全文获取类型
收费全文 | 2526篇 |
免费 | 356篇 |
国内免费 | 372篇 |
专业分类
测绘学 | 916篇 |
大气科学 | 295篇 |
地球物理 | 545篇 |
地质学 | 750篇 |
海洋学 | 212篇 |
天文学 | 36篇 |
综合类 | 260篇 |
自然地理 | 240篇 |
出版年
2024年 | 5篇 |
2023年 | 14篇 |
2022年 | 56篇 |
2021年 | 68篇 |
2020年 | 96篇 |
2019年 | 83篇 |
2018年 | 97篇 |
2017年 | 121篇 |
2016年 | 134篇 |
2015年 | 151篇 |
2014年 | 158篇 |
2013年 | 178篇 |
2012年 | 156篇 |
2011年 | 167篇 |
2010年 | 134篇 |
2009年 | 141篇 |
2008年 | 155篇 |
2007年 | 178篇 |
2006年 | 204篇 |
2005年 | 145篇 |
2004年 | 111篇 |
2003年 | 99篇 |
2002年 | 93篇 |
2001年 | 93篇 |
2000年 | 70篇 |
1999年 | 53篇 |
1998年 | 49篇 |
1997年 | 43篇 |
1996年 | 32篇 |
1995年 | 29篇 |
1994年 | 26篇 |
1993年 | 22篇 |
1992年 | 12篇 |
1991年 | 21篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
排序方式: 共有3254条查询结果,搜索用时 15 毫秒
101.
Salim Lamine Sudhir Kumar Singh Szilárd Szabó Nour El Islam Bachari Prashant K. Srivastava 《国际地球制图》2018,33(8):862-878
This study aims to quantify the landscape spatio-temporal dynamics including Land Use/Land Cover (LULC) changes occurred in a typical Mediterranean ecosystem of high ecological and cultural significance in central Greece covering a period of 9 years (2001–2009). Herein, we examined the synergistic operation among Hyperion hyperspectral satellite imagery with Support Vector Machines, the FRAGSTATS® landscape spatial analysis programme and Principal Component Analysis (PCA) for this purpose. The change analysis showed that notable changes reported in the experimental region during the studied period, particularly for certain LULC classes. The analysis of accuracy indices suggested that all the three classification techniques are performing satisfactorily with overall accuracy of 86.62, 91.67 and 89.26% in years 2001, 2004 and 2009, respectively. Results evidenced the requirement for taking measures to conserve this forest-dominated natural ecosystem from human-induced pressures and/or natural hazards occurred in the area. To our knowledge, this is the first study of its kind, demonstrating the Hyperion capability in quantifying LULC changes with landscape metrics using FRAGSTATS® programme and PCA for understanding the land surface fragmentation characteristics and their changes. The suggested approach is robust and flexible enough to be expanded further to other regions. Findings of this research can be of special importance in the context of the launch of spaceborne hyperspectral sensors that are already planned to be placed in orbit as the NASA’s HyspIRI sensor and EnMAP. 相似文献
102.
遥感岩性制图是地质填图中的重要工作,基于光谱特征的岩性分类易受到色调、纹理等因素影响导致精度不佳。前人进行岩性自动分类研究多关注影像的光谱特征,而忽略空间特征,笔者等基于甘肃北山白峡尼山地区ASTER影像,将支持向量机、极限学习机两种机器学习分类方法与基于空间特征的快速漂移算法相结合进行岩性分类。结果表明支持向量机分类总体精度为89.17%;极限学习机不但具有需调节参数少的优势,且分类精度和速度均优于支持向量机,分类总体精度达96.70%;利用快速漂移算法提取的影像空间特征可有效减少错分区,提升岩性分类效果。研究证实将基于光谱特征的极限学习机和基于空间特征的快速漂移算法结合的岩性分类方法具有客观、高效、高精度等优势,可为后续地质填图和找矿勘查工作提供可靠数据支撑,在遥感岩性分类领域具有较高的推广价值。 相似文献
103.
由于加筋土界面作用的复杂性,加筋土工程建设中铺设土工格栅时往往采用经验的方法,很大程度上造成了土工格栅的浪费及工程安全隐患,理清不同填料筋土界面作用的影响范围,有助于确定加筋土结构的合理加筋间距。为了揭示不同填料筋土界面作用的影响范围,采用4种不同类型的砂土与格栅在不同法向应力下进行了一系列的拉拔试验,并结合数字图像量测技术,分析了不同类型砂土下界面剪切带厚度、颗粒位移矢量、格栅拉拔阻力峰值及应变等演变规律。研究表明:界面剪切带厚度H随法向应力σv与砂土平均粒径d50的增加而增大,通过多变量拟合的方法,得到了H、σv与d50三者之间的函数表达式;格栅在拉拔过程中,砂土颗粒位移矢量以土工格栅为界有着显著的差别,格栅上部的颗粒位移矢量明显大于下部颗粒,且在格栅上下一定范围内会形成颗粒位移矢量集中带;拉拔阻力峰值随σv及d50的增加而增大;不同类型砂土各区段的格栅应变均表现出由前向后依次递减的趋势。 相似文献
104.
