全文获取类型
收费全文 | 8916篇 |
免费 | 998篇 |
国内免费 | 1046篇 |
专业分类
测绘学 | 2338篇 |
大气科学 | 1468篇 |
地球物理 | 2277篇 |
地质学 | 1646篇 |
海洋学 | 599篇 |
天文学 | 55篇 |
综合类 | 796篇 |
自然地理 | 1781篇 |
出版年
2024年 | 58篇 |
2023年 | 163篇 |
2022年 | 422篇 |
2021年 | 495篇 |
2020年 | 454篇 |
2019年 | 480篇 |
2018年 | 297篇 |
2017年 | 362篇 |
2016年 | 387篇 |
2015年 | 399篇 |
2014年 | 467篇 |
2013年 | 581篇 |
2012年 | 569篇 |
2011年 | 519篇 |
2010年 | 413篇 |
2009年 | 474篇 |
2008年 | 467篇 |
2007年 | 583篇 |
2006年 | 488篇 |
2005年 | 417篇 |
2004年 | 351篇 |
2003年 | 332篇 |
2002年 | 269篇 |
2001年 | 261篇 |
2000年 | 197篇 |
1999年 | 180篇 |
1998年 | 199篇 |
1997年 | 129篇 |
1996年 | 99篇 |
1995年 | 96篇 |
1994年 | 102篇 |
1993年 | 63篇 |
1992年 | 49篇 |
1991年 | 26篇 |
1990年 | 25篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1979年 | 2篇 |
1977年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
43.
44.
45.
46.
一种裸露土壤湿度反演方法 总被引:1,自引:0,他引:1
针对目前土壤湿度反演方法研究较少且缺少实时性的现状,该文提出一种土壤湿度反演方法——最小二乘支持向量机技术。以积分方程模型为正向算法,数值模拟不同雷达参数(频率、入射角及极化)下后向散射系数随土壤含水量和地表粗糙度的变化情况。经过数据敏感性分析,选取C-波段和X-波段、小入射角下的同极化后向散射系数作为支持向量回归的训练样本信息;经过适当的训练,利用支持向量回归技术对土壤含水量进行了反演研究;并考虑通过多频率、多极化、多入射角数据的组合,消除地表粗糙度的影响,提高反演精度。模拟结果表明,该方法反演土壤湿度具有较高的精度和较好的实时性;同时,与人工神经网络方法的结果比较,证明了该方法的有效性,为土壤湿度的反演研究提供了一种方法。 相似文献
47.
针对港珠澳大桥沉管隧道的安全贯通问题,该文从隧道外GPS网的布设及施测方法出发,通过多期复测数据评价GPS网具有可靠的测量精度;通过对GPS网点的稳定性分析,阐明了沉管隧道地面控制点位无法长期保留、稳定性差的特点;最后就GPS网引起的贯通误差影响值进行了估算。 相似文献
48.
Pattern recognition in road networks can be used for different applications, including spatiotemporal data mining, automated map generalization, data matching of different levels of detail, and other important research topics. Grid patterns are a common pattern type. This paper proposes and implements a method for grid pattern recognition based on the idea of mesh classification through a supervised learning process. To train the classifier, training datasets are selected from worldwide city samples with different cultural, historical, and geographical environments. Meshes are subsequently labeled as composing or noncomposing grids by participants in an experiment, and the mesh measures are defined while accounting for the mesh’s individual characteristics and spatial context. The classifier is generated using the C4.5 algorithm. The accuracy of the classifier is evaluated using Kappa statistics and the overall rate of correctness. The average Kappa value is approximately 0.74, which corresponds to a total accuracy of 87.5%. Additionally, the rationality of the classifier is evaluated in an interpretation step. Two other existing grid pattern recognition methods were also tested on the datasets, and comparison results indicate that our approach is effective in identifying grid patterns in road networks. 相似文献
49.
Erik H. Schmidt Budhendra L. Bhaduri Nicholas Nagle Bruce A. Ralston 《地理信息系统科学与遥感》2018,55(6):860-879
For many researchers, government agencies, and emergency responders, access to the geospatial data of US electric power infrastructure is invaluable for analysis, planning, and disaster recovery. Historically, however, access to high quality geospatial energy data has been limited to few agencies because of commercial licenses restrictions, and those resources which are widely accessible have been of poor quality, particularly with respect to reliability. Recent efforts to develop a highly reliable and publicly accessible alternative to the existing datasets were met with numerous challenges – not the least of which was filling the gaps in power transmission line voltage ratings. To address the line voltage rating problem, we developed and tested a basic methodology that fuses knowledge and techniques from power systems, geography, and machine learning domains. Specifically, we identified predictors of nominal voltage that could be extracted from aerial imagery and developed a tree-based classifier to classify nominal line voltage ratings. Overall, we found that line support height, support span, and conductor spacing are the best predictors of voltage ratings, and that the classifier built with these predictors had a reliable predictive accuracy (that is, within one voltage class for four out of the five classes sampled). We applied our approach to a study area in Minnesota. 相似文献
50.
The identification and analysis of natural channel networks from digital elevation models are discussed from the point of view of their environmental applications. An interactive, graphical software package that implements some of the most widely used techniques for the automatic recognition of channel networks and for the computation of some useful geomorphologic indices and functions is presented. 相似文献