全文获取类型
收费全文 | 5321篇 |
免费 | 711篇 |
国内免费 | 714篇 |
专业分类
测绘学 | 371篇 |
大气科学 | 720篇 |
地球物理 | 1787篇 |
地质学 | 1866篇 |
海洋学 | 670篇 |
天文学 | 112篇 |
综合类 | 212篇 |
自然地理 | 1008篇 |
出版年
2024年 | 46篇 |
2023年 | 102篇 |
2022年 | 185篇 |
2021年 | 224篇 |
2020年 | 234篇 |
2019年 | 234篇 |
2018年 | 206篇 |
2017年 | 209篇 |
2016年 | 247篇 |
2015年 | 214篇 |
2014年 | 331篇 |
2013年 | 465篇 |
2012年 | 252篇 |
2011年 | 267篇 |
2010年 | 209篇 |
2009年 | 267篇 |
2008年 | 321篇 |
2007年 | 312篇 |
2006年 | 294篇 |
2005年 | 258篇 |
2004年 | 203篇 |
2003年 | 199篇 |
2002年 | 184篇 |
2001年 | 142篇 |
2000年 | 145篇 |
1999年 | 131篇 |
1998年 | 112篇 |
1997年 | 112篇 |
1996年 | 105篇 |
1995年 | 87篇 |
1994年 | 71篇 |
1993年 | 70篇 |
1992年 | 61篇 |
1991年 | 36篇 |
1990年 | 35篇 |
1989年 | 31篇 |
1988年 | 32篇 |
1987年 | 22篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 11篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1971年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有6746条查询结果,搜索用时 31 毫秒
991.
992.
993.
This paper examines the use of terrestrial photogrammetry as a technique for measuring bank erosion in a rapidly changing fluvial environment. It has been recognized that there are a number of advantages when applying photogrammetric techniques to geomorphological situations. In this study the enhancement of spatial sampling combined with the ability to capture additional information, such as soil moisture, on film, is of particular importance in enabling the identification of specific processes involved in bank erosion as well as detailed volumetric analysis of losses. Metric terrestrial photography was taken of the river bank on several dates, and data were abstracted by the use of analytical photogrammetry. This enabled the generation of digital terrain models from which morphological and volumetric changes could be assessed. © 1997 John Wiley & Sons, Ltd. 相似文献
994.
995.
随着共建“一带一路”倡议的推进,中国对沿线国家矿业投资稳步增长。由于部分国家特殊的政治、经济、文化背景,使得投资活动常面临众多风险,因此,有必要对矿业投资风险进行科学评价,以提高风险抵御能力。以国际投资风险一体化分析框架为基础,提出从社会一般环境与行业一般环境2个维度展开的矿业投资风险评价“四层次”指标体系,然后运用灰色系统理论的重要工具灰色关联分析对指标体系进行了实证研究。研究结果表明,政局稳定程度是影响矿业投资风险的首要因素,其次为外交关系情况、基础设施建设、矿产资源禀赋、对外单项投资和国民教育水平;以国别进行风险评价的研究结果表明,越南、印度尼西亚等为低风险国家,哈萨克斯坦、波兰等为中风险国家,巴基斯坦、伊朗等为高风险国家。 相似文献
996.
塔里木盆地下志留统塔塔埃尔塔格组沉积体系及沉积模式 总被引:6,自引:3,他引:3
通过对塔里木盆地众多野外剖面详细观测及钻井岩心的详细观察,研究区下志留统塔塔埃尔塔格组主要沉积一套以砂、泥岩为主的碎屑岩,根据岩性特征、生物标志和地球物理特征,将塔里木盆地塔塔埃尔塔格组划分为滨岸和陆棚沉积体系,滨岸沉积体系进一步划分为无障壁海岸和有障壁潮坪沉积,陆棚沉积体系以浅水陆棚沉积为特征,并就各微相的岩石类型、粒度分布和沉积构造特征进行了详细的研究。在此基础上,分析了塔塔埃尔塔格沉积期的平面展布特征,总体格局为南北向分带,东西向展布为特征。同时建立了塔里木盆地塔塔埃尔塔格组滨岸、潮坪和滨岸-潮坪的沉积模式,在研究区以滨岸-潮坪的沉积模式为主。 相似文献
997.
The disruption of a transportation network can have a high social and economic impact on the welfare of a society, as it can significantly affect the daily routines of a community. Although many studies have focused on the estimation of physical risk in the components that compose these networks, only a limited number have analyzed their interconnections and impact in the traffic flow. The present study analyzes how earthquake damage can disrupt the road network in an urban environment, and how this will influence the ability of the population to travel. Traffic due to daily commutes is modeled for different layouts of the network, corresponding to possible disruptions caused by earthquake damage. The duration and length of each trip were calculated both for the undamaged network conditions and for the disrupted network. The increase in the median duration and length of each trip allows estimating the economic loss for each event due to drivers' delay. By combining the probability of a specific road being blocked with its number of users, the average number of affected vehicles was estimated, and the most critical segments identified. The methodology was applied to a case study concerning the road network of the area around the Italian city of Messina in Sicily. The results were calculated for both a repetition of the well-known historical event of 1908 and a set of simulated earthquakes consistent with the national probabilistic seismic hazard model of Italy. 相似文献
998.
