全文获取类型
收费全文 | 4671篇 |
免费 | 784篇 |
国内免费 | 815篇 |
专业分类
测绘学 | 1141篇 |
大气科学 | 637篇 |
地球物理 | 1507篇 |
地质学 | 1694篇 |
海洋学 | 465篇 |
天文学 | 61篇 |
综合类 | 406篇 |
自然地理 | 359篇 |
出版年
2024年 | 14篇 |
2023年 | 46篇 |
2022年 | 111篇 |
2021年 | 129篇 |
2020年 | 161篇 |
2019年 | 231篇 |
2018年 | 188篇 |
2017年 | 207篇 |
2016年 | 231篇 |
2015年 | 213篇 |
2014年 | 287篇 |
2013年 | 318篇 |
2012年 | 281篇 |
2011年 | 311篇 |
2010年 | 237篇 |
2009年 | 293篇 |
2008年 | 284篇 |
2007年 | 292篇 |
2006年 | 294篇 |
2005年 | 251篇 |
2004年 | 237篇 |
2003年 | 206篇 |
2002年 | 179篇 |
2001年 | 146篇 |
2000年 | 139篇 |
1999年 | 126篇 |
1998年 | 156篇 |
1997年 | 115篇 |
1996年 | 112篇 |
1995年 | 77篇 |
1994年 | 76篇 |
1993年 | 78篇 |
1992年 | 42篇 |
1991年 | 35篇 |
1990年 | 30篇 |
1989年 | 18篇 |
1988年 | 22篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 16篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 10篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有6270条查询结果,搜索用时 955 毫秒
51.
52.
53.
Sediment movement under unidirectional flows: an assessment of empirical threshold curves 总被引:1,自引:0,他引:1
D. Paphitis 《Coastal Engineering》2001,43(3-4)
A selection of threshold data sets from previous investigations is used to re-examine, selectively, some of the empirical curves that are most commonly used for the prediction of sediment threshold. Simple analytical formulae are derived describing single line curves for mean threshold values and envelopes for the initial movement of discrete particles and the commencement of mass sediment transport phases of the critical condition.The complexity of factors governing the prevailing conditions at threshold, together with the randomness of events, limits deterministic solutions; however, the curves provide an approach that can be used readily in practical applications. In contrast, the envelope presentation is more of a stochastic approach to the definition of threshold. It recognises that there is no definitive threshold condition under which a particular particle size can be displaced, but rather that a range of threshold values exist.The empirical threshold relationships were derived on the basis of a relatively large amount of data. However, the sources utilised have interpreted the ‘critical condition’ in various ways, implying that the formulae must be treated with caution in applications. 相似文献
54.
本文讨论Bowen数的意义、功能和计算法。同时,依据多年水文气象实测资料作统计,计算出东中国海的Bo值。其结果绘制成1月至12月的月平均分布图,从而对本海域的Bo分布特点作详细分析介绍。 相似文献
55.
56.
以低成本陆用航姿参考系统(AHRS)/GPS紧耦合系统为研究对象,建立了基于伪距、伪距率、航向角的组合观测数学模型。引入强跟踪滤波(STF)算法,利用渐消因子的作用,增强滤波器对状态快速变化的跟踪能力。同时,针对微惯性器件漂移过大的缺点,采用二阶EKF(QEKF)方法,通过对Hessian矩阵的求解,补偿系统观测方程线性化的二阶截断误差。仿真表明:STQEKF方法可高速准确地逼近系统非线性模型,实现陆地载体导航控制,在传感器精度有限的情况下,使姿态和位置的控制效果较标准EKF分别提高了约8.9%-38.2%和48.7%-54.4% 相似文献
57.
J A Vargas-Guzmán 《中国地质大学学报(英文版)》2008,19(1):85-92
An analysis of statistical expected values for transformations is performed in this study to quantify the effect of heterogeneity on spatial geological modeling and evaluations. Algebraic transformations are frequently applied to data from logging to allow for the modeling of geological properties. Transformations may be powers, products, and exponential operations which are commonly used in well-known relations (e.g., porosity-permeability transforms). The results of this study show that correct computations must account for residual transformation terms which arise due to lack of independence among heterogeneous geological properties. In the case of an exponential porosity-permeability transform, the values may be positive. This proves that a simple exponential model back-transformed from linear regression underestimates permeability. In the case of transformations involving two or more properties, residual terms may represent the contribution of heterogeneous components which occur when properties vary together, regardless of a pair-wise linear independence. A consequence of power- and product-transform models is that regression equationswithin those transformations need corrections via residual cumulants. A generalization of this result isthat transformations of multivariate spatial attributes require multiple-point random variable relations. This analysis provides practical solutions leading to a methodology for nonlinear modeling using correct back transformations in geology. 相似文献
58.
59.
A. Shamshad C.S. LeowA. Ramlah W.M.A. Wan HussinS.A. Mohd. Sanusi 《International Journal of Applied Earth Observation and Geoinformation》2008
The study evaluated the performance and suitability of AnnAGNPS model in assessing runoff, sediment loading and nutrient loading under Malaysian conditions. The watershed of River Kuala Tasik in Malaysia, a combination of two sub-watersheds, was selected as the area of study. The data for the year 2004 was used to calibrate the model and the data for the year 2005 was used for validation purposes. Several input parameters were computed using methods suggested by other researchers and studies carried out in Malaysia. The study shows that runoff was predicted well with an overall R2 value of 0.90 and E value of 0.70. Sediment loading was able to produce a moderate result of R2 = 0.66 and E = 0.49, nitrogen loading predictions were slightly better with R2 = 0.68 and E = 0.53, and phosphorus loading performance was slightly poor with an R2 = 0.63 and E = 0.33. The erosion map developed was in agreement with the erosion risk map produced by the Department of Agriculture, Malaysia. Rubber estates and urban areas were found to be the main contributors to soil erosion. The simulation results showed that AnnAGNPS has the potential to be used as a valuable tool for planning and management of watersheds under Malaysian conditions. 相似文献
60.
Jianqiang Ren Zhongxin Chen Qingbo Zhou Huajun Tang 《International Journal of Applied Earth Observation and Geoinformation》2008,10(4):403
The significance of crop yield estimation is well known in agricultural management and policy development at regional and national levels. The primary objective of this study was to test the suitability of the method, depending on predicted crop production, to estimate crop yield with a MODIS-NDVI-based model on a regional scale. In this paper, MODIS-NDVI data, with a 250 m resolution, was used to estimate the winter wheat (Triticum aestivum L.) yield in one of the main winter-wheat-growing regions. Our study region is located in Jining, Shandong Province. In order to improve the quality of remote sensing data and the accuracy of yield prediction, especially to eliminate the cloud-contaminated data and abnormal data in the MODIS-NDVI series, the Savitzky–Golay filter was applied to smooth the 10-day NDVI data. The spatial accumulation of NDVI at the county level was used to test its relationship with winter wheat production in the study area. A linear regressive relationship between the spatial accumulation of NDVI and the production of winter wheat was established using a stepwise regression method. The average yield was derived from predicted production divided by the growing acreage of winter wheat on a county level. Finally, the results were validated by the ground survey data, and the errors were compared with the errors of agro-climate models. The results showed that the relative errors of the predicted yield using MODIS-NDVI are between −4.62% and 5.40% and that whole RMSE was 214.16 kg ha−1 lower than the RMSE (233.35 kg ha−1) of agro-climate models in this study region. A good predicted yield data of winter wheat could be got about 40 days ahead of harvest time, i.e. at the booting-heading stage of winter wheat. The method suggested in this paper was good for predicting regional winter wheat production and yield estimation. 相似文献