首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   251篇
  免费   13篇
  国内免费   22篇
测绘学   32篇
大气科学   27篇
地球物理   27篇
地质学   37篇
海洋学   59篇
天文学   69篇
综合类   14篇
自然地理   21篇
  2024年   4篇
  2023年   3篇
  2022年   11篇
  2021年   3篇
  2020年   6篇
  2019年   12篇
  2018年   1篇
  2017年   8篇
  2016年   7篇
  2015年   17篇
  2014年   16篇
  2013年   13篇
  2012年   16篇
  2011年   8篇
  2010年   14篇
  2009年   14篇
  2008年   19篇
  2007年   15篇
  2006年   26篇
  2005年   5篇
  2004年   5篇
  2003年   5篇
  2002年   9篇
  2001年   7篇
  2000年   6篇
  1999年   1篇
  1998年   2篇
  1997年   3篇
  1996年   6篇
  1995年   6篇
  1994年   2篇
  1993年   1篇
  1992年   2篇
  1991年   5篇
  1990年   3篇
  1989年   2篇
  1987年   1篇
  1985年   1篇
  1981年   1篇
排序方式: 共有286条查询结果,搜索用时 0 毫秒
121.
基于PDA的公交信息数据库设计与查询算法   总被引:2,自引:0,他引:2  
设计了一个基于PDA的、移动的公交信息数据库并描述了四种面向不同需要的公交信息查询方式及相应的算法。除了提供三种传统的公交查询方式外,还系统提供了以设备持有者为当前所处位置为中心的、公交线路信息的查询。  相似文献   
122.
传统的单点实时定位简单灵活,但精度低,一般的差分定位需要布设基站,作用距离短。广域精密单点定位技术恰好集成了普通单点定位和差分定位的优点,克服了各自的缺点。通过星基增强系统的卫星信号代替地面GPS基准站的差分信号的方法进行海上探测作业,提高了作业效率,进一步证实了此方法的有效性。  相似文献   
123.
A photometric analysis of the short-period Algol eclipsing binary system BF Velorum based for the first time on complete BVRI CCD light curves is presented. The new photometric solution obtained with the Wilson-Devinney program reveals that BF Vel is a near-contact semi-detached system with its secondary component filling its Roche lobe. Moreover, we discovered pulsations of one member of the binary system, the analysis of which shows main frequencies of 44.9386 and 33.6731 cycles/day. Absolute elements of the system were calculated, and the evolutionary status of its members was estimated.   相似文献   
124.
张腾腾 《北京测绘》2021,35(2):212-216
以研究区1∶10 000DEM和DLG数据为基础,分别提出基于DLG数据和DEM数据的精细化处理方法,并对比分析了最近邻域法、双线性内插法以及三次卷积法三种重采样方法对DEM精细化处理效果的影响,结果表明:双线性内插法在时间效率、数据精度以及图像效果方面均适中,故决定采用双线性内插法将研究区5m格网DEM数据通过重采样生成为2m格网DEM数据,最后通过可视化和检查点法验证了精细化处理的精度均满足相关要求。相关研究成果可为地形特征等数据的提取提供精度较高的基础数据。  相似文献   
125.
文章利用数字高程剖面将青藏高原东缘分为4个大尺度地貌单元,即青藏高原地貌区、龙门山高山地貌区、山前冲积平原区(成都盆地)和四川盆地东部隆起区。根据数字高程剖面中的最高海拔高程点剖面与最低海拔高程点剖面之间的高差,定量计算了该地区河流下切深度;结合成都盆地岷江最古老冲积扇沉积物提供的青藏高原东缘河流形成的时间(3.6MaB.P.),定量计算了河流下切速率为1.29mm/a;在约束局部侵蚀基准面和气候变化对河流下切速率控制作用的基础上,建立了青藏高原东缘河流下切速率与表面隆升速率之间的定量关系,结果表明河流下切速率约为表面隆升速率的4倍。基于龙门山在表面隆升速率和下切速率等方面均大于青藏高原内部,认为青藏高原东缘的边缘山脉是剥蚀隆升和构造隆升两者叠加的产物。  相似文献   
126.
