全文获取类型
收费全文 | 2320篇 |
免费 | 311篇 |
国内免费 | 357篇 |
专业分类
测绘学 | 145篇 |
大气科学 | 394篇 |
地球物理 | 625篇 |
地质学 | 814篇 |
海洋学 | 149篇 |
天文学 | 425篇 |
综合类 | 108篇 |
自然地理 | 328篇 |
出版年
2024年 | 6篇 |
2023年 | 27篇 |
2022年 | 43篇 |
2021年 | 69篇 |
2020年 | 63篇 |
2019年 | 100篇 |
2018年 | 63篇 |
2017年 | 95篇 |
2016年 | 110篇 |
2015年 | 98篇 |
2014年 | 150篇 |
2013年 | 231篇 |
2012年 | 125篇 |
2011年 | 118篇 |
2010年 | 122篇 |
2009年 | 192篇 |
2008年 | 179篇 |
2007年 | 119篇 |
2006年 | 129篇 |
2005年 | 145篇 |
2004年 | 88篇 |
2003年 | 90篇 |
2002年 | 63篇 |
2001年 | 54篇 |
2000年 | 41篇 |
1999年 | 35篇 |
1998年 | 18篇 |
1997年 | 21篇 |
1996年 | 59篇 |
1995年 | 118篇 |
1994年 | 138篇 |
1993年 | 11篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 16篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
排序方式: 共有2988条查询结果,搜索用时 15 毫秒
471.
《The Journal of geography》2012,111(6):213-223
Abstract This study uses survey and focus group methods to explore attitudes toward and experiences of nature among millennial-aged students in northern Colorado. First, results confirm that young people possess a strong interest in the outdoors yet time, transportation, and new technologies hamper their ability to visit public lands and outdoor spaces. Second, respondents experience nature in ways more mediated by new technologies such as cell phones and nature DVDs. Third, citizen science and active learning show promise as techniques for returning young people to the outdoors. The challenge lies in teaching young people to reimagine the outdoors not as elsewhere or out there but as a backyard geographic space. 相似文献
472.
473.
Giorgos Mountrakis 《International journal of geographical information science》2013,27(11):1389-1412
Moose–vehicle collisions (MVCs) pose a serious safety and environmental concern in many regions of Europe and North America. For example, in the state of Vermont, one‐third of all reported MVCs resulted in motorist injury or fatality while collisions have increased from two in 1982 to 164 in 2002. Our work used a MVC dataset from 1983 to 1999 in the Northeastern Highlands of Vermont (four major roads) to perform space, time and spatiotemporal analyses and guide future mitigation strategies. An adapted kernel density estimator was implemented for exploratory analyses to detect high density collision hotspots on roads. The kernel in space showed seven major density peaks which varied in magnitude and spread between roads. The kernel estimator in time for all roads showed an exponentially increasing trend with annual periodicity and a seasonal cyclic component, where the majority of collisions occurred from May to October. Spatiotemporal kernel estimation exhibited discontinuous density hotspots in time and space suggesting changing animal movement patterns across roads. We used an adapted Ripley's K‐function to test the hypothesis that MVCs clustering occurred at multiple scales in space, in time and in space–time combined. Statistically significant spatial clustering was evident on all roads at spatial scales from 2 to 10 km. A more consistent clustering in time occurred on all roads at a scale distance of 5 years. Similar to the kernel estimation, annual periodicity was also evident. Positive space–time clustering was present at small spatial (5 km) and temporal scales (2 years) indicating that where MVCs occur is also influenced by when they occur. In retrospect, using multiple road lengths, and the combined kernel estimation and Ripley's K‐function in time and space, provided a powerful methodology to study varying spatiotemporal patterns of wildlife collisions along roads. This can greatly assist transportation planners in identifying optimal mitigation strategies along specific roads, such as deciding on location and spatial length for permanent and expensive measures (e.g. crossing structures and associated fencing) versus less permanent and inexpensive structures (e.g. wildlife signage and reduced speed limits). 相似文献
474.
