全文获取类型
收费全文 | 1025篇 |
免费 | 75篇 |
国内免费 | 99篇 |
专业分类
测绘学 | 93篇 |
大气科学 | 34篇 |
地球物理 | 43篇 |
地质学 | 355篇 |
海洋学 | 464篇 |
天文学 | 12篇 |
综合类 | 71篇 |
自然地理 | 127篇 |
出版年
2023年 | 7篇 |
2022年 | 25篇 |
2021年 | 27篇 |
2020年 | 37篇 |
2019年 | 30篇 |
2018年 | 20篇 |
2017年 | 72篇 |
2016年 | 53篇 |
2015年 | 45篇 |
2014年 | 70篇 |
2013年 | 47篇 |
2012年 | 71篇 |
2011年 | 58篇 |
2010年 | 82篇 |
2009年 | 56篇 |
2008年 | 60篇 |
2007年 | 60篇 |
2006年 | 54篇 |
2005年 | 20篇 |
2004年 | 40篇 |
2003年 | 27篇 |
2002年 | 23篇 |
2001年 | 15篇 |
2000年 | 17篇 |
1999年 | 17篇 |
1998年 | 24篇 |
1997年 | 19篇 |
1996年 | 20篇 |
1995年 | 12篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 10篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 13篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有1199条查询结果,搜索用时 15 毫秒
541.
Community Supported Fishery (CSF) Programs are arrangements between fishermen and consumers where consumers provide upfront payments to fishermen in exchange for scheduled seafood deliveries. They are modeled after the popular Community Supported Agriculture Programs, a form of direct-to-consumer-marketing in which a group of individuals support a farm. There are multiple market and non-market benefits from these programs. Fishermen receive higher prices for fish, are guaranteed a stable income, and can activate political and regulatory support through direct interaction with consumers. Consumers are provided with access to high-quality novel types of fish and benefit from interactions with the producers of their food. CSFs have frequently collaborated with non-governmental organizations to address the challenges associated with these programs. Under the catch share system in the Northeast US groundfish fishery, sectors may be well-positioned to implement a CSF. Direct marketing through a CSF is not likely to completely replace traditional markets for fishermen, but can be a valuable supplement to their operations. 相似文献
542.
Improving the economic performance of fisheries is becoming increasingly important in fisheries management, and in some cases, maximum economic yield (MEY) is set as a key management target. However, recent critics of MEY as a management target have argued that a decline in the level of fishing activity necessary to achieve the target will result in a subsequent loss of economic activity elsewhere in the economy and, potentially, a net loss to society. In this paper, an input–output framework is used to estimate the net economic effects of achieving MEY in several Australian fisheries for which there is information on their short- and long-term performances when moving towards MEY. While overall losses were found in the short term, achieving MEY was found to result in a net economic benefit to society in the longer term. Local coastal communities in particular were found to benefit, although some losses were incurred elsewhere in the economy. 相似文献
543.
根据模糊集原理,提出了一种数量型数据挖掘关联规则的方法,并通过试验证明了算法的合理性. 相似文献
544.
In this study, the nature of basin‐scale hydroclimatic association for Indian subcontinent is investigated. It is found that, the large‐scale circulation information from Indian Ocean is also equally important in addition to the El Niño‐Southern Oscillation (ENSO), owing to the geographical location of Indian subcontinent. The hydroclimatic association of the variation of monsoon inflow into the Hirakud reservoir in India is investigated using ENSO and EQUatorial INdian Ocean Oscillation (EQUINOO, the atmospheric part of Indian Ocean Dipole mode) as the large‐scale circulation information from tropical Pacific Ocean and Indian Ocean regions respectively. Individual associations of ENSO & EQUINOO indices with inflow into Hirakud reservoir are also assessed and found to be weak. However, the association of inflows into Hirakud reservoir with the composite index (CI) of ENSO and EQUINOO is quite strong. Thus, the large‐scale circulation information from Indian Ocean is also important apart form the ENSO. The potential of the combined information of ENSO and EQUINOO for predicting the inflows during monsoon is also investigated with promising results. The results of this study will be helpful to water resources managers due to fact that the nature of monsoon inflow is becoming available as an early prediction. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
545.