The Tungabhadra River is one of the sacred rivers in south India. Industrial and mining operations on its banks generate enormous amounts of effluent that have led to substantial metal and nutrient contamination of the aquatic system. In the present investigation an attempt has been made to assess variation in the physico-chemical characteristics and speciation of zinc in the water and sediment of the Tungabhadra River. Most of the metal ions in the collected samples were in high concentrations compared to the applicable standards. The Individual Contamination Factor (ICF) of Zn was found to be more at Site-B compared to sites A and C. Zinc-bearing particles were characterized by the different morphologies measuring from large colloidal to small particles which were associated with biological structures or may be present in amorphous nature which were observed by Transition electron microscopy (TEM) with X-EDS (Energy dispersive spectrometry). In conclusion, the level of Zn contamination in the Tungabhadra River is increasing at an alarming rate due to industrial activities.  相似文献   
127.
Along with meteorological observations, complementary and systematic oceanographic observations of various physical, biological and chemical parameters have been made at Ocean Station P (OSP) (50°N, 145°W) since the early 1950s. These decadal time scale data have contributed to a better understanding of the physical, biological and chemical processes in the surface layer of the northeastern subarctic region of the Pacific Ocean. These data have demonstrated the importance of the North Pacific in the global carbon cycle and, in particular, the role of biological/chemical processes in the net exchange of CO2 across the air–sea interface. Although we do not fully comprehend how climatic variations influence marine communities or marine biogeochemistry, previous studies have provided some basic understanding of the mechanisms controlling the seasonal and inter-annual variations of biological and chemical parameters (such as phytoplankton, bacteria, nitrate/ammonium concentration) at OSP, and how they affect the carbon cycling in the subarctic North Pacific. In this study, we investigate how these mechanisms might alter the seasonal variations of these parameters at OSP under a 2XCO2 condition. We examine these influences using a new biological model calibrated by the climatological data from OSP. For the 2XCO2 simulation, the biological model is driven off line (i.e., no feedback to the ocean/atmospheric model components) by the climatology plus 2XCO2−1XCO2 outputs from a global surface ocean model and the Canadian GCM. Under the 2XCO2 condition, the upper layer ocean shows an increase in the entrainment rate at the bottom of the mixed layer for OSP during the late autumn and winter seasons, resulting in an increase in the f-ratio. Although there is an overall increase in the primary production (PP) by 3–18%, a decrease in the biomass of small phytoplankton and microzooplankton (due to mesozooplankton grazing) lowers the concentration of dissolved organic matter (DOM) by 4–25%. The model also predicts a significant increase in the concentrations of nitrate and ammonium, and in bacterial production during July and August. Doubling of the atmospheric CO2 from 330 to 660 ppm forces the marine pCO2 to increase by about 63%, much of which is driven by an increased flux of CO2 from the atmosphere to the oceans.  相似文献   
128.
Iceland's nationwide privatized Individual Transferable Quota (ITQ) system is over thirty years old but remains a topic of public and political debate, particularly because of the continued effects on small-scale fisheries. A national survey of small-boat fishermen was distributed to: (1) identify major defining characteristics of participants in ITQ and non-ITQ fisheries, (2) document and examine differences in satisfaction with fisheries management, and (3) evaluate the existing options for newcomers to participate in small-boat fisheries. Survey results indicate that Icelandic small-boat fishermen are engaged in multiple management systems within a wide range of boat sizes. Those who held quota were more satisfied with the current ITQ system compared to those who did not hold quota; however, nearly all fishermen were still critical of fisheries management in Iceland and the two major non-ITQ options of lumpfish and coastal fishing were not perceived to offer significant opportunity for entry-level fishermen. Dissatisfaction stemmed from the lack of decision-making power, a distrust of scientific advice, and the perception that the ITQ system did not serve the purpose of protecting fisheries resources, but was rather oriented only toward economic goals. The dynamic nature of Icelandic small-boat fishing livelihoods and the pervasive negative attitudes thirty years after ITQ implementation demonstrate the need for culturally appropriate and equitable fisheries management schemes where success is measured in social as well as economic and biological terms.  相似文献   
129.
本文划分出澜沧江中下游斜坡结构类型并分析了各斜坡类型的基本特征、变形机制和演化方式,阐述了斜坡变形破坏对该地区人类经济活动的影响,为评价预测澜沧江中下游斜坡稳定性奠定了基础。  相似文献   
130.
后寨河喀斯特流域土地资源合理利用模式研究   总被引:10,自引:2,他引:10  
分析了后寨喀斯特流域土地资源的利用现状、特点及存在的问题,并对流域内五种土地合理利用模式的适用范围和运用状况进行调查分析,提出了各种土地利用模式的推广实施对策。   相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号