Marcelo Roberto Barbosa Mario Guimarães Buratto Leonardo Santana de Oliveira Dias João Paulo Carvalho Raivel Flavio Lobos Martins 《International journal of geographical information science》2013,27(6):873-901
To increase the monitoring potential of forest fires, an alert classification methodology using satellite-mapped hotspots has been established to help forest managers in the prioritization of which hotspot to be verified in the field, thus potentially improving the distribution of fire-fighting resources. A computer application was developed based on web-distributed geographical information technology whose main function is to interact automatically generated satellite hotspots and risk areas indicated in fire-susceptibility maps and classify them into five alert levels. The location of the hotspots is available continuously every 4 h, and a susceptibility map is produced daily through map algebra algorithm, which uses static (topography, vegetation and land use) and dynamic (weather) variables. Every process runs through automated geoprocessing routines. The methodology was tested during the dry period of 2007 in the Carajás National Forest, in the Brazilian Amazon, within an area of 400,000 ha. It is a critical area constantly threatened by fires caused by invasions and deforestation owing to intense agribusiness advances and mining activities in its surroundings. This situation results in observations of many hotspots inside the study area for the same day and almost the same time period, in places of extreme opposites, demanding complex rapid analysis and hindering the decision of the displacement of fire-fighting teams. Further, a major mining company operates within the National Forest area, maintaining actions of protection as part of its environmental mining license. Results are presented under three aspects: (i) the credibility of the daily susceptibility map (algorithm), which showed strong correlation between areas of greatest risks and the confirmed forest fires; (ii) the reliability of hotspots (alert levels), confirming 71% of fires; (iii) accuracy in the decision of which hotspot to be checked, which revealed the same number of verifications at different alert levels, 82% confirmed alert 5 hotspots (maximum) and only 50% from alert 1 (minimum), resulting in faster fire-fighting actions, minimizing burned areas and, in some cases, allowing fire control before its spreading. Therefore, the methodology demonstrated that GIS routines are able to determine the relationship between a reality-based, interpreted susceptibility map of the area and satellite-generated hotspots, highlighting the ones of highest hazard level through the alert classification, becoming an important tool to help decisions from the fire-control center, especially for high-risk regions. The methodology may be extrapolated to other forested areas. 相似文献
475.
在"旅游网站信息流距离衰减形态描述与集中度计算"[1]一文的基础上,从旅游网站性质的角度对旅游网站信息流距离衰减形态与集中性的差异进行指数衰减模型的拟合分析,并应用Zipf定律检验其特征,发现:1)不同性质的旅游网站信息流距离衰减形态存在差异,旅游主管部门网站、旅游企业网站、旅游目的地网站的信息流距离衰减形态分别表现出非常符合指数衰减模型、基本符合指数衰减模型、不符合指数衰减模型特征。2)不同性质的旅游网站信息流距离衰减的集中性存在差异,分别表现出具有明显的本地集中性特征、兼有本地集中性和经济集中性特征、具有明显的经济集中性特征。3)分维值多在0.5左右说明了旅游网站信息流距离衰减集中性较强,分维值的差异说明了不同性质旅游网站信息流距离衰减集中性存在差异。旅游网站信息流的位序-规模均符合Zipf定律,其首位省市均存在于网站所在地或者周边省市,其位序-规模的前四位总体为经济发达省市,说明了旅游网站信息流距离衰减具有本地和经济集中性。4)对旅游网站信息流距离衰减集中性的Zipf检验,证实了空间距离集中性特征的普遍意义。 相似文献
476.
南极洲的泥盆系主要分布于横贯南极山脉的麦克默多和俄亥俄岭-埃尔斯沃思山等两个沉积盆地中。前一个盆地的泥盆系代表从海岸泻湖-河流三角洲到近岸冲积平原的层序;后一个盆地的彭萨科拉山的泥盆系较厚,从非海相冲积扇-冲积平原-浅海相,最后又恢复到非海相沉积环境,但在俄亥俄岭却沉积了厚度不大的浅海相地层,含Malvinokafric生物地理大区的海相双壳类、腹足类、三叶虫、竹节石和鱼类等化石。除了上述两个沉积盆地外,在罗斯海两边却出露了火山岩,说明该地当时处于俯冲带附近的火山弧中。中国华南的曲靖型和西北的祁连山型泥盆系也属于滨海相和非海相沉积,它们与南极洲的泥盆系可资比较,但两者的生物地理区系并不相同 相似文献
477.
中国南极中山站位于南极拉斯曼丘陵地区(λ=76°22′E,φ=69°22′S),1991年3月至1993年2月,用LacosteET型重力仪,在这个地区进行了为期近两年的观测,取得了600多天的有效资料,经调和分析和流变模型、海潮负荷改正后得到:δo1=1.2253±0.0031,Δφo1=0.73°±0.14°;δm2=1.0785±0.0045,Δφm2=-3.55°±0.29°从资料分析的精度来看,观测资料是比较好的;但由于海潮负荷改正缺少附近海域的资料,会对最终结果造成较大的影响 相似文献
478.
479.
China’s tourism industry has witnessed rapid progress in recent years, and is now an important part of global tourism in dealing with climate change. Within a framework of Pressure-State-Response (PSR), this paper focuses on the emission reduction pressure, carbon emission status, and responses of stakeholders in China’s tourism industry. Findings include: 1) The central government’s strategy and rapid growth of the industry scale exert rising pressure on China’s tourism to reduce carbon emissions. 2) Carbon emissions of China's tourism account for 13%-14.6% of global tourism, and about 3% of China’s emissions overall. Chinese tourists’ per capita carbon emission is lower than half of the global level. 3) The Chinese government attaches great importance to energy-saving and carbon emission reduction. In the tourism industry, documents, standards and other regulative measures have been issued to ensure that business practitioners set up green operational and managerial systems. In the field of tourism transportation, China's high-speed rail, new energy vehicles, and urban shared bicycles, have developed very rapidly in recent years, and they have effectively reduced the carbon emissions in traveling. Furthermore, this paper finds that Chinese tourists already have awareness and willingness for low-carbon tourism. 相似文献
480.