全文获取类型
收费全文 | 6295篇 |
免费 | 1104篇 |
国内免费 | 1296篇 |
专业分类
测绘学 | 1115篇 |
大气科学 | 1622篇 |
地球物理 | 1443篇 |
地质学 | 1962篇 |
海洋学 | 673篇 |
天文学 | 80篇 |
综合类 | 479篇 |
自然地理 | 1321篇 |
出版年
2024年 | 23篇 |
2023年 | 87篇 |
2022年 | 230篇 |
2021年 | 316篇 |
2020年 | 327篇 |
2019年 | 431篇 |
2018年 | 278篇 |
2017年 | 313篇 |
2016年 | 347篇 |
2015年 | 404篇 |
2014年 | 421篇 |
2013年 | 490篇 |
2012年 | 411篇 |
2011年 | 456篇 |
2010年 | 352篇 |
2009年 | 409篇 |
2008年 | 402篇 |
2007年 | 445篇 |
2006年 | 376篇 |
2005年 | 300篇 |
2004年 | 268篇 |
2003年 | 236篇 |
2002年 | 176篇 |
2001年 | 183篇 |
2000年 | 154篇 |
1999年 | 135篇 |
1998年 | 109篇 |
1997年 | 110篇 |
1996年 | 95篇 |
1995年 | 82篇 |
1994年 | 83篇 |
1993年 | 70篇 |
1992年 | 44篇 |
1991年 | 29篇 |
1990年 | 21篇 |
1989年 | 22篇 |
1988年 | 19篇 |
1987年 | 11篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 2篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有8695条查询结果,搜索用时 722 毫秒
221.
Journal of Geographical Sciences - The border areas of the Tibetan Plateau and the neighboring mountainous areas have a high incidence of earthquakes with a magnitude greater than Ms 5.0, as well... 相似文献
222.
223.
本研究利用遥感和GIS技术,分析盐城自然保护区1983–2018年的景观格局变化,建立了基于区域自然-社会-经济复合生态系统安全格局的评价指标体系,分析了生态安全变化的空间特征和成因。结果表明,1983–2018年由于人类活动加剧,研究区内景观格局变化强烈,从以自然景观为主发展到自然景观与人工景观并存。在整体生态安全方面,目前保护区的生态安全状况不容乐观,从保护区丹顶鹤的生存状态看,保护区总体上还是适宜丹顶鹤栖息的,但由于人类活动的增加以及政策的调整,丹顶鹤的栖息地呈现破碎化并向核心区和缓冲区等局部区域聚集。我们应从管理、政策制定以及当地居民的环境保护意识上增加宣传和投入,并相应减少保护区内人类活动的强度。 相似文献
224.
Coffee berry necrosis is a fungal disease that, at a high level, significantly affects coffee productivity. With the advent of surface mapping satellites, it was possible to obtain information about the spectral signature of the crop on a time scale pertinent to the monitoring and detection of plant phenological changes. The objective of this paper was to define the best machine learning algorithm that is able to classify the incidence CBN as a function of Landsat 8 OLI images in different atmospheric correction methods. Landsat 8 OLI images were acquired at the dates closest to sampling anthracnose field data at three times corresponding to grain filling period and were submitted to atmospheric corrections by DOS, ATCOR, and 6SV methods. The images classified by the algorithms of machine learning, Random Forest, Multilayer Perceptron and Naive Bayes were tested 30 times in random sampling. Given the overall accuracy of each test, the algorithms were evaluated using the Friedman and Nemenyi tests to identify the statistical difference in the treatments. The obtained results indicated that the overall accuracy and the balanced accuracy index were on an average around 0.55 and 0.45, respectively, for the Naive Bayes and Multilayer Perceptron algorithms in the ATCOR atmospheric correction. According to the Friedman and Nemenyi tests, both algorithms were defined as the best classifiers. These results demonstrate that Landsat 8 OLI images were able to identify an incidence of the coffee berry necrosis by means of machine learning techniques, a fact that cannot be observed by the Pearson correlation. 相似文献
225.
To monitor chalk cliff face along the Normandy coast (NW France) which is prone to erosion, we tested the potential of cliff face 3D reconstruction using pairs of images with high angle of incidence at different dates from the agile Pléiades satellites. The verticality aspect of the cliff face brings difficulties in the 3D reconstruction process. Furthermore, the studied area is challenging mainly because the cliff face is north-oriented (shadow). Pléiades images were acquired over several days (multi-date stereoscopic method) with requested incidence angles until 40°. 3D reconstructions of the cliff face were compared using two software: ASP® and ERDAS IMAGINE®. Our results are twofold. Firstly, despite ASP® provides denser point clouds than ERDAS IMAGINE® (an average of 1.60 points/m² from 40° incidence angle stereoscopic pairs on the whole cliff face of Varengeville-sur-Mer against 0.77 points/m² respectively), ERDAS IMAGINE® provides more reliable point clouds than ASP® (precision assessment on the Varengeville-sur-Mer cliff face of 0.31 m ± 2.53 and 0.39 m ± 4.24 respectively), with a better spatial distribution over the cliff face and a better representation of the cliff face shape. Secondly, the quality of 3D reconstructions depends mostly on the amount of noise from raw images and on the shadow intensity on the cliff face (radiometric quality of images). 相似文献
226.
