全文获取类型
收费全文 | 338篇 |
免费 | 35篇 |
国内免费 | 33篇 |
专业分类
测绘学 | 97篇 |
大气科学 | 23篇 |
地球物理 | 103篇 |
地质学 | 94篇 |
海洋学 | 29篇 |
天文学 | 6篇 |
综合类 | 27篇 |
自然地理 | 27篇 |
出版年
2023年 | 4篇 |
2022年 | 4篇 |
2021年 | 11篇 |
2020年 | 10篇 |
2019年 | 11篇 |
2018年 | 11篇 |
2017年 | 10篇 |
2016年 | 17篇 |
2015年 | 15篇 |
2014年 | 9篇 |
2013年 | 26篇 |
2012年 | 10篇 |
2011年 | 21篇 |
2010年 | 8篇 |
2009年 | 15篇 |
2008年 | 20篇 |
2007年 | 18篇 |
2006年 | 14篇 |
2005年 | 15篇 |
2004年 | 19篇 |
2003年 | 12篇 |
2002年 | 6篇 |
2001年 | 8篇 |
2000年 | 3篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 12篇 |
1996年 | 12篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有406条查询结果,搜索用时 15 毫秒
211.
D. C. Boes R. A. Davis S. N. Gupta 《Stochastic Environmental Research and Risk Assessment (SERRA)》1989,3(2):97-110
A class of regression type estimators of the parameterd in a fractionally differencedARMA (p, q) process is introduced. This class is an extension of the estimator considered by Geweke and Porter-Hudak. In a simulation study, we compared three estimators from this class together with two approximate maximum likelihood estimators which are based on two separate approximations to the likelihood. One approximation ignores the determinant term in the likelihood and the other includes a compensating factor for the determinant. When the determinant term is included, the estimate tends to be much less biased and is in general superior to the other estimate. The approximate maximum likelihood estimator out performed, by a large margin, the regression type estimators for pureARIMA (0,d,0) processes. However, forARIMA (1,d,1) processes, a regression type estimator turned out to be the best for realizations of length 400 in 3 out of the 5 cases we tried. 相似文献
212.
蒋家沟泥石流输沙年际变化及其灾变预测 总被引:1,自引:0,他引:1
根据云南东川蒋家沟泥石流年输沙量的时序资料,分析了蒋家沟泥石流输沙的年际变化特征,发现历年输沙量变动幅度大,波动明显且年际变化频繁,用灰色系统理论的灾变预测方法,建立泥石流年输沙量的等维新息灾变预测模型,能够对未来可能出现的灾变年份进行较好的预测。在前期研究基础上,补充最新观测资料,建立了新的灾变时间序列和相应的等维新息灾变预测模型,使蒋家沟泥石流的年输沙量灾变预测得以连续进行,并对预测结果进行了检验,效果较为理想。 相似文献
213.
The paper shows an application of Scale Recursive Estimation (SRE) used to assimilate rainfall rates estimated during a storm event from three remote sensing devices. These are the TMI radiometer and the PR radar, carried on board of the TRMM satellite and the KNQA Memphis Weather Surveillance radar, belonging to the NEXRAD network, each one providing rain rate estimates at a different spatial scale. The variability of rain rate process in scales is modeled as a multiplicative random cascade, including spatial intermittence. The observational noise in the estimates is modeled according to a multiplicative error. System estimation, including process and observational noise, is carried out using Maximum Likelihood Estimation implemented by a scale recursive Expectation Maximization (EM) algorithm. As a result, new rainfall rate estimates are obtained that feature decreased estimation error as compared to those coming from each device alone. The performance of the SRE-EM approach is compared with that of the latest methods proposed for data fusion of multisensor estimates. The proposed approach improves the current methods adopted for SRE and provides an alternative for data fusion in the field of precipitation. 相似文献
214.
Estimability analysis of variance and covariance components 总被引:1,自引:1,他引:1
Although variance and covariance components have been extensively investigated and a number of elegant formulae to compute
them have been derived, nothing is known, without any ambiguity, about their estimability in the case of a fully unknown variance–covariance
matrix. We prove that variance and covariance components in this case are not estimable, thus clarifying the ambiguity of
the literature on the topic and correcting some erroneous statements in the literature. We also give a new theorem on the
estimability of a linear function of variance and covariance components. Then we propose a new method to estimate the variance–covariance
matrix with special structure, which can presumably be represented by, at most, r(r + 1)/2 independent parameters to guarantee its estimability in such a subspace, by directly implementing the positive definiteness
of the matrix as constraint to the restricted maximum likelihood method, where r is the number of redundant measurements. Therefore, our estimates of the variance and covariance components always reconstruct
a positive definite matrix and are always physically meaningful. 相似文献
215.
基于HMRF先验模型的HBE卫星遥感图像超分辨率重建 总被引:1,自引:0,他引:1
提出了一种在Bayes概率统计框架下的混合Bayes超分辨率重建算法,该算法采用Huber马尔可夫随机场(Huber Markov random field,HMRF)模型对理想图像进行先验建模,可以较好地突出重建图像的不连续边缘特征信息。实验结果表明,该算法克服了极大后验概率估计(maximum a posteriori,MAP)算法中的若干缺陷,取得了良好的重建结果,图像边缘特征清晰,纹理信息突出。 相似文献
216.
A new companion for Mercator 总被引:1,自引:1,他引:0
Waldo Tobler 《制图学和地理信息科学》2018,45(3):284-285
The inappropriate use of the Mercator projection has declined but still occasionally occurs. One method of contrasting the Mercator projection is to present an alternative in the form of an equal area projection. The map projection derived here is thus not simply a pretty Christmas tree ornament: it is instead a complement to Mercator’s conformal navigation anamorphose and can be displayed as an alternative. The equations for the new map projection preserve the latitudinal stretching of the Mercator while adjusting the longitudinal spacing. This allows placement of the new map adjacent to that of Mercator. The surface area, while drastically warped, maintains the correct magnitude. 相似文献
217.
基于2013年2—3月南极普里兹湾南极大磷虾(Euphausia superba)样品的复眼直径和体长数据,使用Normsep方法对大磷虾种群年龄结构组成进行分离研究,结果表明:利用复眼直径和体长数据分离出的大磷虾年龄组均为1+—4+龄,群体年龄以2+龄为主;复眼直径的分离效果优于体长,其结果更为可靠;1+—3+龄磷虾复眼直径平均值分布范围为1.21—1.40 mm、1.59—1.76 mm和1.93—2.08 mm,4+龄复眼直径约为2.23 mm;从20世纪80年代至今,普里兹湾附近海域大磷虾各年龄组的平均体长分布变化不大,表明大磷虾种群体长特征稳定,其资源状况较好。 相似文献
218.
219.
220.
等位移理论估计高层建筑结构非弹性地震反应的进一步研究 总被引:1,自引:0,他引:1
为进一步研究用等位移理论估计高层建筑结构非弹性地震反应,选用五个钢筋混凝土高层结构实例,输入二十条不同场地条件的地震波,采用结构弹性和非弹性地震时程反应分析方法,研究了结构弹性和非弹性最大顶点位移以及最大层间位移角之间的关系。提出可以直接用弹性最大顶点位移估计非弹性最大顶点位移。对于非弹性层间位移角反应,在中、弱非线陛阶段宜可直接用弹性反应结果估计,而在强非线性阶段则需进行一定修正。 相似文献