全文获取类型
收费全文 | 239篇 |
免费 | 32篇 |
国内免费 | 57篇 |
专业分类
测绘学 | 84篇 |
大气科学 | 35篇 |
地球物理 | 19篇 |
地质学 | 97篇 |
海洋学 | 29篇 |
天文学 | 6篇 |
综合类 | 29篇 |
自然地理 | 29篇 |
出版年
2023年 | 1篇 |
2022年 | 3篇 |
2021年 | 7篇 |
2020年 | 3篇 |
2019年 | 11篇 |
2018年 | 2篇 |
2017年 | 12篇 |
2016年 | 8篇 |
2015年 | 9篇 |
2014年 | 15篇 |
2013年 | 13篇 |
2012年 | 13篇 |
2011年 | 19篇 |
2010年 | 13篇 |
2009年 | 17篇 |
2008年 | 16篇 |
2007年 | 12篇 |
2006年 | 14篇 |
2005年 | 13篇 |
2004年 | 21篇 |
2003年 | 14篇 |
2002年 | 13篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1999年 | 9篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1989年 | 4篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有328条查询结果,搜索用时 46 毫秒
141.
142.
143.
城市体系时空演化的广义维数分析——刻划城市资源分享空间的理论基础、计算方法与应用实例 总被引:3,自引:2,他引:1
根据城市组成元素和城市体系构成要素的异速生长性质发展了一套城市体系时空演化的广义维数分析方法。只要城市和城市体系满足异速生长或准异速生长关系,就可以据之建立基于广义维数比值的分享系数矩阵,通过矩阵的特征向量计算出城市元素和各个城市在某种测度下的相对发展速度权重,进而利用先前的结果算出综合权重数值。将有关数值予以适当的处理可以揭示城市体系时空演化的复杂动态,从而借之实现城市体系的功能调控和结构优化。本方法的数学处理过程与Saaty的递阶分析方法(AHP)异曲同工,但其理论前提、建模技巧和应用方向等等都与后者具有严格的本质性区别。 相似文献
144.
向家坝进水口高边坡系统锚杆受力状况研究 总被引:2,自引:0,他引:2
向家坝水电站进水口高边坡地质条件较好,坡形较合理,设计拟采用系统锚杆对其进行加固。由于系统锚杆作用的被动性和边坡岩体变形的非均匀性,使得不同位置的锚件的应力出现较大的差别。利用有三维弹黏塑性有限单元法对向家坝进水口边坡中系统锚杆的受力状况进行了模拟和分析,从计算结果可以清晰地看出:边坡岩体不同部位的锚杆受力与被锚岩体的位移变形有一定的关系,而且设计采用的逐步开挖逐步锚固的施工步骤,使得先期锚入的锚杆受力随着后期开挖锚固的进行而逐渐加大。其计算结果为优化设计提供了依据。 相似文献
145.
像点偏移是影响光束法区域网平差结果精度的主要因素之一。克服平差过程中像点偏移的影响,以提高区域网平差精度,一直受到业界高度重视。当前,克服像点偏移的常规做法是借助特定的参数模型对其进行描述,构建基于附加参数模型的自检校光束法区域网平差模型,在区域网平差的同时求解引入的参数,进而实现对系统误差的补偿。Bauer模型、Ebner模型、Brown模型、光学畸变模型是现有算法中描述像点偏移的常用模型。为了对上述4种模型的系统误差进行验证,本文首先在经典的光束法区域网平差模型基础上分别设计了与4种模型相对应的自检校光束法区域网平差模型,并予以编程实现;然后通过实例对上述4种模型的算法运行结果进行了验证、比较及分析。试验结果表明,Ebner模型和Brown模型的引入可有效提高光束法区域网平差的精度;Bauer模型的引入对小型区域网平差精度的提升更加有效;光学畸变模型的引入对区域网平差结果的精度提升作用相对较小。 相似文献
146.
147.
Sandy beaches have been identified as threatened ecosystems but despite the need to conserve them, they have been generally overlooked. Systematic conservation planning (SCP) has emerged as an efficient method of selecting areas for conservation priority. However, SCP analyses require digital shapefiles of habitat and species diversity. Mapping these attributes for beaches from field data can take years and requires exhaustive resources. This study thus sought to derive a methodology to classify and map beach morphodynamic types from satellite imagery. Since beach morphodynamics is a strong predictor of macrofauna diversity, they could be considered a good surrogate for mapping beach biodiversity. A dataset was generated for 45 microtidal beaches (of known morphodynamic type) by measuring or coding for several physical characteristics from imagery acquired from Google Earth. Conditional inference trees revealed beach width to be the only factor that significantly predicted beach morphodynamic type, giving four categories: dissipative, dissipative-intermediate, intermediate and reflective. The derived model was tested by using it to predict the morphodynamic type of 28 other beaches of known classification. Model performance was good (75% prediction accuracy) but misclassifications occurred at the three breaks between the four categories. For beaches around these breaks, consideration of surf zone characteristics in addition to beach width ameliorated the misclassifications. The final methodology yielded a 93% prediction accuracy of beach morphodynamic type. Overlaying other considerations on this classification scheme could provide additional value to the layer, such that it also describes species’ spatial patterns. These could include: biogeographic regions, estuarine versus sandy beaches and short versus long beaches. The classification scheme was applied to the South African shoreline as a case study. The distribution of the beach morphodynamic types was partly influenced by geography. Most of the long, dissipative beaches are found along the west coast of the country, the south coast beaches are mostly dissipative-intermediate, and the east coast beaches range from short, estuarine pocket and embayed beaches in the former Transkei (south east), to longer intermediate and reflective beaches in KwaZulu-Natal (in the north east). Once combined with the three biogeographic regions, and distinguishing between estuarine and sandy shores, the South African coast comprised 24 different beach types. Representing shorelines in this form opens up potential for numerous spatial analyses that can not only further our understanding of sandy beach ecology at large spatial scales but also aid in deriving conservation strategies for this threatened ecosystem. 相似文献
148.
DCMT的主测微器采用一种活动V形光栅扫描模式。根据对人造准直光源像的测量分析,研究了主测微器扫描测量的系统精度。研究结果表明,主测微器系统精度优于0″.009,为天体位置的高精度测量提供了良好的条件。 相似文献
149.
DAIChang-lei CHIBao-ming YIShu-ping LIZhi-jun 《东北亚地学研究》2004,7(1):64-69
South Yishu geosyncline is 50 km southeast of Changchun City of Jilin Province, where an aquifer is thick,surface runoff is abundant and it has potential to develop water resources preferably. By means of system analysis, the authors analyse the structural characteristics, I/O characteristics, function characteristics and boundary and environment characteristics of the groundwater system, so as to search for a way of optimizing water resources arrangement and enhancing water resources‘bearing capacity. Based on the analysis results, the authors abstract conceptual model and mathematical model of the groundwater system. The simulation results certify and enrich the knowledge about south Yishu geosyncline. 相似文献
150.