全文获取类型
收费全文 | 81352篇 |
免费 | 1204篇 |
国内免费 | 609篇 |
专业分类
测绘学 | 2106篇 |
大气科学 | 5641篇 |
地球物理 | 16442篇 |
地质学 | 28446篇 |
海洋学 | 7213篇 |
天文学 | 18495篇 |
综合类 | 235篇 |
自然地理 | 4587篇 |
出版年
2021年 | 789篇 |
2020年 | 888篇 |
2019年 | 1034篇 |
2018年 | 2053篇 |
2017年 | 1945篇 |
2016年 | 2312篇 |
2015年 | 1260篇 |
2014年 | 2203篇 |
2013年 | 3912篇 |
2012年 | 2472篇 |
2011年 | 3267篇 |
2010年 | 2961篇 |
2009年 | 3834篇 |
2008年 | 3425篇 |
2007年 | 3502篇 |
2006年 | 3182篇 |
2005年 | 2358篇 |
2004年 | 2372篇 |
2003年 | 2265篇 |
2002年 | 2207篇 |
2001年 | 1933篇 |
2000年 | 1810篇 |
1999年 | 1584篇 |
1998年 | 1586篇 |
1997年 | 1548篇 |
1996年 | 1286篇 |
1995年 | 1245篇 |
1994年 | 1116篇 |
1993年 | 997篇 |
1992年 | 876篇 |
1991年 | 919篇 |
1990年 | 930篇 |
1989年 | 876篇 |
1988年 | 819篇 |
1987年 | 926篇 |
1986年 | 865篇 |
1985年 | 1060篇 |
1984年 | 1203篇 |
1983年 | 1128篇 |
1982年 | 1050篇 |
1981年 | 983篇 |
1980年 | 894篇 |
1979年 | 853篇 |
1978年 | 833篇 |
1977年 | 799篇 |
1976年 | 696篇 |
1975年 | 711篇 |
1974年 | 684篇 |
1973年 | 748篇 |
1972年 | 502篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Station coordinates are combined with velocities estimated by space geodesy techniques to produce the International Terrestrial
Reference System. The input is sets of coordinates and velocities calculated by International Earth Rotation Service analysis
centers using space geodesy techniques. The working reference system of individual analysis centers is generally conventionally
defined. However, the implications of such processing can have an effect on the resulting combined set. The problem of datum
definition as a function of coordinate combinations is reviewed. In particular, the problem of minimum constraints is clearly
emphasized and the reference system effect is defined. The goal is to build a process that could be used generally to remove
uncertainties in the underlying coordinate system without disturbing the underlying information with additional unnecessary
information.
Received: 25 January 1999 / Accepted: 20 September 2000 相似文献
992.
Kieran P. Donaghy 《Journal of Geographical Systems》2001,3(3):257-270
This paper presents and demonstrates a general approach to solving spatial dynamic models in continuous space and continuous
time that characterize the behaviour of intertemporally and interspatially optimizing agents and estimating from discrete
data the parameters of such models. The approach involves the use of a projection method to solve the models and a quasi-Newton
algorithm to update quasi-FIML parameter estimates.
Received: 26 July 2000 / Accepted: 31 January 2001 相似文献
993.
B C Sarkar B S Deota P L N Raju D K Jugran 《Journal of the Indian Society of Remote Sensing》2001,29(3):151-164
GIS. a potential tool for facilitating the generation and use of thematic information, has been applied to groundwater potentiality of the Shamri micro-watershed in Shimla Taluk. The role of various parameters, namely, drainage. lineament. lithology . slope and landuse have been emphasised for delineation of groundwater potential iones. IRS-I C IAN and LISS Ill FCC merged satellite images on 1:25000 scale and Topographic map no. 53L/4 SI together with field traverses have been used as the data source. A multi-criteria evaluation following probability weighted approach has been applied for overlay analysis that allows a linear combination of weights of each thematic map with the individual capability value. 1 he resultant map indicates a high groundwater potentiality in the flood plains, river terraces and river channels in the vicinity of the Shamri nala. Other sites of high potentiality include places showing break in slopes and criss-crossing of lineaments. 相似文献
994.
