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   731篇
  免费   98篇
  国内免费   73篇
测绘学   109篇
大气科学   122篇
地球物理   176篇
地质学   207篇
海洋学   131篇
天文学   38篇
综合类   40篇
自然地理   79篇
  2024年   3篇
  2023年   8篇
  2022年   23篇
  2021年   30篇
  2020年   24篇
  2019年   33篇
  2018年   15篇
  2017年   26篇
  2016年   20篇
  2015年   19篇
  2014年   22篇
  2013年   36篇
  2012年   29篇
  2011年   32篇
  2010年   29篇
  2009年   62篇
  2008年   55篇
  2007年   55篇
  2006年   37篇
  2005年   37篇
  2004年   24篇
  2003年   28篇
  2002年   30篇
  2001年   28篇
  2000年   12篇
  1999年   16篇
  1998年   29篇
  1997年   25篇
  1996年   18篇
  1995年   14篇
  1994年   23篇
  1993年   19篇
  1992年   8篇
  1991年   7篇
  1990年   5篇
  1989年   8篇
  1988年   4篇
  1987年   1篇
  1986年   3篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
排序方式: 共有902条查询结果,搜索用时 15 毫秒
571.
We collected polychaete diversity and abundance data at a range of impacted and reference sites near an alumina refinery in Melville Bay, northern Australia. The aims were to measure the impact of sediment modified by the alumina refinery discharge on polychaete communities and secondly to gather baseline data from which to measure future changes. Polychaete communities in both soft-bottom habitats and subtidal areas adjacent to mangrove forests were studied. We also developed and deployed an artificial substratum device to sample polychaetes associated with hard-substrate habitats. For each habitat, polychaete community composition was different between impacted and reference sites and at multiple time points. The impact of future changes either from bioremediation or management practices can be measured against these baseline data. Indicator species analysis was used to identify polychaete species that were significantly different at the locations tested, and we discuss their potential as indicator species.  相似文献   
572.
The present study analyses the composition, structure and trophic function of epibenthic assemblages in two artificial reefs (ARs) 16 years after deployment and in nearby natural reefs (NRs), aiming at providing insights on the complementarity between both habitats. Current findings suggest that after 16 years the ARs (concrete blocks), located in southern Portugal, do not act as surrogates for NRs, as epibenthic assemblages differed between reef types in composition, structure and trophic function. NRs showed higher diversity and complementarity (i.e. beta-diversity) than ARs, evidencing higher redundancy. Higher heterogeneity within NRs was also evidenced by the multi-dimensional scaling analysis based on abundance, biomass and trophic composition. NRs presented higher abundance of molluscs and biomass of sponges, resulting in differences in the trophic function: suspension-feeding dominated the NRs, while within ARs there was an ascendency of carnivory. Although not acting as surrogates for NRs and provided that no adverse effects (e.g. establishment of non-native species) were detected, ARs may have a significant contribution for the increase of regional diversity, as evidenced by the highest complementarity levels observed between assemblages in both reefs.  相似文献   
573.
张院  孙颖  王新娟 《城市地质》2013,8(1):51-53
北京地区开展人工回灌有了近50年的历史,经历了试验和工程实施阶段。试验阶段包括深井、砂石坑、大口井回灌等试验,工程实施包括城市雨洪利用和再生水回灌利用等。  相似文献   
574.
岛屿岸线的确定是海岛地物信息遥感调查中重要的一步,对舟山岙山岛的Quickbird高分辨率遥感影像进行地物地理信息分析,提取出岛屿岸线及港口码头的空间基本地物特征,在此基础上运用面向对象分类方法和数学形态学的知识,在ENVI+IDL图形图像处理平台上实现对岛屿岸线及港口码头的自动提取,提取结果与实际情况基本符合,并且可以为人工智能提所有海岛地物信息的取做准备。  相似文献   
575.
旅游业已经成为我国国民经济的重要支柱,但是各景区普遍存在服务能力有限、方法欠佳等问题,导致游客满意度不高,投诉事件屡见不鲜,景区管理难度较大。LBS是近年来国内外在导航应用领域中的一个热点,应用比较广泛。本文介绍了基于北斗的LBS的功能及应用前景,并在此基础上,研究了景区智能导游系统,分析了系统结构和工作模式,给出了系统建设方案,探讨了该系统在景区管理中的作用。  相似文献   
576.
Collective intelligence is the idea that under the right circumstances collections of individuals are smarter than even the smartest individuals in the group, that is a group has an “intelligence” that is independent of the intelligence of its members. The ideology of collective intelligence undergirds much of the enthusiasm about the use of “volunteered” or crowd-sourced geographic information. Literature from a variety of fields makes clear that not all groups possess collective intelligence, this article identifies four pre-conditions for the emergence of collective intelligence and then examines the extent to which collectively generated mapping systems satisfy these conditions. However, the “intelligence” collectively generated maps is hard to assess because there are two difficult to reconcile perspectives on map quality – the credibility perspective and the accuracy perspective. Much of the current literature on user-generated maps focuses on assessing the quality of individual contributions. However, because user-generated maps are complex social systems and because the quality of a contribution is difficult to assess this strategy may not yield an “intelligent” end product. The existing literature on collective intelligence suggests that the structure of groups is more important than the intelligence of group members. Applying this idea to user-generated maps suggests that systems should be designed to foster conditions known to produce collective intelligence rather than privileging particular contributions/contributors. The article concludes with some design recommendations and by considering the implications of collectively generated maps for both expert knowledge and traditional state sponsored mapping programs.  相似文献   
577.
Metonymically used location names (toponyms) refer to other, related entities and thus possess a meaning different from their literal, geographic sense. Metonymic uses are to be treated differently to improve the performance of geographic information retrieval (GIR). Statistics on toponym senses show that 75.06% of all location names are used in their literal sense, 17.05% are used metonymically, and 7.89% have a mixed sense. This article presents a method for disambiguating location names in texts between literal and metonymic senses, based on shallow features.

