全文获取类型
收费全文 | 12466篇 |
免费 | 2393篇 |
国内免费 | 3321篇 |
专业分类
测绘学 | 1032篇 |
大气科学 | 2645篇 |
地球物理 | 3150篇 |
地质学 | 6720篇 |
海洋学 | 1530篇 |
天文学 | 520篇 |
综合类 | 1183篇 |
自然地理 | 1400篇 |
出版年
2024年 | 113篇 |
2023年 | 361篇 |
2022年 | 715篇 |
2021年 | 864篇 |
2020年 | 665篇 |
2019年 | 720篇 |
2018年 | 794篇 |
2017年 | 740篇 |
2016年 | 787篇 |
2015年 | 666篇 |
2014年 | 778篇 |
2013年 | 761篇 |
2012年 | 686篇 |
2011年 | 685篇 |
2010年 | 723篇 |
2009年 | 692篇 |
2008年 | 638篇 |
2007年 | 556篇 |
2006年 | 540篇 |
2005年 | 459篇 |
2004年 | 335篇 |
2003年 | 347篇 |
2002年 | 360篇 |
2001年 | 325篇 |
2000年 | 371篇 |
1999年 | 490篇 |
1998年 | 430篇 |
1997年 | 432篇 |
1996年 | 380篇 |
1995年 | 309篇 |
1994年 | 293篇 |
1993年 | 215篇 |
1992年 | 191篇 |
1991年 | 150篇 |
1990年 | 100篇 |
1989年 | 116篇 |
1988年 | 106篇 |
1987年 | 64篇 |
1986年 | 50篇 |
1985年 | 37篇 |
1984年 | 21篇 |
1983年 | 26篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 23篇 |
1979年 | 12篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1958年 | 13篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
在铁路测量中,长度投影变形要求小于2.5cm/km。但是由于铁路为典型的线性工程,通常横跨多个投影带,因此采用国家标准3。带难以满足铁路测量精度要求,本文通过分析高斯投影长度变形,提出分段建立独立坐标系的方法,控制长度投影变形,满足精度要求,最后介绍利用GlobalMapper软件实现两个独立坐标系的转换。 相似文献
112.
The research work has been seldom done about cloverleaf junction expression in a 3-dimensional city model (3DCM). The main reason is that the cloverleaf junction is often in a complex and enormous construction. Its main body is bestraddle in air, and has aerial intersections between its parts. This complex feature made cloverleaf junction quite different from buildings and terrain, therefore, it is difficult to express this kind of spatial objects in the same way as for buildings and terrain. In this paper, authors analyze spatial characteristics of cloverleaf junction, propose an all-constraint points TIN algorithm to partition cloverleaf junction road surface, and develop a method to visualize cloverleaf junction road, surface using TIN. In order to manage cloverleaf junction data efficiently, the authors also analyzed the mechanism of 3DCM data management, extended BLOB type in relational database, and combined R-tree index to manage 3D spatial data. Based on this extension, an appropriate data structure for cloverleaf junction in 3DCM is proposed. 相似文献
113.
点模式及其趋同研究是揭示地学现象的产生、发展与演变,量化空间相似性分布、诠释空间分布成因的重要方式。目前,点模式研究侧重于已知频率与随机分布的一元独立性检验、距离测度下单观测值的二元相关性分析,而针对集聚过程相关性,空间拓扑与非拓扑邻近、综合多观测值的点模式趋同量化研究顾及不足。据此,以空间邻近性聚类、局部相关的多指标评价为切入点,本文提出了一种Voronoi邻近关系支持下的点模式趋同提取方法。首先,以Voronoi邻近相关表集聚算法剖分出空间独立性点模式;其次,依据Voronoi邻近关系指数测度、样本分布均值与分布方差的趋同假设,使用拉普拉斯平滑算子评价趋同度;最后,依据λ截矩阵,提取出Voronoi邻近、非Voronoi邻近关系支持下的强趋同点模式。试验以云南省腾冲市居民点数据为算例,经与点模式构建的聚类方法对比、趋同度计算与强趋同提取,验证了该方法的可行性与有效性。 相似文献
114.
115.
通过对某一大型的石油储油罐进行实地观测,阐述了采用徕卡TM30对大型储油罐进行变形观测的工作原理及简单操作流程。用拟合的方法拟合出基准圆及其他每一圈板的圆心,编程实现观测数据的自动化处理,求解出需要的几何参数,节约了工作成本并提高了效率。最后对油罐的变形程度做出分析和评价。 相似文献
116.
ABSTRACTSocial, economic, and environmental statistical data associated with geographic points are currently globally available in large amounts. When conventional thematic maps, such as proportional symbol maps or point diagram maps, are used to represent these data, the maps appear cluttered if the point data volumes are relatively large or cover a relatively dense region. To overcome these limitations, we propose a new type of thematic map for statistical data associated with geographic points: the point grid map. In a point grid map, an input point data set is transformed into a grid in which each point is represented by a square grid cell of equal size while preserving the relative position of each point, which leads to a clear and uncluttered appearance, and the grid cells can be shaded or patterned with symbols or diagrams according to the attributes of the points. We present an algorithm to construct a point grid map and test it with several simulated and real data sets. Furthermore, we present some variants of the point grid map. 相似文献
117.
118.
Selective omission is necessary for road network generalisation. This study investigates the use of supervised learning approaches for selective omission in a road network. To be specific, at first, the properties to measure the importance of a road in the network are viewed as input attributes, and the decision of such a road is retained or not at a specific scale is viewed as an output class; then, a number of samples with known input and output are used to train a classifier; finally, this classifier can be used to determine whether other roads to be retained or not. In this study, a total of nine supervised learning approaches, i.e., ID3, C4·5, CRT, Random Tree, support vector machine (SVM), naive Bayes (NB), K-nearest neighbour (KNN), multilayer perception (MP) and binary logistic regression (BLR), are applied to three road networks for selective omission. The performances of these approaches are evaluated by both quantitative assessment and visual inception. Results show that: (1) in most cases, these approaches are effective and their classification accuracy is between 70% and 90%; (2) most of these approaches have similar performances, and they do not have any statistically significant difference; (3) but sometimes, ID3 and BLR performs significantly better than NB and SVM; NB and KNN perform significantly worse than MP, SVM and BLR. 相似文献
119.
120.