首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1321篇
  免费   18篇
  国内免费   8篇
测绘学   29篇
大气科学   14篇
地球物理   224篇
地质学   417篇
海洋学   21篇
天文学   562篇
综合类   1篇
自然地理   79篇
  2021年   13篇
  2020年   9篇
  2019年   14篇
  2018年   8篇
  2017年   35篇
  2016年   24篇
  2015年   29篇
  2014年   34篇
  2013年   56篇
  2012年   25篇
  2011年   38篇
  2010年   35篇
  2009年   87篇
  2008年   54篇
  2007年   78篇
  2006年   57篇
  2005年   67篇
  2004年   68篇
  2003年   56篇
  2002年   59篇
  2001年   42篇
  2000年   64篇
  1999年   62篇
  1998年   55篇
  1997年   24篇
  1996年   19篇
  1995年   16篇
  1994年   19篇
  1993年   8篇
  1992年   8篇
  1990年   7篇
  1989年   15篇
  1988年   7篇
  1987年   9篇
  1986年   4篇
  1985年   10篇
  1984年   5篇
  1983年   11篇
  1982年   11篇
  1981年   10篇
  1980年   12篇
  1979年   9篇
  1978年   9篇
  1977年   11篇
  1976年   6篇
  1975年   6篇
  1974年   8篇
  1973年   8篇
  1971年   4篇
  1970年   7篇
排序方式: 共有1347条查询结果,搜索用时 31 毫秒
51.
The Romanian Danube-Black Sea Canal, opened in 1984, is the latest development in the European network of Class 6 waterways. Construction began in 1978 and involved the mobilisation of labour and equipment on an unprecedented scale to expedite work in four construction zones between the Danube at Cernavoda and the Black Sea at South Constanta-Agigea. The paper outlines the construction work and also discusses three other themes. First the history of abortive canal projects which extends back for more than a century, including the notorious scheme of 1949–1953 in the Stalin era, which has to be seen in the context of available waterways through the Danube delta and a complex of economic, political and technical issues. Second the interest of the present Romanian government in pressing ahead with the project at a time of severe economic difficulty: the changing economic geography of the country is thoroughly examined. And third the likely impact of the canal which is very difficult to assess in an international context in view of the alternative route through the Danube delta at Sulina. However the hinterland of Constanta has been greatly improved and further canal schemes in Romania will generate more traffic for water transport in the years ahead.  相似文献   
52.
We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°-78°. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71° by the time of the expansion phase onset. A westward electrojet, centred at 65.4°, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1° at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the “distant” neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.  相似文献   
53.
In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of these waves has been investigated and it has been shown that there are large differences between the polarization azimuth distributions on either side of noon. In the morning the azimuth distributions are spread evenly across all possible orientations whereas in the afternoon they are predominantly in a North-South orientation. The difference appears to be the result of a specific polarization pattern that occurred across the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all occurred in the morning sector, close to the dawn terminator. It is suggested that this special polarization pattern is a consequence of ionospheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarization variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.  相似文献   
54.
55.
56.
Alan C. McKinnon 《Geoforum》1983,14(4):389-399
Over the past decade there has been a growing recognition by the business community, central government and local planners of the economic importance of warehousing. This importance can be measured in terms of the value of stock held in warehouses, their very rapid growth relative to other types of commercial property and the employment they create. This article reviews changes in business and planning attitudes to warehousing, accounts for the recent growth of warehouse floorspace in England and examines the spatial distribution of this growth.  相似文献   
57.
A crustal seismic velocity model for the UK, Ireland and surrounding seas   总被引:1,自引:0,他引:1  
A regional model of the 3-D variation in seismic P -wave velocity structure in the crust of NW Europe has been compiled from wide-angle reflection/refraction profiles. Along each 2-D profile a velocity–depth function has been digitised at 5 km intervals. These 1-D velocity functions were mapped into three dimensions using ordinary kriging with weights determined to minimise the difference between digitised and interpolated values. An analysis of variograms of the digitised data suggested a radial isotropic weighting scheme was most appropriate. Horizontal dimensions of the model cells are optimised at 40 × 40 km and the vertical dimension at 1 km. The resulting model provides a higher resolution image of the 3-D variation in seismic velocity structure of the UK, Ireland and surrounding areas than existing models. The construction of the model through kriging allows the uncertainty in the velocity structure to be assessed. This uncertainty indicates the high density of data required to confidently interpolate the crustal velocity structure, and shows that for this region the velocity is poorly constrained for large areas away from the input data.  相似文献   
58.
