全文获取类型
收费全文 | 27468篇 |
免费 | 3349篇 |
国内免费 | 4789篇 |
专业分类
测绘学 | 9735篇 |
大气科学 | 851篇 |
地球物理 | 1789篇 |
地质学 | 17085篇 |
海洋学 | 1835篇 |
天文学 | 652篇 |
综合类 | 1531篇 |
自然地理 | 2128篇 |
出版年
2024年 | 171篇 |
2023年 | 726篇 |
2022年 | 895篇 |
2021年 | 1036篇 |
2020年 | 793篇 |
2019年 | 1020篇 |
2018年 | 757篇 |
2017年 | 749篇 |
2016年 | 837篇 |
2015年 | 948篇 |
2014年 | 1631篇 |
2013年 | 1124篇 |
2012年 | 1493篇 |
2011年 | 1386篇 |
2010年 | 1182篇 |
2009年 | 1219篇 |
2008年 | 1306篇 |
2007年 | 1202篇 |
2006年 | 1319篇 |
2005年 | 1132篇 |
2004年 | 1050篇 |
2003年 | 1160篇 |
2002年 | 1015篇 |
2001年 | 1014篇 |
2000年 | 890篇 |
1999年 | 838篇 |
1998年 | 1013篇 |
1997年 | 1047篇 |
1996年 | 955篇 |
1995年 | 863篇 |
1994年 | 790篇 |
1993年 | 760篇 |
1992年 | 934篇 |
1991年 | 730篇 |
1990年 | 656篇 |
1989年 | 545篇 |
1988年 | 88篇 |
1987年 | 54篇 |
1986年 | 41篇 |
1985年 | 35篇 |
1984年 | 37篇 |
1983年 | 18篇 |
1982年 | 38篇 |
1981年 | 14篇 |
1980年 | 15篇 |
1979年 | 13篇 |
1977年 | 9篇 |
1975年 | 7篇 |
1972年 | 6篇 |
1948年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
241.
基于坐标归一化和奇异值分解的直接线性变换解法 总被引:2,自引:0,他引:2
直接线性变换方法是数字摄影测量和计算机视觉领域最常用的解析处理方法之一,特别适用于未检校数码相机的三维测量,但直接线性参数间的相关性、物方控制的约束和设计矩阵元素数量级的较大差异,均可导致法矩阵严重病态,从而影响解的稳定性。本文借鉴改进的八点基本矩阵估计算法,采用基于坐标归一化和奇异值分解的解法,即首先将像点和物点坐标进行相似变换得到归一化坐标后组成法矩阵,其次利用矩阵奇异值分解方法代替常规的最小二乘方法,模拟和真实数据表明,此方法可以有效提高解算精度和稳定性。 相似文献
242.
Floating tephra was deposited together with ice core,snow layer,abyssal sediment,lake sediments,and other geological records.It is of great significance to interpret the impact on the climate change of volcanic eruptions from these geological records.It is the first time that volcanic glass was discovered from the peat of Jinchuan(金川)Maar,Jilin(吉林)Province,China.And it is in situ sediments from a near-source explosive eruption according to particle size analysis and identification results.The tephra were neither from Tianchi(天池)volcano eruptions,Changbai(长白)Mountain,nor from Jinlongdingzi(金龙顶子)volcano about 1 600 aBP eruption,but maybe from an unknown eruption of Longgang(龙岗)volcano group according to their geochemistry and distribution.Geochemical characters of the tephra are similar to those of Jingiongdingzi,which are poor in s.Jica,deficient in alkali,Na20 content is more than K20 content,and are similar to distribution patterns of REE and incompatible elements,which helps to speculate that they originated from the same mantle magma with rare condemnation,and from basaltic explosive eruption of Longgang volcano group.The tephra,from peat with age proved that the eruption possibly happened in 15 BC-26 AD,is one of Longgang volcano group eruption that was not recorded and is earlier than that of Jinglongdingzi about 1 600 aBP eruption.And the sedimentary time of tephra is during the period of low temperature alteration.which may be the influence of eruption toward the local climate according to the correlativity of eruption to local temperature curve of peat cellulose oxygen isotope. 相似文献
243.
Calculating Pollution Indices by Heavy Metals in Ecological Geochemistry Assessment and a Case Study in Parks of Beijing 总被引:4,自引:0,他引:4
Pollution index is a powerful tool for ecological geochemistry assessment. The commonly used pollution indices by heavy metals in soils and sediments were classified as two types of single index and integrated index in an algorithm point of view. Four single indices of contamination factor (or concentration factor), ecological risk factor, enrichment factor, and index of geo-accumulation were illustrated, and the reference values for calculating single indices were distinguished into background levels and threshold pollution values. Eight integrated indices were divided into two groups. One group is suitable for the normal distribution single indices including the sum, average, weighted average,vector modulus, and Nemerow pollution indices, and the other for log-normal distribution including the product, root of product, and weighted power product pollution indices. Using background levels as reference values, five contamination classes were divided, and the terminologies are suggested for the single and integrated indices to unify the assessment results. Software of EGAPI was developed in a single document interface to calculate the four single and eight integrated indices by heavy metals to assess the quality of soil and sediment ecological geochemistry. Pollution indices by heavy metals of Cu,Pb, and Zn in soils in parks of Beijing were calculated using EGAPI software, and these five contamination classes and terminologies suggested in this study were evaluated and used. Results ofintegrated indices of Cu, Pb, and Zn in soils indicated that the soil qualities are unpolluted as a whole and varied from low polluted to unpolluted status from the center to the outskirts of Beijing City. 相似文献
244.