WEN Boyu XU Chong HE Xiangli MA Siyuan SHAO Xiaoyi LI Kai ZHANG Zhongjian LI Zhengfang 《中国地震研究》2020,34(1):64-80
An MW6.6 earthquake occurred in eastern Hokkaido, Japan on September 6th, 2018. Based on the pre-earthquake image from Google Earth and the post-earthquake image from high resolution (3 m) planet satellite, we manually interpret 9 293 coseismic landslides and select 7 influencing factors of seismic landslide, such as elevation, slope, slope direction, road distance, flow distance, peak ground acceleration (PGA) and lithology. Then, 9 293 landslide points are randomly divided into training samples and validation samples with a proportion of 7:3. In detail, the training sample has 6 505 landslide points and the validation sample has 2 788 landslide points. The hazard risk assessment of seismic landslide is conducted by using the information value method and the study area is further divided into five risk grades, including very low risk area, low risk area, moderate risk area high risk area and very high risk area. The results show that there are 7 576 landslides in high risk area and very high risk area, accounting for 81.52% of the total landslide number, and the landslide area is 22.93 km2, accounting for 74.35% of the total area. The hazard zoning is in high accordance with the actual situation. The evaluation results are tested by using the curve of cumulative percentage of hazardous area and cumulative percentage of landslides number. The results show that the success rate of the information value method is 78.50% and the prediction rate is 78.43%. The evaluation results are satisfactory, indicating that the hazard risk assessment results based on information value method may provide scientific reference for landslide hazard risk assessment as well as the disaster prevention and mitigation in the study area. 相似文献
999.
The importance of large wood (LW) to riverine functions is well established scientifically and increasingly recognized by river managers in many countries. However, public perceptions largely associate LW with elevated danger and/or need for intervention. Such perspectives are amplified amongst recreational river users (defined here as any individuals that recreate by floating on the water surface of a river) who interact more directly with rivers than the general public and commonly view wood in life-or-death terms. Given that human life occupies a highest-order charge for river managers, they are left in a difficult position when safety appears to conflict with environmental services. LW deficits are perpetuated partly because wood removal, often in the name of safety, is far easier than placing wood in rivers. Further, river restoration practitioners are frequently burdened with expectations and liability unparalleled in built environments. A fundamentally different mindset is necessary to achieve desired ecologic outcomes when working with rivers. Based on two decades of experience as boaters, LW practitioners, and emergency responders, we (1) discuss LW hazard and risk from recreational and management viewpoints, (2) discretize objective and measurable physical properties of LW hazards, and (3) propose a decision framework that implicitly addresses risk by considering LW hazards relative to river use and ambient hazards. The approach is structured to increase objectivity in LW hazard mitigation and diminish asymmetric biases that favor LW removal. Our intent is to build understanding and rational flexibility among risk-averse management, regulatory, and funding entities to facilitate implementation of scientific understanding without undue risk to river users. © 2020 John Wiley & Sons, Ltd. 相似文献
1000.
Álvaro Gómez-Gutiérrez Trent Biggs Napoleon Gudino-Elizondo Paz Errea Esteban Alonso-González Estela Nadal Romero José Juan de Sanjosé Blasco 《地球表面变化过程与地形》2020,45(11):2524-2539
Image network geometry, including the number and orientation of images, impacts the error, coverage, and processing time of 3D terrain mapping performed using structure-from-motion and multiview-stereo (SfM-MVS). Few studies have quantified trade-offs in error and processing time or ways to optimize image acquisition in diverse topographic conditions. Here, we determine suitable camera locations for image acquisition by minimizing the occlusion produced by topography. Viewshed analysis is used to select the suitable images, which requires a preliminary digital elevation model (DEM), potential camera locations, and sensor parameters. One aerial and two ground-based image collections were used to analyse differences between SfM-MVS models produced using: (1) all available images (ALL); (2) images selected using conventional methods (CON); and (3) images selected using the viewshed analysis (VIEW). The resulting models were compared with benchmark point clouds acquired by a terrestrial laser scanner (TLS) and TLS-derived DEMs. The VIEW datasets produced denser point clouds (28–32% more points) and DEMs with up to 66% reduction in error compared with CON datasets due to reduction of gaps in the DEM. VIEW datasets reduced processing time by 37–76% compared with ALL, with no reduction in coverage or increase in error. DEMs produced with ALL and VIEW datasets had similar slope and roughness, while slight differences that may be locally important were observed for the CON dataset. The new method helps optimize SfM-MVS image collection strategies that significantly reduce the number of images required with minimal loss in coverage or accuracy over complex surfaces. © 2020 John Wiley & Sons, Ltd. 相似文献