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   1288篇
  免费   42篇
  国内免费   8篇
测绘学   61篇
大气科学   100篇
地球物理   269篇
地质学   420篇
海洋学   90篇
天文学   250篇
综合类   5篇
自然地理   143篇
  2021年   9篇
  2020年   9篇
  2019年   12篇
  2018年   29篇
  2017年   29篇
  2016年   31篇
  2015年   19篇
  2014年   24篇
  2013年   74篇
  2012年   47篇
  2011年   50篇
  2010年   48篇
  2009年   59篇
  2008年   59篇
  2007年   56篇
  2006年   41篇
  2005年   42篇
  2004年   34篇
  2003年   38篇
  2002年   37篇
  2001年   23篇
  2000年   41篇
  1999年   22篇
  1998年   22篇
  1997年   20篇
  1996年   21篇
  1995年   21篇
  1994年   13篇
  1993年   22篇
  1992年   20篇
  1991年   14篇
  1990年   23篇
  1989年   16篇
  1988年   18篇
  1987年   18篇
  1986年   14篇
  1985年   14篇
  1984年   22篇
  1983年   26篇
  1982年   19篇
  1981年   15篇
  1980年   16篇
  1977年   10篇
  1976年   10篇
  1975年   12篇
  1974年   10篇
  1973年   9篇
  1972年   19篇
  1971年   12篇
  1969年   7篇
排序方式: 共有1338条查询结果,搜索用时 296 毫秒
501.
502.
This paper expands the human capital model to compare the migration propensities of Cubans, Mexicans, and Puerto Ricans within the U.S. between 1985 and 1990. Using the 5% PUMS from the 1990 U.S. Census, both aggregate migration streams and micro-level migration propensities are estimated for Cubans, Mexicans, and Puerto Ricans. The effects of personal factors, the economic environment, and the presence of fellow nationals are examined in the context of larger geographic patterns, and discussed in terms of each nationality's immigration history and cultural context.

While many factors effect migration behavior similarly, there are notable differences in the way education, English fluency, and unemployment rate affect migration propensity of the native-born and foreign-born of each nationality. Differences are also apparent at the macro-level. Puerto Ricans show signs of dispersing out of New York; Mexicans are redistributing within the Southwest; and Cubans are re-concentrating in Florida. Both levels of analysis point to how immigration history, settlement patterns, and cultural context influence migration behavior. The results demonstrate the benefits of using an expanded human capital approach to explain migration differences, and highlight the diversity of population redistribution occurring within Hispanic nationalities.  相似文献   
503.
In this article, we examine graffiti abatement policies and programs in three West Coast U.S. cities: Portland, San Francisco, and Seattle. Through this analysis, several similarities in the graffiti abatement efforts of each city emerged. Since the 1990s, graffiti abatement efforts in these cities have become increasingly formalized and coordinated. By the early 2000s, the cities of Portland, San Francisco, and Seattle adopted zero-tolerance approaches toward handling graffiti and those caught making graffiti. City governments have placed increasing responsibility for cleaning up graffiti on private property onto property owners. Despite the increased coordination and effort, annual abatement estimates are hard to obtain because tracking spending on graffiti abatement across agencies and bureaus is difficult and often the cities do not comprehensively compile these data.  相似文献   
504.
505.
Matthew W. Wilson 《Geoforum》2012,43(6):1266-1275
The production and consumption of geographic information is becoming a more mobile practice, with more corporate actors challenging the traditional stronghold of Esri- and government-based geospatial developments. What can be considered a geographic information system has expanded to include web-based technologies like Google Earth/Maps, as well as more recent developments of Microsoft’s Bing Maps and the mobile version of ArcGIS available for the iPhone. In addition to these developments, a discursive shift toward ‘location’ is occurring across the Internet industry. Location has become the new buzzword for social-spatial strategies to target consumers. As reported in 2010, venture capitalists have, since 2009, invested $115 million into ‘location start-ups’ – software companies that provide location-based services to mobile computing consumers (Miller and Wortham, 2010). Applications like Foursquare, Loopt, Gowalla, and most recently, Facebook Places allow users to ‘check-in’ at restaurants, bars, gyms, retail outlets, and offices, thereby sharing their location within their social network. These developments enable consumers to (re)discover their proximities to products, while feeding a desire for making known one’s everyday movements. Here, I discuss the development of location-based services as the proliferation of a peculiar form of geographic information: conspicuous mobility. Through discussion of a recent gathering of location-aware software professionals and through analysis of discourses that emerge over a battle between ‘check in’ companies, I sketch an area of study that explores the implications of these emerging geographic information ‘systems’, and new everyday cartographers.  相似文献   
506.
This paper presents the background, objectives, and preliminary outcomes from the first year of activities of the Polish–Norwegian project CHIHE (Climate Change Impact on Hydrological Extremes). The project aims to estimate the influence of climate changes on extreme river flows (low and high) and to evaluate the impact on the frequency of occurrence of hydrological extremes. Eight “twinned” catchments in Poland and Norway serve as case studies. We present the procedures of the catchment selection applied in Norway and Poland and a database consisting of near-natural ten Polish and eight Norwegian catchments constructed for the purpose of climate impact assessment. Climate projections for selected catchments are described and compared with observations of temperature and precipitation available for the reference period. Future changes based on those projections are analysed and assessed for two periods, the near future (2021–2050) and the far-future (2071–2100). The results indicate increases in precipitation and temperature in the periods and regions studied both in Poland and Norway.  相似文献   
507.
Inversion of gravity and/or magnetic data attempts to recover the density and/or magnetic susceptibility distribution in a 3D earth model for subsequent geological interpretation. This is a challenging problem for a number of reasons. First, airborne gravity and magnetic surveys are characterized by very large data volumes. Second, the 3D modelling of data from large‐scale surveys is a computationally challenging problem. Third, gravity and magnetic data are finite and noisy and their inversion is ill posed so regularization must be introduced for the recovery of the most geologically plausible solutions from an infinite number of mathematically equivalent solutions. These difficulties and how they can be addressed in terms of large‐scale 3D potential field inversion are discussed in this paper. Since potential fields are linear, they lend themselves to full parallelization with near‐linear scaling on modern parallel computers. Moreover, we exploit the fact that an instrument’s sensitivity (or footprint) is considerably smaller than the survey area. As multiple footprints superimpose themselves over the same 3D earth model, the sensitivity matrix for the entire earth model is constructed. We use the re‐weighted regularized conjugate gradient method for minimizing the objective functional and incorporate a wide variety of regularization options. We demonstrate our approach with the 3D inversion of 1743 line km of FALCON gravity gradiometry and magnetic data acquired over the Timmins district in Ontario, Canada. Our results are shown to be in good agreement with independent interpretations of the same data.  相似文献   
508.
GRACE's spatial aliasing error   总被引:2,自引:0,他引:2  
  相似文献   
509.
510.
AzTEC is a mm-wavelength bolometric camera utilizing 144 silicon nitride micromesh detectors. Here, we describe the AzTEC instrument architecture and its use as an astronomical instrument. We report on several performance metrics measured during a three-month observing campaign at the James Clerk Maxwell Telescope and conclude with our plans for AzTEC as a facility instrument on the Large Millimetre Telescope.  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