Land cover and land use change (LCLUC) is a global phenomenon, and LCLUC in urbanizing regions has substantial impacts on humans and their environments. In this paper, a semi-automatic approach to identifying the type and starting time of urbanization was developed and tested based on dense time series of Vegetation-Impervious-Soil (V-I-S) maps derived from Landsat surface reflectance imagery. The accuracy of modeled V-I-S fractions and the estimated time of initial change in impervious cover were assessed. North Taiwan, one of the regions of the island of Taiwan that experienced the greatest urban LCLUC, was chosen as a test area, and the study period is 1990 to 2015, a period of substantial urbanization. In total, 295 dates of Landsat imagery were used to create 295 V-I-S fraction maps that were used to construct fractional cover time series for each pixel. Root Mean Square Error (RMSE)s for the modeled Vegetation, Impervious, and Soil were 25 %, 22 %, 24 % respectively. The time of Urban Expansion is estimated by logistic regression applied to Impervious cover time series, while the time of change for Urban Renewal is determined by the period of brief Soil exposure. The identified location and estimated time for newly urbanized lands were generally accurate, with 80% of Urban Expansion estimated within ±2.4 years. However, the accuracy of identified Urban Renewal was relatively low. Our approach to identifying Urban Expansion with dense time series of Landsat imagery is shown to be reliable, while Urban Renewal identification is not. 相似文献
227.
Most studies have the achieved rapid and accurate determination of soil organic carbon (SOC) using laboratory spectroscopy; however, it remains difficult to map the spatial distribution of SOC. To predict and map SOC at a regional scale, we obtained fourteen hyperspectral images from the Gaofen-5 (GF-5) satellite and decomposed and reconstructed the original reflectance (OR) and the first derivative reflectance (FDR) using discrete wavelet transform (DWT) at different scales. At these different scales, as inputs, we selected the 3 optimal bands with the highest weight coefficient using principal component analysis and chose the normalized difference index (NDI), ratio index (RI) and difference index (DI) with the strongest correlation with the SOC content using a contour map method. These inputs were then used to build regional-scale SOC prediction models using random forest (RF), support vector machine (SVM) and back-propagation neural network (BPNN) algorithms. The results indicated that: 1) at a low decomposition scale, DWT can effectively eliminate the noise in satellite hyperspectral data, and the FDR combined with DWT can improve the SOC prediction accuracy significantly; 2) the method of selecting inputs using principal component analysis and a contour map can eliminate the redundancy of hyperspectral data while retaining the physical meaning of the inputs. For the model with the highest prediction accuracy, the inputs were all derived from the wavelength range of SOC variations; 3) the differences in prediction accuracy among the different prediction models are small; and 4) the SOC prediction accuracy using hyperspectral satellite data is greatly improved compared with that of previous SOC prediction studies using multispectral satellite data. This study provides a highly robust and accurate method for predicting and mapping regional SOC contents. 相似文献
228.
为实现多光谱数据对煤矿区水体信息提取,以萍乡市芦新岭及周边煤矿为试验区,在国内外学者对水体指数法提取水体信息和应用的启发下,对试验区不同地物的波谱特征进行分析。本文首先利用数学统计方法建立一种ASTER数据综合归一化差异水体指数模型SWI ASTERxy,然后利用这一模型对试验区进行水体信息提取。试验表明,该模型能够有效地提取矿区各类水体,还可以运用在ASTER数据(或其他多光谱数据)对不同区域不同地物的提取,在遥感地物定量提取上具有很好的效果和潜力,具有一定的推广价值。 相似文献
229.
选取贵州省纳雍县为易地扶贫迁入地研究对象,构建了评价指标体系,对迁入地资源环境压力进行了评价;同时应用层次分析法确定指标权重,计算资源环境压力,并将结果划分为相对高、中、低压力区3个等级。分析结果表明,纳雍县整体资源较为贫乏,生态环境较为脆弱,人均资源量较少,造成资源环境整体压力较大;由贫困发生率与资源环境评价值作相关分析,资源环境压力与贫困发生率呈弱负相关性。 相似文献
230.
袁慧洁 《测绘与空间地理信息》2020,(3):113-117,123
近年来,全国各地进行了大范围的土地利用调查,随着无人机遥感技术越来越成熟,无人机影像分析技术已深入应用到土地利用调查中,其中最多的用途是地物分类。本文选择昭通市昭阳区某乡镇区域为研究区,对采集到的无人机影像进行预处理,生成对应的正射影像;基于多种可见光植被指数,计算每3种指数合并得到影像的OIF指数,确定最佳波段组合;采用基于规则和基于样本两种面向对象分类方法,提取房屋、道路、植被等简单地物及背景。分析结果:两种方法的提取精度均达到90%以上,基于规则的面向对象分类方法精度较高,但耗时较长;基于样本的面向对象方法耗时较短,精度相对较低。两种方法相结合的全自动分类提取是下一步研究的目标。 相似文献