全文获取类型
收费全文 | 26985篇 |
免费 | 4610篇 |
国内免费 | 6681篇 |
专业分类
测绘学 | 5054篇 |
大气科学 | 4782篇 |
地球物理 | 6502篇 |
地质学 | 11654篇 |
海洋学 | 3753篇 |
天文学 | 305篇 |
综合类 | 2424篇 |
自然地理 | 3802篇 |
出版年
2024年 | 176篇 |
2023年 | 471篇 |
2022年 | 946篇 |
2021年 | 1155篇 |
2020年 | 1313篇 |
2019年 | 1457篇 |
2018年 | 1217篇 |
2017年 | 1396篇 |
2016年 | 1533篇 |
2015年 | 1655篇 |
2014年 | 1742篇 |
2013年 | 1960篇 |
2012年 | 1814篇 |
2011年 | 1799篇 |
2010年 | 1448篇 |
2009年 | 1649篇 |
2008年 | 1672篇 |
2007年 | 1763篇 |
2006年 | 1655篇 |
2005年 | 1464篇 |
2004年 | 1292篇 |
2003年 | 1112篇 |
2002年 | 1023篇 |
2001年 | 883篇 |
2000年 | 810篇 |
1999年 | 727篇 |
1998年 | 689篇 |
1997年 | 605篇 |
1996年 | 536篇 |
1995年 | 481篇 |
1994年 | 450篇 |
1993年 | 366篇 |
1992年 | 230篇 |
1991年 | 196篇 |
1990年 | 140篇 |
1989年 | 120篇 |
1988年 | 104篇 |
1987年 | 72篇 |
1986年 | 38篇 |
1985年 | 32篇 |
1984年 | 16篇 |
1983年 | 2篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
提出用B样条函数求解曲线、曲面上重磁位场的向上延拓,水平、垂向导数计算,磁异常分量互换的方法。该方法的特点是:原理简明,程序通用性强,计算精度高。 相似文献
32.
33.
Darío R. Minkoff Mauricio Escapa Flix E. Ferramola Silvio D. Maraschín Jorge O. Pierini Gerardo M.E. Perillo Claudio Delrieux 《Estuarine, Coastal and Shelf Science》2006,69(3-4):403
The Bahía Blanca Estuary (38° 50′ S, and 62° 30′ W) presents salt marshes where interactions between the local flora (Sarcocornia perennis) and fauna (Chasmagnathus granulatus) generate some kind of salt pans that alter the normal water circulation and condition its flow and course towards tidal creeks. The crab–vegetation dynamics in the salt marsh presents variations that cannot be quantified in a reasonable period of time. The interaction between S. perennis plant and C. granulatus crab is based on simple laws, but its result is a complex biological mechanism that causes an erosive process on the salt marsh and favors the formation of tidal creeks. To study it, a Cellular Automata model is proposed, based on the laws deduced from the observation of these phenomena in the field, and then verified with measurable data within macroscale time units. Therefore, the objective of this article is to model how the interaction between C. granulatus and S. perennis modifies the landscape of the salt marsh and influences the path of tidal creeks. The model copies the basic laws that rule the problem based on purely biological factors.The Cellular Automata model proved capable of reproducing the effects of the interaction between plants and crabs in the salt marsh. A study of the water drainage of the basins showed that this interaction does indeed modify the development of tidal creeks. Model dynamics would likewise follow different laws, which would provide a different formula for the probability of patch dilation. The patch shape can be obtained changing the pattern that dilates. 相似文献
34.
The phase one project of the twin jetties at the Tao-er Estuary yields no effect on the flow, and there is a large area of deposition. A numerical model is employed to simulate the flow field, and the scouring and silting in the channel be-tween the two jetties are analyzed. The results show that the effective tidal flux between the two jetties is reduced due to the notch located at the heel of the east jetty, and the effect of contraction-flush is not obvious. As for the regulation of this kind of estuary under the tide action, the capability of tidal flux should be fully used. 相似文献
35.
36.
37.
38.
39.
Modelling Positional Uncertainty of Line Features by Accounting for Stochastic Deviations from Straight Line Segments 总被引:1,自引:0,他引:1
Sytze de Bruin 《Transactions in GIS》2008,12(2):165-177
The assessment of positional uncertainty in line and area features is often based on uncertainty in the coordinates of their elementary vertices which are assumed to be connected by straight lines. Such an approach disregards uncertainty caused by sampling and approximation of a curvilinear feature by a sequence of straight line segments. In this article, a method is proposed that also allows for the latter type of uncertainty by modelling random rectangular deviations from the conventional straight line segments. Using the model on a dense network of sub‐vertices, the contribution of uncertainty due to approximation is emphasised; the sampling effect can be assessed by applying it on a small set of randomly inserted sub‐vertices. A case study demonstrates a feasible way of parameterisation based on assumptions of joint normal distributions for positional errors of the vertices and the rectangular deviations and a uniform distribution of missed sub‐vertices along line segments. Depending on the magnitudes of the different sources of uncertainty, not accounting for potential deviations from straight line segments may drastically underestimate the positional uncertainty of line features. 相似文献
40.
利用UML模型构建Geodatabase的方法与实践 总被引:2,自引:0,他引:2
利用UML模型生成Geodatabase数据库便于数据库的更新及移植,同时能很好地反映数据库结构及对象之间的关系。论文分析了利用VISIO建模工具构建Geodatabase的UML模型以及在此基础上利用ArcGIS Desktop相关模块生成Geodatabase数据库的方法,并以上海市绿化林业信息数据库为例,演示了Geodatabase的创建及更新过程。 相似文献