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   26733篇
  免费   4911篇
  国内免费   6407篇
测绘学   1170篇
大气科学   5896篇
地球物理   7201篇
地质学   13346篇
海洋学   2917篇
天文学   1379篇
综合类   3039篇
自然地理   3103篇
  2024年   125篇
  2023年   503篇
  2022年   1151篇
  2021年   1337篇
  2020年   1110篇
  2019年   1199篇
  2018年   1510篇
  2017年   1333篇
  2016年   1599篇
  2015年   1254篇
  2014年   1627篇
  2013年   1487篇
  2012年   1353篇
  2011年   1392篇
  2010年   1501篇
  2009年   1462篇
  2008年   1321篇
  2007年   1271篇
  2006年   1019篇
  2005年   884篇
  2004年   771篇
  2003年   797篇
  2002年   739篇
  2001年   690篇
  2000年   919篇
  1999年   1388篇
  1998年   1118篇
  1997年   1114篇
  1996年   1004篇
  1995年   880篇
  1994年   827篇
  1993年   696篇
  1992年   542篇
  1991年   438篇
  1990年   324篇
  1989年   303篇
  1988年   268篇
  1987年   171篇
  1986年   147篇
  1985年   114篇
  1984年   70篇
  1983年   57篇
  1982年   70篇
  1981年   56篇
  1980年   30篇
  1979年   29篇
  1978年   10篇
  1977年   4篇
  1976年   7篇
  1958年   27篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Up to now there is no specification about the allowance of lateral breakthrough error for super long tunnel from 20 km to 50 km. On the basis of the design of GPS networks located outside and inside tunnel traverse network, we propose a method for calculating the influence value caused by control surveying errors. Through a lot of simulative calculations and combination with piercing practice of super tunnels in Wan Jiazai Project, Shanxi province, we present an allowance table of lateral breakthrough error for super long tunnels from 20 km to 50 km.  相似文献   
72.
On the basis of the authors' experiences of setting up an NGDC Web site, this paper attempts to present some significant aspects about the security of NGDC based on ASP. They include data storing, database maintenance, new technical support and so on. Firstly, this paper discusses how to provide the security of data which is saved in the hosts of NGDC. The security model of “Networks-DB Server-DB-DB Object” is also presented. In Windows NT Server, Internet Information Server (i. e., IIS) is in charge of transferring message and the management of Web sites. ASP is also based on IIS. The advantages of virtual directory technique provided by IIS are emphasized.  相似文献   
73.
74.
This paper focuses on a series of quantitative analysis models, such as grey relational analysis model, hierarchical cluster analysis model, principal component analysis model, linear regression model and elastic coefficient model. These models are used to analyze the comprehensive function and effect of driving forces systemically, including analysis on features, analysis for differentiating the primary and the secondary, analysis on comprehensive effects, analysis of elasticity, analysis of prediction. The primary and characteristic factors can be extracted by analysis of features and analysis for differentiating the primary and the secondary. Analysis on prediction and elasticity can predict the area of cultivated land in the future and find out which factors exert great influence on the cultivated land supply.  相似文献   
75.
This paper endeavours to put the discussion on errors and uncertainties in geographical information systems (GISs) in a more systematic way by examining the strength and weakness of discrete objects and continous fields, the two distinct schools of spatial data modelling. In doing so, it argues that neither discrete objects nor continous fields alone provide objective and complete representations of highly complex geographical phenomena, though there are good reasons for asserting that continuous fields are better suited to modelling spatial dependence, heterogeneity and fuzzines significant in geographical reality than discrete objects. Thus, there seems to be merit in adopting an integrated model incorporating analytical capabilities of fields and generalization functions of objects, for which extended TIN (triangulated irregular network) models along with their duals (Voronoi diagrams) provide a pragmatical solution.  相似文献   
76.
