全文获取类型
收费全文 | 2966篇 |
免费 | 305篇 |
国内免费 | 408篇 |
专业分类
测绘学 | 1391篇 |
大气科学 | 263篇 |
地球物理 | 551篇 |
地质学 | 632篇 |
海洋学 | 348篇 |
天文学 | 83篇 |
综合类 | 283篇 |
自然地理 | 128篇 |
出版年
2024年 | 25篇 |
2023年 | 61篇 |
2022年 | 132篇 |
2021年 | 177篇 |
2020年 | 165篇 |
2019年 | 198篇 |
2018年 | 121篇 |
2017年 | 199篇 |
2016年 | 163篇 |
2015年 | 175篇 |
2014年 | 202篇 |
2013年 | 237篇 |
2012年 | 239篇 |
2011年 | 198篇 |
2010年 | 111篇 |
2009年 | 172篇 |
2008年 | 151篇 |
2007年 | 165篇 |
2006年 | 131篇 |
2005年 | 125篇 |
2004年 | 98篇 |
2003年 | 103篇 |
2002年 | 51篇 |
2001年 | 49篇 |
2000年 | 37篇 |
1999年 | 36篇 |
1998年 | 21篇 |
1997年 | 36篇 |
1996年 | 20篇 |
1995年 | 20篇 |
1994年 | 12篇 |
1993年 | 13篇 |
1992年 | 4篇 |
1991年 | 10篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1984年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有3679条查询结果,搜索用时 31 毫秒
781.
782.
二温式多重PCR检测鉴别对虾白斑综合征病毒(WSSV)和传染性皮下及造血器官坏死病毒(IHHNV)的研究与应用 总被引:7,自引:0,他引:7
建立了一种同时检测对虾白斑综合征病毒(WSSV)、传染性皮下和造血器官坏死病毒(IHHNV)的二温式多重PCR技术。根据基因库中WSSV(AF369029)和IHHNV(NC002190)基因序列,设计了两对分别与WSSV和IHHNV某段保守基因序列互补的引物,用这两对引物对同一样品中的WSSV和IHHNVDNA模板进行多重PCR扩增,结果均同时得到了两条大小与实验设计相符的593bp(WSSV)和356bp(1HHNV)特异性多重PCR扩增条带,而对其他对虾疾病病原的PCR扩增结果均为阴性;敏感性测定结果表明,该多重PCR技术最低能检测到WSSV和IHHNVDNA各100pg。用该多重PCR对400份临床病料进行检测,结果230份检出WSSV,阳性率57.5%;96份检出IHHNV,阳性率24%;56份同时检出WSSV和IHHNV,阳性率14%。18份为阴性,阴性率为4.5%。结果提示了WSSV和IHHNV广泛存在于中国南方的养殖对虾中,作者建立的二温式多重PCR可以用于这两种病毒的临床快速检测和鉴别诊断。 相似文献
783.
Sea surface temperature observation by Global Imager (GLI)/ADEOS-II: Algorithm and accuracy of the product 总被引:1,自引:0,他引:1
Futoki Sakaida Kohtaro Hosoda Masao Moriyama Hiroshi Murakami Akira Mukaida Hiroshi Kawamura 《Journal of Oceanography》2006,62(3):311-319
A sea surface temperature (SST) retrieval algorithm for Global Imager (GLI) aboard the ADEOS-II satellite has been developed.
The algorithm is used to produce the standard SST product in the Japan Aerospace Exploration Agency (JAXA). The algorithm
for cloud screening is formed by combinations of various types of tests to detect cloud-contaminated pixels. The combination
is changed according to the solar zenith angle, which enables us to detect clouds even in the sun glitter region in daytime.
The parameters in the cloud-detection tests have been tuned using the GLI global observations. SST is calculated by the Multi-Channel
SST (MCSST) technique from the detected clear pixels. Using drifting buoy measurements, match-up data are produced to derive
the coefficients of the MCSST equations and to examine their performance. The bias and RMSE of the GLI SST are 0.03 K and
0.66 K for daytime and, −0.01 K and 0.70 K for nighttime, respectively. 相似文献
784.
运用专业地震解释及构造建模软件,综合利用焦作地区地震数据、钻孔数据、地质信息对活断层二维浅层地震剖面进行层位与断层解释,同时对断层平面组合与空间展布规律进行研究,实现地质资料与地震数据的有机结合,构建近地表Q3地层底界面、Q地层底界面、N地层底界面分布模型和第四纪断层模型。以第四纪断层模型为基本骨架,以近地表地层分布为主要分层,模拟Q-Q3地质体、N-Q地质体的构造形态和断层发育情况,系统反映近地表N地层底界面至Q3地层底界面的空间结构特征。 相似文献
785.
