全文获取类型
收费全文 | 69077篇 |
免费 | 10877篇 |
国内免费 | 15388篇 |
专业分类
测绘学 | 7116篇 |
大气科学 | 8636篇 |
地球物理 | 15168篇 |
地质学 | 37773篇 |
海洋学 | 8095篇 |
天文学 | 3200篇 |
综合类 | 4736篇 |
自然地理 | 10618篇 |
出版年
2024年 | 389篇 |
2023年 | 1037篇 |
2022年 | 1998篇 |
2021年 | 2344篇 |
2020年 | 2568篇 |
2019年 | 2971篇 |
2018年 | 2245篇 |
2017年 | 2766篇 |
2016年 | 2967篇 |
2015年 | 3131篇 |
2014年 | 3862篇 |
2013年 | 4090篇 |
2012年 | 4105篇 |
2011年 | 4357篇 |
2010年 | 3651篇 |
2009年 | 4471篇 |
2008年 | 4430篇 |
2007年 | 4820篇 |
2006年 | 4634篇 |
2005年 | 3909篇 |
2004年 | 3666篇 |
2003年 | 3411篇 |
2002年 | 2962篇 |
2001年 | 2601篇 |
2000年 | 2421篇 |
1999年 | 2240篇 |
1998年 | 2020篇 |
1997年 | 1808篇 |
1996年 | 1603篇 |
1995年 | 1367篇 |
1994年 | 1353篇 |
1993年 | 1181篇 |
1992年 | 911篇 |
1991年 | 654篇 |
1990年 | 566篇 |
1989年 | 482篇 |
1988年 | 365篇 |
1987年 | 234篇 |
1986年 | 162篇 |
1985年 | 124篇 |
1984年 | 57篇 |
1983年 | 41篇 |
1982年 | 51篇 |
1981年 | 38篇 |
1980年 | 41篇 |
1979年 | 40篇 |
1978年 | 47篇 |
1977年 | 43篇 |
1976年 | 26篇 |
1954年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Tongyu County in Northeast China is highly prone to land degradation due to its fragile physical settings characterized by a flat topography, a semi-arid climate, and a shallow groundwater table. This study aims to determine the causes of land degradation through detecting the long-term trend of land cover changes. Degraded lands were mapped from satellite images recorded in 1992 and 2002. These land cover maps revealed that the area subject to land degradation in the form of soil salinization, waterlogging and desertification increased from 2400 to 4214 km2, in sharp contrast to most severely degraded land that decreased by 122.5 km2. Newly degraded land stems from productive farmland (263 km2), harvested farmland (551 km2), and grassland (468 km2). Therefore, the worsened degradation situation is attributed to excessive reclamation of grassland for farming, over cultivation, overgrazing, and deforestation. Mechanical, biological, ecological and engineering means should be adopted to rehabilitate the degraded land. 相似文献
992.
Ross S. Lunetta Yang Shao Jayantha Ediriwickrema John G. Lyon 《International Journal of Applied Earth Observation and Geoinformation》2010
The Moderate Resolution Imaging Spectrometer (MODIS) Normalized Difference Vegetation Index (NDVI) 16-day composite data product (MOD12Q) was used to develop annual cropland and crop-specific map products (corn, soybeans, and wheat) for the Laurentian Great Lakes Basin (GLB). The crop area distributions and changes in crop rotations were characterized by comparing annual crop map products for 2005, 2006, and 2007. The total acreages for corn and soybeans were relatively balanced for calendar years 2005 (31,462 km2 and 31,283 km2, respectively) and 2006 (30,766 km2 and 30,972 km2, respectively). Conversely, corn acreage increased approximately 21% from 2006 to 2007, while soybean and wheat acreage decreased approximately 9% and 21%, respectively. Two-year crop rotational change analyses were conducted for the 2005–2006 and 2006–2007 time periods. The large increase in corn acreages for 2007 introduced crop rotation changes across the GLB. Compared to 2005–2006, crop rotation patterns for 2006–2007 resulted in increased corn–corn, soybean–corn, and wheat–corn rotations. The increased corn acreages could have potential negative impacts on nutrient loadings, pesticide exposures, and sediment-mediated habitat degradation. Increased in US corn acreages in 2007 were related to new biofuel mandates, while Canadian increases were attributed to higher world-wide corn prices. Additional study is needed to determine the potential impacts of increases in corn-based ethanol agricultural production on watershed ecosystems and receiving waters. 相似文献
993.