The Apuseni Mountains constitute one of the most interesting parts of the Carpathians in terms of landscape, biodiversity and culture. Yet dislocations are now severe and there are development threats which also require a significant response. On the one hand there are some severe pollution problems associated with mining areas developed in the communist period which require remedial action and the cases of Roşia Montana and Zlatna are discussed. On the other hand, there are forest and pasture zones – especially the Padiş Plateau – which are coming under heavy pressure from farmers seeking to enhance their incomes following the decline of mining and manufacturing which has left many households without salaries. At the same time, tourist pressure is growing. Actions are already being taken to limit pollution damage and develop sustainable landuse practices. There is a national park project linked with nature reserves comprising picturesque landscapes, a rare flora and fauna and distinct geological and palaeotological formations (including fossil ice). There are major tourist attractions and with proper management they could become part of a development programme of the countryside. However it is essential that all stakeholders adopt sustainable practices and this means that official programmes – including the future national park authority administration's work in environment protection and tourism management – are backed up by Environmental Non-Governmental Organisations (ENGOs). The paper reviews their diverse activities and concludes that they are making a very substantial contribution in ensuring that the potentials of isolated regions can be utilised in a manner consistent with nature conservation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
59.
Industrial archaeology has developed as an important aspect of conservation during the second half of the twentieth century in the Western world and there have been great opportunities in transition countries since 1989 on account of economic restructuring and the importance given to 'niche' tourism. While the core interest lies with machines and industrial buildings, attention is also given to transport systems and the social context, with respect to relations between owners, managers and workers. Moreover, the desirability of maintaining old industrial installations within their traditional landscape settings enables industrial archaeology to make a contribution to sustainable development. This paper considers the prospects for Romania, with particular reference to the Reşiţa area of the Banat Carpathians. The mineral resources of this region gave rise to a metallurgical industry which was established under Habsburg administration in the eighteenth century and continues today. In addition to the principal factories in Anina and Reşiţa, the mining installations, transport systems and power stations contribute to a diverse industrial region in a distinctive physical and cultural setting. The development of the complex is summarised and the opportunities for conservation are evaluated. The tourist infrastructure of the area is also examined with regard to the range of accommodation available. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
60.
Propagating effects of database generalization on the viewshed   总被引:2,自引:0,他引:2  
Few studies have systematically examined the effects of different possible generalization methods on the products of GIS operations. In this paper the effects of generalizing a Digital Elevation Model (DEM) on the area which is determined to be visible (the viewshed) is examined. Among the many different methods of generalization which are possible, a limited set of operators is examined here. First, they all fit geometrically from one resolution to another, and they can be grouped into two types: regular sampling and statistical summaries. In the latter class four different operations are used: for any cell at the target resolution, elevations are determined, from the arithmetic mean, the maximum, the minimum, and the maximum difference from the mean of the cells within the kernel at the original scale. Changes in resolution of 0.5, 0.33, 0.25, and 0.2 of the original study area are studied at 100 viewing points in each of two study areas. At the original resolution a 120 × 120 pixel area was examined, giving generalizations to 60 × 60, 40 × 40, 30 × 30, and 24 × 24 pixels. The viewsheds determined over these different resolution DEMs are compared with a number of possible viewsheds derived by generalization of the viewshed over the original DEM. Of those tested, the maximum deviation from the mean within kernel emerges as the most reliable estimate of the pattern and area of the viewshed at all resolutions. The importance of this conclusion is that different generalization operators yield more or less faithful versions of the ungeneralized product (the viewshed), and it seems indicative that a similar variety in stability of the product with generalization operator will result in many, if not all, complex products of spatial analysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号