准确可靠的中长期径流预报是支撑水资源科学调配、提高水资源利用效率的关键。本研究采用AdaBoost模型(AdB)、随机森林模型(RF)和支持向量机模型(SVM)进行淮河流域王家坝和蚌埠站当年11月至次年10月共12个月的中长期径流预报研究。采用置换准确度重要性度量法从130项气象-气候因子及前期降雨/流量构建的1 562个因子变量中筛选出影响各月径流的关键因子,构建了基于AdB、RF和SVM模型的各月径流预报模型,模型参数采用随机搜索技术并结合交叉验证方式确定。采用变幅误差合格率和等级(五级)预报合格率指标对模型的预报精度进行了评估。变幅误差合格率指标表明,王家坝12个月的平均合格率分别为99.8%(AdB)、96.6%(RF)和95.9%(SVM),蚌埠站分别为100%(AdB)、94.8%(RF)和93.8%(SVM);等级预报合格率指标表明,王家坝12个月的平均合格率分别为79.0%(AdB)、76.4%(RF)和79.9%(SVM),蚌埠站分别为81.0%(AdB)、75.6%(RF)和76.6%(SVM)。模型均具有较好的预报效果,但RF和SVM模型对于高流量值的预报存在偏低现象,AdB模型整体上优于RF和SVM模型。 相似文献
105.
全国第二次土地调查于2007年下半年启动,2009年完成,安徽省于2008年4月率先启动了芜湖县的试点工作,并自行购买了ALOS1B1级数据制作DOM,套合上矢量的原始土地利用现状图,以便于外业调绘的全面开展.下面,是笔者对利用ALOS1B1级数据制作正射影像图以及外业调绘用图的方法进行了探讨,研究发现,此方法易于掌握,调绘底图的生产快速,质量可靠,效果显著. 相似文献
106.
基料一滤层利用数码可视化跟踪技术、计算机信息实时处理技术和土体变形无标点量测技术,对宽级配基料一滤层系统进行了渗透室内模型试验。探讨了滤层准则的适用性,研究了不同滤层厚度和加压速率对管涌发生发展的影响。结果表明:传统太沙基滤层准则不适用于宽级配砂土,建议采用可移动颗粒中的d85s进行滤层准则设计;增加滤层厚度可提高系统的抗渗透破坏能力,但提高滤层厚度对提高整个系统抵抗渗透能力作用不大;水力梯度增加较快不利于系统的稳定。 相似文献
107.
108.
109.
The cooperative and conflictual interactions between the United States,Russia, and China: A quantitative analysis of event data 总被引:1,自引:0,他引:1
YUAN Lihua SONG Changqing CHENG Changxiu SHEN Shi CHEN Xiaoqiang WANG Yuanhui 《地理学报(英文版)》2020,30(10):1702-1720
The United States, Russia and China are militarily and economically among the most powerful countries in the post-Cold War period, and the interactions between the three powers heavily influence the international system. However, different conclusions about this question are generally made by researchers through qualitative analysis, and it is necessary to objectively and quantitatively investigate their interactions. Monthly-aggregated event data from the Global Data on Events, Location and Tone(GDELT) to measure cooperative and conflictual interactions between the three powers, and the complementary cumulative distribution function(CCDF) and the vector autoregression(VAR) method are utilized to investigate their interactions in two periods: January, 1991 to September, 2001, and October, 2001 to December, 2016. The results of frequencies and strengths analysis showed that: the frequencies and strengths of USA-China interactions slightly exceeded those of USA-Russia interactions and became the dominant interactions in the second period. Although that cooperation prevailed in the three dyads in two periods, the conflictual interactions between the USA and Russia tended to be more intense in the second period, mainly related to the strategic contradiction between the USA and Russia, especially in Georgia, Ukraine and Syria. The results of CCDF indicated that similar probabilities in the cooperative behaviors between the three dyads, but the differences in the probabilities of conflictual behaviors in the USA-Russia dyad showed complicated characteristic, and those between Russia and China indicated that Russia had been consistently giving China a hard time in both periods when dealing with conflict. The USA was always an essential factor in affecting the interactions between Russia and China in both periods, but China's behavior only played a limited role in influencing the interactions between the USA-Russia dyad. Our study provides quantitative insight into the direct cooperative and conflictual interactions between the three dyads since the end of the Cold War and helps to understand their interactions better. 相似文献
110.
This paper examines the potential of least‐square support vector machine (LSVVM) in the prediction of settlement of shallow foundation on cohesionless soil. In LSSVM, Vapnik's ε‐insensitive loss function has been replaced by a cost function that corresponds to a form of ridge regression. The LSSVM involves equality instead of inequality constraints and works with a least‐squares cost function. The five input variables used for the LSSVM for the prediction of settlement are footing width (B), footing length (L), footing net applied pressure (P), average standard penetration test value (N) and footing embedment depth (d). Comparison between LSSVM and some of the traditional interpretation methods are also presented. LSSVM has been used to compute error bar. The results presented in this paper clearly highlight that the LSSVM is a robust tool for prediction of settlement of shallow foundation on cohesionless soil. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献