基于数学和分析力学角度分别推导了航空矢量重力测量的数学模型,得到了一致的模型公式;给出了矢量模型的3个分量形式,其中垂直方向的分量就是标量重力测量的数学模型;简要介绍了我国研制成功的航空标量重力测量系统CHAGS的数据处理的过程,分析了标量重力测量中测线交叉点和重复测线的重力异常的精度;根据实测数据计算的结果表明:测线交叉点重力异常不符值的标准差约为5×10-5ms-2左右,重复测线的内符合精度优于5×10-5ms-2,达到了预期的要求。 相似文献
245.
特种罐在能源的储备和运输中具有重要意义,在研究特种罐现有容量计量技术的基础上,自主创新提出了基于放样原理和测量机器人实现特种罐容量快速和精密计量的原理和方法。将特种罐容量计量转化为点位放样问题,较好地克服了全站仪现有测量技术的缺点,并利用软件实现了特种罐容量计量的数据采集、数据计算、数据修正和报表输出的一体化。通过实际测量数据,分析了该方法的测量精度和测量效率。 相似文献
246.
Error sources which decrease the accuracy of GPS in absolute velocity determination have been changed since SA was turned off. Firstly, quantities of all kinds of error sources that influence velocity determination are analyzed. The potential accuracy of GPS absolute velocity determination is derived from both theory and field GPS data simulation. After that, two tests were carried out to evaluate the performance of GPS absolute velocity determination in the case of a static and an airborne GPS receiver and INS (Inertial Navigation System) instrument in kinematic mode. In static mode, the receiver velocity has been estimated to be several mm/s with the carrier-phase derived Doppler measurements, and several cm/s with the receiver generated Doppler measurements. In kinematic mode, GPS absolute velocity estimates are compared with the synchronized measurements from the high accuracy INS. The root mean square statistics of the velocity discrepancies between GPS and INS come up to dm/s. Moreover, it has a strong correlation with the acceleration or jerk of the aircraft. 相似文献
247.
An algorithm (differential mode) is presented for the improvement of harmonic tidal analysis along T/P tracks, in which the differences between the observed sea surface heights at adjacent points are taken as observations. Also, the observation equations are constrained with the results of the crossover analysis; the parameter estimations are performed at 0.1° latitude intervals by the least squares. Cycle 10 to 330 T/P altimeter data covering the China Sea and the Northwest Pacific Ocean (2°-50° N,105°-150° E) are adopted for a refined along-track harmonic tidal analysis, and harmonic constants of 12 constituents in 8 474 points are obtained, which indicates that the algorithm can efficiently remove non-tidal effects in the altimeter observations, and improve the precision of tide parameters. Moreover, parameters along altimetry tracks represent a smoother distribution than those obtained by traditional algorithms. The root mean squares of the fitting errors between the tidal height model and the observations reduce from 11 cm to 1.3 cm. 相似文献
248.
植被波谱空间尺度效应及尺度转换方法初步研究 总被引:8,自引:1,他引:7
基于遥感图像的地物识别需要大量地物波谱数据的支持,而现有地物波谱库中收集到的同一地物的波谱,由于其测量尺度和方法的差异,波谱也存在很大差异.以冬小麦为例,首先介绍了材料波谱、端元波谱和像元波谱这3种不同尺度波谱的概念,并以实验数据分析了不同测量尺度下波谱的差别,以此说明波谱尺度转换的必要性.然后利用物理模型和统计模型建立不同测量尺度下的波谱转换关系.分别验证了SAILH模型和线性光谱混合模型在波谱转换中的精度.研究表明,在大尺度上采用统计模型,在小尺度上采用非线性的物理模型可以解释不同尺度观测植被波谱之间的差异. 相似文献
249.
全球土壤变化暨生态系统长期试验国际研讨会侧记 总被引:3,自引:0,他引:3
2007年12月10~14日, 由美国Duke大学Nicholas环境与地学学院发起, 美国国家科学基金会地学与生物学部领域研究网络化计划(Research Coordination Network)、临界层探测计划(Critical Zone Exploratory Network)和美国农业部土壤过程计划(USDA Soil Process Program)资助的"全球土壤变化暨生态系统长期试验研讨会"在美国Duke大学召开. 相似文献
250.