Single-frequency precise point positioning (SF-PPP) is a potential precise positioning technique due to the advantages of the high accuracy in positioning after convergence and the low cost in operation. However, there are still challenges limiting its applications at present, such as the long convergence time, the low reliability, and the poor satellite availability and continuity in kinematic applications. In recent years, the achievements in the dual-frequency PPP have confirmed that its performance can be significantly enhanced by employing the slant ionospheric delay and receiver differential code bias (DCB) constraint model, and the multi-constellation Global Navigation Satellite Systems (GNSS) data. Accordingly, we introduce the slant ionospheric delay and receiver DCB constraint model, and the multi-GNSS data in SF-PPP modular together. In order to further overcome the drawbacks of SF-PPP in terms of reliability, continuity, and accuracy in the signal easily blocking environments, the inertial measurements are also adopted in this paper. Finally, we form a new approach to tightly integrate the multi-GNSS single-frequency observations and inertial measurements together to ameliorate the performance of the ionospheric delay and receiver DCB-constrained SF-PPP. In such model, the inter-system bias between each two GNSS systems, the inter-frequency bias between each two GLONASS frequencies, the hardware errors of the inertial sensors, the slant ionospheric delays of each user-satellite pair, and the receiver DCB are estimated together with other parameters in a unique Kalman filter. To demonstrate its performance, the multi-GNSS and low-cost inertial data from a land-borne experiment are analyzed. The results indicate that visible positioning improvements in terms of accuracy, continuity, and reliability can be achieved in both open-sky and complex conditions while using the proposed model in this study compared to the conventional GPS SF-PPP.  相似文献   
77.
The Global Navigation Satellite System presents a plausible and cost-effective way of computing the total electron content (TEC). But TEC estimated value could be seriously affected by the differential code biases (DCB) of frequency-dependent satellites and receivers. Unlike GPS and other satellite systems, GLONASS adopts a frequency-division multiplexing access mode to distinguish different satellites. This strategy leads to different wavelengths and inter-frequency biases (IFBs) for both pseudo-range and carrier phase observations, whose impacts are rarely considered in ionospheric modeling. We obtained observations from four groups of co-stations to analyze the characteristics of the GLONASS receiver P1P2 pseudo-range IFB with a double-difference method. The results showed that the GLONASS P1P2 pseudo-range IFB remained stable for a period of time and could catch up to several meters, which cannot be absorbed by the receiver DCB during ionospheric modeling. Given the characteristics of the GLONASS P1P2 pseudo-range IFB, we proposed a two-step ionosphere modeling method with the priori IFB information. The experimental analysis showed that the new algorithm can effectively eliminate the adverse effects on ionospheric model and hardware delay parameters estimation in different space environments. During high solar activity period, compared to the traditional GPS + GLONASS modeling algorithm, the absolute average deviation of TEC decreased from 2.17 to 2.07 TECu (TEC unit); simultaneously, the average RMS of GPS satellite DCB decreased from 0.225 to 0.219 ns, and the average deviation of GLONASS satellite DCB decreased from 0.253 to 0.113 ns with a great improvement in over 55%.  相似文献   
78.
本文在分析国内地质资料信息化服务现状的基础上,以地勘基金成果数据为管理对象,搭建了地勘基金成果管理系统,并以贵州省地勘基金成果数据为例,实现了该地区地勘基金项目、整装勘查项目地质数据的有效管理与查询,为地质资料的管理与应用提供了科学的决策支持.  相似文献   
79.
针对智慧城市背景下提出的新要求,本文在对天地图·潍坊已有建设成果进行总结分析的基础上,结合智慧潍坊时空信息云平台试点项目建设,对天地图·潍坊进行改版升级为智慧潍坊时空信息云平台公众版,介绍了平台架构、功能模块、应用示范等方面的内容,根据实际结果表明,该平台能够为全市公众、企事业单位提供权威、统一、可靠的时空地理信息服务.  相似文献   
80.
地铁隧道结构变形监测信息管理系统的开发   总被引:2,自引:0,他引:2  
地铁隧道结构变形监测的特殊性、周期性和长期性,使其信息量非常庞大.信息管理是地铁隧道结构变形监测中一项重要的工作,现有的管理方式效率很低.为了高效、准确地管理监测信息,及时分析预报地铁隧道结构的稳定状况,本文结合南京地铁运营期隧道结构变形监测实例,开发了一套具有变形监测资料存储、预处理、管理分析、可视化分析、预测预报及限值预警等功能的信息管理系统,保证了准确及时快速的数据处理和信息反馈,具有良好的运用和推广前景.  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