全文获取类型
收费全文 | 1276篇 |
免费 | 137篇 |
国内免费 | 196篇 |
专业分类
测绘学 | 507篇 |
大气科学 | 117篇 |
地球物理 | 91篇 |
地质学 | 525篇 |
海洋学 | 47篇 |
综合类 | 122篇 |
自然地理 | 200篇 |
出版年
2024年 | 3篇 |
2023年 | 11篇 |
2022年 | 35篇 |
2021年 | 53篇 |
2020年 | 56篇 |
2019年 | 57篇 |
2018年 | 39篇 |
2017年 | 86篇 |
2016年 | 79篇 |
2015年 | 74篇 |
2014年 | 96篇 |
2013年 | 80篇 |
2012年 | 83篇 |
2011年 | 80篇 |
2010年 | 79篇 |
2009年 | 82篇 |
2008年 | 60篇 |
2007年 | 71篇 |
2006年 | 90篇 |
2005年 | 52篇 |
2004年 | 57篇 |
2003年 | 48篇 |
2002年 | 46篇 |
2001年 | 36篇 |
2000年 | 31篇 |
1999年 | 24篇 |
1998年 | 19篇 |
1997年 | 16篇 |
1996年 | 15篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 12篇 |
1990年 | 5篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有1609条查询结果,搜索用时 15 毫秒
391.
在地图符号中公路类符号的绘制具有其特殊性,必须进行特殊处理才能正确显示。文中首先介绍了常见的两种处理方法,并分析了其存在的问题;然后针对地图出皈的要求,在全数字地图出版系统中,利用STL标准模板库技术,对公路要素进行批处理绘制,很好地解决了公路类符号的绘制问题。 相似文献
392.
Step–pool morphology characterizes many high‐gradient streams in a variety of natural settings, but formative processes and evolutionary dynamics are still poorly understood. In this paper, natural step–pool geometry is compared with steep alluvial channels where grade‐control structures such as check‐dams and bed sills make the stream profile resemble a natural stepped stream. Along these channels, local scouring due to falling jets forms plunge pools under each structure, analogous to natural steps determining the formation of pools. In order to test the hypothesis that natural pools are analogous to pools formed below grade‐control works with respect to their dimensions, shape and formative dynamics, 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern Italian Alps. Pools below grade‐control works featured a transitional zone between the scour hole and the downstream sloping bed, marked by a depositional berm. When geometric parameters such as maximum pool depth, length and step–berm distances are normalized to the jet virtual energy, no statistically significant differences were detected between natural and artificial systems. These results lend support to an upstream‐forced cascade model for step–pool formation, where the energy of falling jets controls the geometry of the pools, and is therefore regarded as the most important scaling‐independent variable. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
393.
394.
395.
Mary Ann Madej Elizabeth A. Eschenbach Carlos Diaz Rebecca Teasley Kristine Baker 《地球表面变化过程与地形》2006,31(13):1643-1656
Many forested steeplands in the western United States display a legacy of disturbances due to timber harvest, mining or wildfires, for example. Such disturbances have caused accelerated hillslope erosion, leading to increased sedimentation in fish‐bearing streams. Several restoration techniques have been implemented to address these problems in mountain catchments, many of which involve the removal of abandoned roads and re‐establishing drainage networks across road prisms. With limited restoration funds to be applied across large catchments, land managers are faced with deciding which areas and problems should be treated first, and by which technique, in order to design the most effective and cost‐effective sediment reduction strategy. Currently most restoration is conducted on a site‐specific scale according to uniform treatment policies. To create catchment‐scale policies for restoration, we developed two optimization models – dynamic programming and genetic algorithms – to determine the most cost‐effective treatment level for roads and stream crossings in a pilot study basin with approximately 700 road segments and crossings. These models considered the trade‐offs between the cost and effectiveness of different restoration strategies to minimize the predicted erosion from all forest roads within a catchment, while meeting a specified budget constraint. The optimal sediment reduction strategies developed by these models performed much better than two strategies of uniform erosion control which are commonly applied to road erosion problems by land managers, with sediment savings increased by an additional 48 to 80 per cent. These optimization models can be used to formulate the most cost‐effective restoration policy for sediment reduction on a catchment scale. Thus, cost savings can be applied to further restoration work within the catchment. Nevertheless, the models are based on erosion rates measured on past restoration sites, and need to be updated as additional monitoring studies evaluate long‐term basin response to erosion control treatments. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
396.
基于遗传算法的道路网综合模型 总被引:2,自引:1,他引:2
提出了一种基于遗传算法的道路网综合模型。实验表明,该模型在保持道路网的空间分布性和实用性方面具有庭好的效果。 相似文献
397.
398.
基于专家评分模型和GIS的滑坡预警分析开发研究 总被引:2,自引:0,他引:2
利用层次分析法,结合专家评分模型,构建了滑坡预警评价指标体系。在GIS技术的支持下,开发出滑坡预警分析系统。采用该系统不但能预测滑坡灾害的空间发展趋势,而且还可反映出某一时段内其发生发展态势。文中以河北省太行山地区为例,利用该系统,实现了该区的滑坡预警评价分析。其结果与实地滑坡分布规律存在很好的一致性。 相似文献
399.
本文介绍了用载波相位平滑伪距观测数据和自适应抗差Kalman滤波算法进行差分动态定位的方法,用试验数据说明了车载GPS道路测量系统所能达到的精度水平。 相似文献
400.
Systems Enginee 《地球空间信息科学学报》2006,(2)
IntroductionIf let roadway entities be denoted by arcs and in-tersections be denoted by nodes, then a road net-work can be represented with node-arc model. Be-cause the designation of nodes can be determinedby arcs according to the definition of node-arc … 相似文献