Automatic extraction of urban features from high resolution satellite images is one of the main applications in remote sensing. It is useful for wide scale applications, namely: urban planning, urban mapping, disaster management, GIS (geographic information systems) updating, and military target detection. One common approach to detecting urban features from high resolution images is to use automatic classification methods. This paper has four main objectives with respect to detecting buildings. The first objective is to compare the performance of the most notable supervised classification algorithms, including the maximum likelihood classifier (MLC) and the support vector machine (SVM). In this experiment the primary consideration is the impact of kernel configuration on the performance of the SVM. The second objective of the study is to explore the suitability of integrating additional bands, namely first principal component (1st PC) and the intensity image, for original data for multi classification approaches. The performance evaluation of classification results is done using two different accuracy assessment methods: pixel based and object based approaches, which reflect the third aim of the study. The objective here is to demonstrate the differences in the evaluation of accuracies of classification methods. Considering consistency, the same set of ground truth data which is produced by labeling the building boundaries in the GIS environment is used for accuracy assessment. Lastly, the fourth aim is to experimentally evaluate variation in the accuracy of classifiers for six different real situations in order to identify the impact of spatial and spectral diversity on results. The method is applied to Quickbird images for various urban complexity levels, extending from simple to complex urban patterns. The simple surface type includes a regular urban area with low density and systematic buildings with brick rooftops. The complex surface type involves almost all kinds of challenges, such as high dense build up areas, regions with bare soil, and small and large buildings with different rooftops, such as concrete, brick, and metal.Using the pixel based accuracy assessment it was shown that the percent building detection (PBD) and quality percent (QP) of the MLC and SVM depend on the complexity and texture variation of the region. Generally, PBD values range between 70% and 90% for the MLC and SVM, respectively. No substantial improvements were observed when the SVM and MLC classifications were developed by the addition of more variables, instead of the use of only four bands. In the evaluation of object based accuracy assessment, it was demonstrated that while MLC and SVM provide higher rates of correct detection, they also provide higher rates of false alarms. 相似文献
786.
The annual timing of river flows might indicate changes that are climate related. In this study, trends in timing of low flows for the Reference Hydrometric Basin Network were investigated under three different hypotheses namely: independence, short‐term persistence (STP) and long‐term persistence (LTP). Both summer and winter time series were characterized with scaling behaviour providing strong evidence of LTP. The Mann–Kendall trend test was modified to account for STP and LTP, and used to detect trends in timing of low flows. It was found that considering STP and LTP resulted in a significant decrease in the number of detected trends. Numerical analysis showed that the timing of summer 7‐day low flows exhibited significant trends in 16, 9 and 7% of stations under independence, STP and LTP assumptions, respectively. Timing of summer low flow shifted toward later dates in western Canada, whereas the majority of stations in the east half of the country (except Atlantic Provinces) experienced a shift toward earlier dates. Timing of winter low flow experienced significant trends in 20, 12, and 6% of stations under independence, STP and LTP assumptions, respectively. Shift in timing of winter low flow toward earlier dates was dominant all over the country where it shifted toward earlier dates in up to 3/4 of time series with significant trends. There are local patterns of upward significant/insignificant trends in southeast, southwest and northern Canada. This study shows that timing of low flows in Canada is time dependent; however, addressing the full complexity of memory properties (i.e. short term vs long term) of a natural process is beyond the scope of this study. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
787.
Wen Dai Jiaming Na Nan Huang Guanghui Hu Guoan Tang 《International journal of geographical information science》2020,34(3):484-503
ABSTRACTAgricultural terraces are important for agricultural production and soil-and-water conservation. They comprise treads and risers that require manual construction and maintenance. If managed improperly, risers will collapse, causing soil loss, gully erosion, and cultivation threats. However, mapping terrace risers remains a challenge. This study presents a novel approach to automatically map terrace risers by combining remote sensing images and digital elevation models (DEMs). First, a terraced hillslope was extracted via a hill-shading method and edges in the image were detected using a Canny edge detector. Next, the DEM was used to generate the contour direction, and edges along this direction were searched and coded as candidate terrace risers via directional detection. Finally, the results of directional detection and the edge image obtained from the Canny detector were overlaid to backtrack complete terrace risers. The approach was validated using four study areas with different topographic characteristics in the Loess Plateau, China. The results verify that the approach achieves outstanding performance and robustness in mapping terrace risers. The precision, recall, and F-measure were 90.81%–97.57%, 88.53%–94.10%, and 90.13%–95.80%, respectively. This approach is flexible and applicable with freely available images and DEM sources. 相似文献
789.
本文提出了一种利用方向性特征进行显著性建模的MRI脑图像肿瘤自动检测算法.该模型首先将MRI脑图像进行预处理,去除掉图像中头骨区域的干扰;然后使用基于方向特征的显著性检测增加病变区域的对比度,实现更准确地提取肿瘤图像区域.本文算法在脑图像数据集上进行了大量的实验,并且与主流的肿瘤自动检测方法进行了对比,证明了本文算法的有效性,并为医生提供可靠的辅助诊断和临床参考. 相似文献
790.