The evaluation of this method is two‐fold. First, we use a memory‐based learner (TiMBL) to train a classifier and determine standard evaluation measures such as F‐score and accuracy. The classifier achieved an F‐score of 0.842 and an accuracy of 0.846 for identifying toponym senses in a subset of the CoNLL (Conference on Natural Language Learning) data.

Second, we perform retrieval experiments based on the GeoCLEF data (newspaper article corpus and queries) from 2005 and 2006. We compare searching location names in a database index containing both their literal and metonymic senses with searching in an index containing their literal senses only. Evaluation results indicate that removing metonymic senses from the index yields a higher mean average precision (MAP) for GIR. In total, we observed a significant gain in MAP: an increase from 0.0704 to 0.0715 MAP for the GeoCLEF 2005 data, and an increase from 0.1944 to 0.2100 MAP for the GeoCLEF 2006 data.  相似文献   
578.

This work examines the use of an annually updated metropolitan data system as a tool for evaluating benign racial location policies during the first year of the Section 8 Existing Rent Supplement program. Case-study evidence shows that the rent supplement program (with benign locational policies) is ineffective in reducing the isolation faced by minority groups, and that the information used to monitor compliance of the program with benign locational policies lags badly behind the true state of affairs. As a result, new areas of racial concentration have been encouraged and augmented.  相似文献   
579.
Response surface methodology (RSM) and artificial neural networks (ANNs) based on a multivariate central composite design (CCD) were applied to model and optimize the photocatalytic degradation of N,N‐diethyl‐m‐toluamide (DEET). The individual and interaction effects of three main operating factors (mass of TiO2, initial DEET concentration, and irradiation intensity) on process efficiency were estimated, proving their important effect on % DEET removal. Among the independent variables, TiO2 concentration displayed the highest effect on DEET degradation followed by initial DEET concentration and UV intensity. The optimization and prediction capabilities of ANNs and RSM were compared on the basis of root mean squared error, mean absolute error, absolute average deviation, and correlation coefficient values. Results proved the usefulness and capability of the experimental design strategy for successful investigation and modeling of the photocatalytic process. Moreover, the selected ANN gave better estimation capabilities throughout the range of variables than RSM. Based on the models and the related experimental conditions, the optimal values of each parameter were determined. Under optimum conditions, DEET and total organic carbon (TOC) followed pseudo‐first order kinetics. Nearly complete degradation of DEET took place within 15 min whereas high TOC removal percentages (>85%) was achieved after 90 min irradiation time.  相似文献   
580.
ENVI遥感图像监督分类方法比较   总被引:2,自引:0,他引:2  
贾建峰 《西部资源》2014,(6):133-136
鉴于遥感监督分类方法的普遍应用,本文介绍了五种常用的监督分类方法:平行六面体法、最小距离法、马氏距离法、最大似然法和人工神经网络分类法。就同一地区TM影像应用这五种方法进行了土地利用分类,对比分析了这五种方法的分类精度,发现人工神经网络对土地覆盖与利用的分类精度高于最大似然法,最大似然法分类精度优于平行六面体法、最小距离法和马氏距离法。所得结论对有关遥感图像分类工作具有指导和借鉴意义。  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