全文获取类型
收费全文 | 1213篇 |
免费 | 121篇 |
国内免费 | 150篇 |
专业分类
测绘学 | 124篇 |
大气科学 | 218篇 |
地球物理 | 340篇 |
地质学 | 281篇 |
海洋学 | 261篇 |
天文学 | 4篇 |
综合类 | 61篇 |
自然地理 | 195篇 |
出版年
2024年 | 9篇 |
2023年 | 16篇 |
2022年 | 27篇 |
2021年 | 36篇 |
2020年 | 42篇 |
2019年 | 57篇 |
2018年 | 31篇 |
2017年 | 59篇 |
2016年 | 57篇 |
2015年 | 48篇 |
2014年 | 80篇 |
2013年 | 99篇 |
2012年 | 64篇 |
2011年 | 89篇 |
2010年 | 71篇 |
2009年 | 84篇 |
2008年 | 89篇 |
2007年 | 68篇 |
2006年 | 58篇 |
2005年 | 46篇 |
2004年 | 40篇 |
2003年 | 41篇 |
2002年 | 36篇 |
2001年 | 40篇 |
2000年 | 28篇 |
1999年 | 30篇 |
1998年 | 26篇 |
1997年 | 14篇 |
1996年 | 20篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1992年 | 15篇 |
1991年 | 10篇 |
1990年 | 5篇 |
1989年 | 10篇 |
1988年 | 1篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有1484条查询结果,搜索用时 15 毫秒
101.
城市土地利用综合效益评价:城际比较 总被引:18,自引:0,他引:18
应用系统分析和层次分析法,建立了深圳市土地利用效益评价指标体系,评价了深圳和广州的土地利用效益.结果表明,从1996~2003年,深圳市土地的社会效益、经济效益、生态环境效益和综合效益都呈现增加的趋势,其中经济效益增长最快;与广州市相比,深圳市土地利用的单项效益和综合效益都较高,特别是经济效益和综合效益;但与香港或其它国外发达城市相比,其土地利用效益仍较低,还有较大的提升空间;深圳市各区域之间土地效益差异较大,宝安和龙岗两区的土地利用效益较低,是效益深圳建设的主要突破口.建议:深圳市在社会经济高速发展的同 相似文献
102.
基于时间序列叶面积指数稀疏表示的作物种植区域提取 总被引:3,自引:0,他引:3
以华北平原黄河以北地区为研究区域,以时间序列叶面积指数LAI(Leaf Area Index)傅里叶变换的谐波特征作为不同作物识别的数据源,利用稀疏表示的分类方法识别2007年—2016年冬小麦、春玉米、夏玉米等主要农作物种植区域。首先利用上包络线Savitzky-Golay滤波分别对2007年—2016年的时间序列MODIS LAI曲线进行重构,进而对重构的年时间序列LAI进行傅里叶变换,以0—5级谐波振幅、1—5级谐波相位作为作物识别的依据,基于各类地物的训练样本,通过在线字典学习算法构建稀疏表示方法的判别字典,对每个待测样本利用正交匹配追踪算法求解稀疏系数,从而计算对应于各类地物的重构误差,根据最小重构误差判定待测样本的作物类型,并对作物识别结果的位置精度进行验证。结果表明,2007年—2016年作物识别的总体精度为77.97%,Kappa系数为0.74,表明本文提出的方法可以用于研究区域主要作物种植区域的提取。 相似文献
103.
104.
Despite progress being advanced with spatial approaches to crime and crime control, the geography of crime harm has to date received little attention. The recent development of “Crime Harm Indices”, which weight crimes by an estimate of the relative harm they cause, offers an opportunity to improve on volume based spatial analysis approaches to identify where crime harm concentrates.This study aims to address this issue via the use of a Crime Harm Index (CHI) developed for New Zealand. By contrast to localized ‘harm-spotting’ analysis, we apply a census unit based approach to identify, at a macro level, the neighborhoods and wider communities suffering the highest crime harm in New Zealand. This approach enables harm to be viewed not only as a total Index but as a rate controlled for population and allows for the identification of census based sociodemographic factors which predict harm. Specifically, this paper compares the CHI with the New Zealand Priority Locations Index (PLI), an existing census unit based crime analysis tool which combines crime and demographic variables to identify communities vulnerable to crime and disorder issues.In this study CHI and PLI scores were calculated for Census Area Units (normally containing 3000–5000 population) across New Zealand. Bivariate correlations and a general linear model were used to determine the relationships between the CHI and PLI and additional population related variables. The CHI and PLI were weakly correlated, with population size and urban/rural categorization also accounting for CHI variance. Mapping techniques are used to illustrate outlier locations where the CHI and PLI differ widely and to identify location features which may assist in explaining CHI/PLI differences.This work exemplifies a novel geographic approach to the problem of crime harm with implications for resource allocation at national through to local levels. Wider implications for the theory and practice of crime and crime harm control are discussed, along with limitations of the study and areas for further research. 相似文献
105.