Xia Yao Yan ZhuYongChao Tian Wei FengWeiXing Cao 《International Journal of Applied Earth Observation and Geoinformation》2010
Hyperspectral sensing can provide an effective means for fast and non-destructive estimation of leaf nitrogen (N) status in crop plants. The objectives of this study were to design a new method to extract hyperspectral spectrum information, to explore sensitive spectral bands, suitable bandwidth and best vegetation indices based on precise analysis of ground-based hyperspectral information, and to develop regression models for estimating leaf N accumulation per unit soil area (LNA, g N m−2) in winter wheat (Triticum aestivum L.). Three field experiments were conducted with different N rates and cultivar types in three consecutive growing seasons, and time-course measurements were taken on canopy hyperspectral reflectance and LNA under the various treatments. Then, normalized difference spectral indices (NDSI) and ratio spectral indices (RSI) based on the original spectrum and the first derivative spectrum were constructed within the range of 350–2500 nm, and their relationships with LNA were quantified. The results showed that both LNA and canopy hyperspectral reflectance in wheat changed with varied N rates, with consistent patterns across different cultivars and seasons. The sensitive spectral bands for LNA existed mainly within visible and near infrared regions. The best spectral indices for estimating LNA in wheat were found to be NDSI (R860, R720), RSI (R990, R720), NDSI (FD736, FD526) and RSI (FD725, FD516), and the regression models based on the above four spectral indices were formulated as Y = 26.34x1.887, Y = 5.095x − 6.040, Y = 0.609 e3.008x and Y = 0.388x1.260, respectively, with R2 greater than 0.81. Furthermore, expanding the bandwidth of NDSI (R860, R720) and RSI (R990, R720) from 1 nm to 100 nm at 1 nm interval produced the LNA monitoring models with similar performance within about 33 nm and 23 nm bandwidth, respectively, over which the statistical parameters of the models became less stable. From testing of the derived equations, the model for LNA estimation on NDSI (R860, R720), RSI (R990, R720), NDSI (FD736, FD526) and RSI (FD725, FD516) gave R2 over 0.79 with more satisfactory performance than previously reported models and physical models in wheat. It can be concluded that the present hyperspectral parameters of NDSI (R860, R720), RSI (R990, R720), NDSI (FD736, FD526) and RSI (FD725, FD516) can be reliably used for estimating LNA in winter wheat. 相似文献
994.
提出了一种有效的高分辨率SAR目标特征提取与识别方法,根据SAR图像目标对多尺度Gabor滤波器组的不同响应,充分利用多尺度信息及尺度间的相依性提取新的多尺度特征,该特征综合考虑了SAR图像目标的宏观和微观固有的尺度特性,从而更能反映目标的本质特征;并利用Fisher核映射使得非线性变换比线性变换更能刻画实际中的复杂模式。实验给出了MSTAR数据库中三类目标特征的空间聚类及分类情况,结果表明了该多尺度特征的有效性。 相似文献
995.
一般相位解缠算法通常在相位解缠前首先需要借助滤波算法进行噪声消除,随后才能相位解缠,以保证解缠的质量和顺利实施。卡尔曼滤波将相位解缠问题转化为状态估计问题,实现相位解缠与噪声消除一并处理,简化了数据处理过程。利用真实干涉SAR数据进行实验,采用卡尔曼滤波相位解缠算法进行处理,并与其他几种较常用相位解缠算法的结果从目视和定量两方面进行比较和分析。验证了卡尔曼滤波相位解缠方法在滤波和解缠效果方面的有效性和可行性,可以获得较为可靠的解缠结果。 相似文献
996.
提出了利用同一研究区不同时相且无地质灾害发生情况下数字高程模型(DEM)的差异来计算冰雪覆盖体量变化的方法。设计实现了两种DEM主要表达形式(规则格网DEM和TIN)的冰雪覆盖体量变化计算方法,提出基于三维道格拉斯-普克算法的DEM数据综合方法,减少离散点数量,从而提高建立TIN网的速度以及体积变化计算速度。最后以天山乌鲁木齐河流域数据为例对两种计算方法进行了实验验证。 相似文献
997.
998.
介绍如何利用遥感技术生成的三维可视化图进行相关的地质要素判读和铁路选线,分析对比线路工程地质条件,有一定的实际应用参考价值。 相似文献
999.
1000.