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   1066篇
  免费   151篇
  国内免费   124篇
测绘学   379篇
大气科学   53篇
地球物理   186篇
地质学   280篇
海洋学   170篇
天文学   21篇
综合类   101篇
自然地理   151篇
  2024年   3篇
  2023年   11篇
  2022年   35篇
  2021年   40篇
  2020年   48篇
  2019年   71篇
  2018年   46篇
  2017年   57篇
  2016年   72篇
  2015年   64篇
  2014年   71篇
  2013年   99篇
  2012年   69篇
  2011年   87篇
  2010年   51篇
  2009年   64篇
  2008年   57篇
  2007年   68篇
  2006年   66篇
  2005年   43篇
  2004年   24篇
  2003年   16篇
  2002年   13篇
  2001年   19篇
  2000年   16篇
  1999年   15篇
  1998年   19篇
  1997年   22篇
  1996年   22篇
  1995年   4篇
  1994年   12篇
  1993年   7篇
  1992年   10篇
  1991年   7篇
  1990年   2篇
  1989年   2篇
  1988年   1篇
  1987年   2篇
  1986年   2篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
排序方式: 共有1341条查询结果,搜索用时 156 毫秒
911.
This paper presents a novel approach to feature subset selection using genetic algorithms. This approach has the ability to accommodate multiple criteria such as the accuracy and cost of classification into the process of feature selection and finds the effective feature subset for texture classification. On the basis of the effective feature subset selected, a method is described to extract the objects which are higher than their surroundings, such as trees or forest, in the color aerial images. The methodology presented in this paper is illustrated by its application to the problem of trees extraction from aerial images.  相似文献   
912.
Habitat selection by ungulates is determined by the quantity, quality and distribution of trophic resources as well as by predation risk. It may also vary in relation to species-specific reproductive strategies and social organization. The guanaco (Lama guanicoe), a highly social and sexually size-monomorphic wild camelid typical of arid lands, is ideal for evaluating behavioural responses of this type, since most studies have done on dimorphic ungulates in temperate environments, where trophic resources are abundant. We recorded the group size and social structure of guanaco during both dry and wet seasons of 2005–2007 in an Argentinean desert, where pumas (Puma concolor) are the sole predators. Remote sensing data were used to calculate five variables that reflected trophic availability and terrain morphology for each guanaco group and for an equivalent number of random controls. Habitat use did not differ between types of social groups but differed between seasons. Guanacos used less productive and less steep areas during the breeding season, irrespective of juvenile:adult ratios in the family groups, and larger groups occupied flatter areas. Overall, guanaco habitat selection prioritizes reducing predation risk to the extent that animals occupy areas offering the minimum productivity capable of meeting their energy requirements.  相似文献   
913.
郭庄煤矿七采区北翼涌水点标高-94m,排水管出水口标高-143m,涌水口与出水口垂直落差49m。根据流体力学知识,计算得出排水管路的扬程损失小于49m,满足水自流要求,安设煤矿井下用聚氯乙烯型排水管路两趟,实现绿色、环保、节能排水。  相似文献   
914.
????????????????????????????????????????????????????з?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????Ч???  相似文献   
915.
针对钢筋混凝土高层建筑抗震时程分析输入地震波选择问题,以《建筑抗震设计规范》(GB 50011-2016)设计谱为目标谱,将满足谱匹配原则的加权调幅选波方法与国内学者建议的其它输入地震波选择方法进行了对比研究。以3栋钢筋混凝土高层建筑(15层、30层和44层)为实例,针对8度罕遇地震作用和Ⅱ类场地条件,将上述方法建议的各7条地震波输入结构进行弹塑性时程分析。以结构最大层间位移角均值沿楼层分布为比较参数。结果表明:加权调幅法可用于钢筋混凝土高层建筑抗震时程分析,可以较好地降低结构地震反应均值的离散性。在8度罕遇地震作用条件下,以不同学者建议选择的地震波为输入,高层建筑时程分析结果仍呈现出较大的不同。  相似文献   
916.
高光谱遥感能提供数十至数百个窄波段的光谱信息,从而能够依据地物的诊断性光谱特征进行地物识别。然而,高光谱遥感在提供丰富光谱信息的同时,波段间的相关性和冗余性制约着高光谱遥感的应用。因此,特征参数选择是高光谱遥感分类中最关键的环节之一。首先讨论EO-1/Hyperion的传感器特征,并对其L1R数据进行辐射校正、去条纹、Smile效应纠正等预处理工作。其次利用从图像中提取的典型地物的光谱曲线,采用光谱重建理论获得用于逼近光谱曲线的基函数及其对应的光谱区间。然后采用逐步增加光谱区间,并调整波段中心位置和宽度的方法,得到稳定的光谱区间。最后将光谱区间内的几个原始高光谱波段合成一个宽的波段,得到几个较宽波段的仿真图像,并对其进行分类。结果表明,基于光谱重建的特征参数选择方法获得的分类,总体精度高达92%,充分说明了该方法的有效性。  相似文献   
917.
Given a set of test criteria that determine a quality specification,the question often arises whether anyof the tests are redundant because of intercorrelations.Simple selection of tests on the basis of partialcorrelations with the other tests is rejected on the basis that the random error in the data may be causingspurious correlations.One method is to use cross-validation to define the systematic principal components and examine thecorrelation structure in this reduced space.It is shown that the presence of principal componentsdominated by individual tests(‘variable specific’PCs),which are indicated by cross-validation as beingnon-systematic,must be taken into account.Having defined the dimensionality,a variable reductionmethod based on Procrustes rotation selects subsets of tests that preserve the structure of the samples inmultivariate space.This is an attractive proposition in the context of maintaining a quality controlspecification.It is also shown that the variable reduction technique can be used to aid the identificationof the true dimensionality of the data space.This approach is applied to a number of routine tests carriedout on aviation turbine fuel.  相似文献   
918.
The accuracy and resolution of surface wave dispersion results depend on the parameters used for acquiring data in the field. The optimized field parameters for acquiring multichannel analysis of surface wave (MASW) dispersion images can be determined if preliminary information on the phase velocity range and interface depth is available. In a case study on a fill slope in Hong Kong, the optimal acquisition parameters were first determined from a preliminary seismic survey prior to a MASW survey. Field tests using different sets of receiver distances and array lengths showed that the most consistent and useful dispersion images were obtained from the optimal acquisition parameters predicted. The inverted S-wave velocities from the dispersion curve obtained at the optimal offset distance range also agreed with those obtained by using direct refraction survey.  相似文献   
919.
920.
通过对福建省永定县昌福山勘探区的构造及煤层进行对比分析,认为该区的勘探类型为三类二型.在分析该勘探类型的勘探阶段划分、勘探手段选择的基础上,对其各勘探阶段工程量进行了最合理布置.经过多年的勘查实践证明,本区选择的勘查方法合理正确,所提交的详终报告获一次性评审通过.希望对该类型勘查区的勘查工程布置有一定的参考意义.  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