅游企业形象。根据Jensen-Verbeke都市旅游产品界定模式,结合广州实际,分析了广州的旅游吸引物,将广州旅游形象定位为(1)商都旅游,包括购物,美食、商务旅游和现代化城市景观的游览;(2)近现代史迹旅游,包括古代史迹和建筑艺术的观赏,其形象口号为“两个中心”,即商贸中心和近现 相似文献
25.
26.
-Based on the feasibility study of developing the navigation resources of the Guanhe River and the model test results of the mouth bar regulation, this paper presents some basic principles for the regulation of the channel on the mouth bar, for instance, the direction of navigation channel should be identical with that of the ebb tide current and the main waves, and perpendicular to the bathymetric contours. The principles for regulating mouth bars are also discussed in this paper. 相似文献
27.
利用图像分形编码中定义域块和最小均方误差一一对应的特点,提出了1种基于分形编码的多姿态、表情的人脸图像检索方法。该方法将待检索图像分割为相同大小的值域块,然后将每一值域块按给定的定义域块进行分形编码,得到最小均方误差,计算该最小均方误差与图像库中最小均方误差的欧氏距离。将待检索图像所有值域块的欧氏距离求平均,此平均欧氏距离较小的几幅图像即为检索出的图像。实验证明该方法能够准确地检索出图像库中存储的同一人的不同姿态、表情的图像。 相似文献
28.
29.
30.
文中介绍陆地影像图与港口航道图镶嵌的制图工艺,影像图与港口航道图镶嵌后的成图质量评价。简单阐述了作者在绘制该图种过程中的一些体会,以及影像图取代现有港口航道图上陆地地形的可行性。 相似文献