Late Devonian to Early Carboniferous stratigraphic units within the ‘Zhulumute’ Formation, Hongguleleng Formation (stratotype), ‘Hebukehe’ Formation and the Heishantou Formation near the Boulongour Reservoir in northwestern Xinjiang are fossil-rich. The Hongguleleng and ‘Hebukehe’ formations are biostratigraphically well constrained by microfossils from the latest Frasnian linguiformis to mid-Famennian trachytera conodont biozones. The Hongguleleng Formation (96.8 m) is characterized by bioclastic argillaceous limestones and marls (the dominant facies) intercalated with green spiculitic calcareous shales. It yields abundant and highly diverse faunas of bryozoans, brachiopods and crinoids with subordinate solitary rugose corals, ostracods, trilobites, conodonts and other fish teeth. The succeeding ‘Hebukehe’ Formation (95.7 m) consists of siltstones, mudstones, arenites and intervals of bioclastic limestone (e.g. ‘Blastoid Hill’) and cherts with radiolarians. A diverse ichnofauna, phacopid trilobites, echinoderms (crinoids and blastoids) together with brachiopods, ostracods, bryozoans and rare cephalopods have been collected from this interval. Analysis of geochemical data, microfacies and especially the distribution of marine organisms, which are not described in detail here, but used for facies analysis, indicate a deepening of the depositional environment at the Boulongour Reservoir section. Results presented here concern mainly the sedimentological and stratigraphical context of the investigated section. Additionally, one Late Devonian palaeo-oceanic and biotic event, the Upper Kellwasser Event is recognized near the section base. 相似文献
106.
脆弱生态环境定量评价方法的研究 总被引:128,自引:4,他引:128
建立了一套系统、完整、客观、灵活且具较强可操作性的脆弱生态环境定量评价指标体系及方法,并通过用此法评价全国26个省、区生态环境脆弱度的方式,对其可操作性和准确性进行了验证。 相似文献
107.
Palmer干旱指数在淮河流域的修正及应用 总被引:3,自引:0,他引:3
Palmer指数是目前研究区域干旱时应用最广泛的指数之一,但由于其空间适用性比较强,所以在应用已修正的Palmer指数描述淮河流域干旱等级和持续时间时和实际情况有较大差异,因此有必要做进一步订正。利用淮河流域开封、信阳、巢湖站1961—2009年逐日降水和气温等常规观测资料,根据1965年Palmer指数原理,在2004、2006年几次修正的基础上对Palmer指数进行修正。淮河流域和美国Kansas、Iowa周边区域均属亚热带季风气候,但在用Palmer指数研究2个区域的旱涝时,其前期旱涝对后期的影响程度存在一定的差异,在淮河流域前期旱涝的影响要小些,因此,在修正Palmer指数过程中加入一个修正系数,用以改善其敏感度。结果表明,修正后的Palmer指数在反映淮河流域的干旱强度和持续时间等方面都得到了很好的改进,可以用于淮河流域干旱的监测和预警。 相似文献
108.
GEV干旱指数及其在气象干旱预测和监测中的应用和检验 总被引:2,自引:0,他引:2
介绍了一种新的基于广义极值分布(Generalized Extreme Value,GEV)的干旱指数的构建过程及其原理。并利用1951—2010年中国地区160个气象台站的逐月降水资料,选择6个代表台站,检验1951_2010年60年间预测的干旱和实际干旱事件发生的吻合程度;利用2009--2010年资料检验GEV指数和实际发生的干旱事件在月尺度上的监测效果。结果表明,利用GEV方法定义的干旱指数能较好地监测干旱事件的发生和发展,与目前广泛使用的ci指数监测结果较为一致。同时这种指数也能用于某一地区干旱事件的预测和干旱等级的划分。对干旱业务监测和预测有重要的参考意义。但该指数对站点疏密有一定的依赖性,站点稀疏的区域存在着一些误差。 相似文献
109.
The identification, remedial treatment, and monitoring of contaminated sediments are among the priorities for managers of the Tampa Bay Estuary. Tampa Bay, as an urbanized estuary, is subject to the input of watershed sources of chemical contaminants, including metals, pesticides, and organic chemicals. Although the use of biological indicators and their incorporation into multi-metric indices is not new, the refinement and applications of such techniques for determining environmental condition still require further development and exploration. We present a single Tampa Bay Benthic Index (TBBI) that was developed specifically for Tampa Bay. Stepwise discriminant analysis was applied to a comprehensive list of potential benthic metrics. Results from the stepwise procedure identified the metrics that best discriminated between "healthy" and "degraded" conditions, as defined by sediment contaminant effect levels and dissolved oxygen. Discriminant analysis was then applied to the resultant three variables to determine the linear combination for the index. 相似文献
110.