Vinod K. Hanna M. S. Ravichandran S. P. S. Kushwaha 《Journal of the Indian Society of Remote Sensing》2001,29(1-2):41-46
This paper deals with corridor analysis in Rajaji-Corbett Elephant Reserve in the Shivaliks of Uttaranchal state. Efforts were made to detect changes in the state of forest covervis- à -vis the status of corridors during the three periods i.e. 1967. 1986 and 1998 using remote sensing and GIS. TheERDAS Imagine digital image processing andArcView GIS software packages were used for this purpose. Temporal satellite imagery and ground observations in the Rajaji-Corbett Elephant Reserve revealed forest loss, degradation and disturbances in the corridor areas, hindering elephant movement and restricting them to forests islands. Motichur-Chilla corridor, despite being a highly favoured habitat for the elephants was found to be highly threatened followed by Kotdwar and Ramnagar corridors. Construction of wide bridges across Kunao-Chilla Canal, recreation of corridors through reforestation, reduction of all kinds of pressures in the corridor areas and providing higher protection to corridors are recommended. 相似文献
995.
M. C. Gupta V. P. Krishnarajan Shailesh Nayak 《Journal of the Indian Society of Remote Sensing》2001,29(1-2):79-83
Aquaculture has emerged as one of the fast growing industries in the developing countries both for domestic consumption as well as for the export. This industry not only generates foreign exchange to the country but also provides employment opportunities to the skilled and unskilled rural poor. On the other hand, aquaculture has also some negative impacts on the environment which are mainly due to conversion of mangroves and agricultural, salination of surface water resources and agricultural land at some places besides causing pollution and diseases. All the major shrimp farming countries of the world have faced environmental problem due to intensification, improper and uncontrolled planning by greed and unlimited profit motives of a section of aquaculture community. In view of this, a proper environmental management is a basic need to sustain the industry in the long run. Selection of a suitable site in the coastal areas is the first and the foremost step which is a crucial factor in determining the success of shrimp farming. The present paper discusses the selection of brackish water aquaculture sites using remote sensing satellite data and GIS on the Cannanore coast. Different aquaculture sites in the area were evaluated using a GIS package. Finally a prioritization map showing the sites in the descending order of their priority was generated. 相似文献
996.
Effects of the spherical terrain on gravity and the geoid 总被引:3,自引:0,他引:3
997.
998.
The success rate and precision of GPS ambiguities 总被引:7,自引:1,他引:7
P. J. G. Teunissen 《Journal of Geodesy》2000,74(3-4):321-326
An application of a theorem on the optimality of integer least-squares (LS) is described. This theorem states that the integer
LS estimator maximizes the ambiguity success rate within the class of admissible integer estimators. This theorem is used
to show how the probability of correct integer estimation depends on changes in the second moment of the ambiguity `float'
solution. The distribution of the `float' solution is considered to be a member of the broad family of elliptically contoured
distributions. Eigenvalue-based bounds for the ambiguity success rate are obtained.
Received: 11 January 1999 / Accepted: 2 November 1999 相似文献
999.
Lawrence E. Band Christina L. Tague Soren E. Brun David E. Tenenbaum & Richard A. Fernandes 《Transactions in GIS》2000,4(3):181-196
The generation, transport and fate of non-point source pollutants in surface water systems is recognized as a major threat to water supplies, aquatic and coastal ecosystems. The transformation and movement of water, carbon and nutrients through watersheds integrates a set of ecosystem processes along hydrologic flowpaths. Human individual and institutional interactions with these processes involve direct addition or abstraction of these substances, or the alteration of land cover and drainage systems. In natural and developed catchments, these processes often vary at granularities ranging from below the level of a hillslope, up through regional watersheds. This suggests the need for the development of hierarchical analysis tools that can address the integration of a set of biophysical, biogeochemical and socioeconomic processes over a spectrum of scales. We describe and illustrate the use of a watershed model implemented as a spatial object hierarchy, representing successively contained landform classes associated with class specific processes as member functions. The model has been linked in a range of looser and tighter couplings with GRASS and ArcView, supplemented by specific terrain analytical functions. We illustrate the data and model system for an instrumented catchment monitored as part of the Baltimore Ecosystem Study (BES), a Long Term Ecological Research (LTER) site centering on integrated carbon, water and nutrient cycling. 相似文献
1000.
This paper introduces the subject of digital sensors for aerial survey by reviewing the use made of small format digital cameras in such an application. The major advantages and disadvantages of employing such consumer technology for aerial survey are highlighted. Finally, a specification is proposed for a minimum requirement for a digital solution based on a single area array sensor. 相似文献