Guocheng Pan 《Mathematical Geology》1995,27(5):609-632
A basic task in earth-science data integration is to quantify variable associations. Although manv measures have been used to determine the associations between quantitative variables, the ability to quantify qualitative attributes (e.g., categorical) is limited. Moreover, most traditional association measures are restricted to linear correlations or similarities, for example, correlation coefficient. The measures proposed in this report are designed on the basis of Shannon's entropy concepts, including directional related information, ordinary related information, and partial related information. The directional related information quantifies the association of one variable in terms of another. The ordinary related information determines the mutual association of two variables. The partial related information characterizes the association of an individual stale of one variable in terms of another variable. The properties of these measures are discussed and their sample estimates are derived from both maximum likelihood and Bayesian methods. The relations between these measures are illustrated by using synthetic examples. Two applications of these measures also are developed, including the selection of variables and evaluation of mineral resources. Finally, a case study is given to demonstrate the use of the measures in mineral resources evaluation. 相似文献
546.
耀星是红矮星早期演化的一个必经阶段,这个阶段的持续时间取决于恒星的质量。在年轻和比较年轻的疏散星闭中,都含有大量耀星,它们的光度分布是疏散星闭年龄的标志。在昴生闭中已发现大量耀星,对它们的自行,测光和分光资料进行深入的统计研究,对于解决疏散星闭及矮星起源和演化问题,有着极其重要的价值。 相似文献
547.
548.
演化经济地理强调与本地的技术关联对技术演化的影响,在建设科技创新中心的过程中,向复杂技术的升级需要充分考虑本地技术结构及演化特点。论文以北京、上海和深圳3个国际科技创新中心建设城市为例,将复杂性和关联性纳入研究框架,比较分析技术关联和复杂性对城市技术演化的影响。研究发现,北京、上海和深圳的技术结构、技术关联度、复杂性和演化路径存在明显的差异。上海的技术关联度较高,而复杂度偏低;深圳的技术关联度最低,技术复杂度最高;北京居中。从演化趋势看,北京经历了明显的向复杂技术演化的过程;深圳的整体技术复杂度上升幅度最大,但在2006年以后出现向低复杂度技术多元化发展的趋势;上海的技术复杂度上升幅度最小。计量分析表明,与本地技术关联越强的技术越有利于成为3个城市下一阶段的优势技术,但复杂度高的技术更容易成为北京下一阶段的优势技术,对上海和深圳的技术演化没有显著影响。未来国际科技创新中心建设需要充分考虑本地技术基础和演化特征,并制定针对性的技术发展策略。 相似文献
549.
水下重力异常相关极值匹配算法 总被引:2,自引:1,他引:1
重力辅助惯性导航是利用地球物理特征信息——重力来修正水下潜器惯性导航误差,其关键技术是匹配算法。基于平均平方差最小准则构造差分降权相关目标函数模型,针对受干扰误差随机影响,基于目标函数模型搜索得到的多个有效位置,均有可能以不同概率密度源于正确位置这一问题,提出概率数据关联滤波重力匹配算法,与最近邻法相比,该算法确定的航迹更接近于真实位置,提高算法的可靠性与抗干扰性;分析探讨序列采样间隔对匹配精度的影响。通过试验区仿真匹配,结果表明,当选择适当的采样间隔与采样长度,该算法能有效修正导航误差。 相似文献
550.
以城镇扩展为核心内容的城市土地利用/土地覆盖变化已经成为目前国内土地利用/土地覆盖变化(LUCC)研究的热点.基于某地2002年至2007年土地利用数据,提取城镇用地信息,利用全局和局部空间关联模型,分析了区域城镇扩展的空间分布特征,得出了相关研究成果. 相似文献