全文获取类型
收费全文 | 7799篇 |
免费 | 692篇 |
国内免费 | 692篇 |
专业分类
测绘学 | 2609篇 |
大气科学 | 627篇 |
地球物理 | 1641篇 |
地质学 | 1479篇 |
海洋学 | 582篇 |
天文学 | 58篇 |
综合类 | 770篇 |
自然地理 | 1417篇 |
出版年
2024年 | 23篇 |
2023年 | 98篇 |
2022年 | 335篇 |
2021年 | 384篇 |
2020年 | 362篇 |
2019年 | 392篇 |
2018年 | 206篇 |
2017年 | 308篇 |
2016年 | 301篇 |
2015年 | 321篇 |
2014年 | 363篇 |
2013年 | 472篇 |
2012年 | 471篇 |
2011年 | 453篇 |
2010年 | 327篇 |
2009年 | 384篇 |
2008年 | 423篇 |
2007年 | 492篇 |
2006年 | 432篇 |
2005年 | 346篇 |
2004年 | 338篇 |
2003年 | 310篇 |
2002年 | 264篇 |
2001年 | 255篇 |
2000年 | 181篇 |
1999年 | 157篇 |
1998年 | 174篇 |
1997年 | 116篇 |
1996年 | 93篇 |
1995年 | 99篇 |
1994年 | 87篇 |
1993年 | 54篇 |
1992年 | 36篇 |
1991年 | 20篇 |
1990年 | 29篇 |
1989年 | 22篇 |
1988年 | 22篇 |
1987年 | 15篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1977年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有9183条查询结果,搜索用时 328 毫秒
91.
92.
本文探讨了附加系统参数平差法在带有测距系统误差的导线网中的应用,说明了当系统误差比较显著时.采用附加系统参数平差法的可行性、必要性及其优越性。 相似文献
93.
Some theory problems affecting parameter estimation are discussed in this paper. Influence and transformation between errors
of stochastic and functional models is pointed out as well. For choosing the best adjustment model, a formula, which is different
from the literatures existing methods, for estimating and identifying the model error, is proposed. On the basis of the proposed
formula, an effective approach of selecting the best model of adjustment system is given.
Project supported by the Open Research Fund Program of the Key Laboratory of Geospace Environment and Geodesy, Ministry of
Education, Wuhan University (No. 905276031-04-10). 相似文献
94.
A generic network design in close range photogrammetry is one where optimal multi-ray intersection geometry is obtained with as few camera stations as practicable. Hyper redundancy is a concept whereby, once the generic network is in place, many additional images are recorded, with the beneficial impact upon object point precision being equivalent to the presence of multiple exposures at each camera position within the generic network. The effective number of images per station within a hyper redundant network might well be in the range of 10 to 20 or more. As is apparent when it is considered that a hyper redundant network may comprise hundreds of images, the concept is only applicable in practice to fully automatic vision metrology systems, where it proves to be a very effective means of enhancing measurement accuracy at the cost of minimal additional work in the image recording phase. This paper briefly reviews the network design and accuracy aspects of hyper redundancy and illustrates the technique by way of the photogrammetric measurement of surface deformation of a radio telescope of 26 m diameter. This project required an object point measurement accuracy of σ = 0·065 mm, or 1/400 000 of the diameter of the reflector. 相似文献
95.
分析了江西农村经济信息网(www.jxagriec.gov.cn)的现状,指出农经网信息服务正面临着挑战,并在此基础上阐述了农经网信息服务的发展思路。 相似文献
96.
介绍广西新一代“区 -地 -县”三级气象信息网络系统在拓扑结构、业务软件和运行管理等方面的基本情况 ,总结了建设、运行管理过程中成功经验以及存在的问题 ,指出 :充分利用现有通讯网络资源 ,强化业务软件的开发应用 ,提高网络传输监控管理水平 ,是经济欠发达地区扩展气象信息网络业务能力的重要途径 相似文献
97.
We investigated the dynamics of upwelling fronts near a coast. This work was first motivated by laboratory experiments [Bouruet-Aubertot, Linden, Dyn. Atmos. Oceans, 2002] in which the front is produced by the adjustment of a buoyant fluid initially confined within a bottomless cylinder. It was shown that cyclonic eddies consisting of coastal waters are enhanced when the front is unstable near the coast (the outer vertical boundary). The purpose of this paper is to provide further insights into this process. We reproduced the experimental configuration using a three-dimensional model of the primitive equations. We first show that for coastal fronts more potential energy, in terms of the maximum available potential energy, is released than for open-ocean fronts. Therefore, waves of larger amplitude are generated during the adjustment and the mean flow that establishes has a higher kinetic energy in the former case. Then as baroclinic instability starts and wave crests reach the boundary, cyclonic eddies are enhanced as in the laboratory experiments and in a similar way. However, in contrast to the laboratory experiments, offshore advection of cyclonic eddies can occur in two stages, depending on the spatial organization of the baroclinic wave. When the baroclinic wave consists of the sum of different modes and is thus highly asymmetric, the offshore advection of cyclonic eddies occurs just after their enhancement at the boundary, as in the laboratory experiments. By contrast, when a single-mode baroclinic wave develops, neighboring cyclonic eddies first merge before being advected offshore. Very different behavior is observed for open-ocean fronts. First a mixed baroclinic–barotropic instability grows. Then the eddies transfer their energy to the mean flow and the barotropic and baroclinic instabilities start again. An excellent agreement is obtained with the main result obtained in the laboratory experiments: the ratio between growth rates of surface cyclonic and anticyclonic vorticity increases as the instability develops nearer to the coast. 相似文献
98.
矿产勘查与评估的统计地球化学方法 总被引:1,自引:0,他引:1
统计地球化学在矿产勘查与评估中的应用,主要包括采术规则,临界品位计算,资源量/储量公式,边界品位确定,网度判别,矿床经济分类以及矿床开发风险等内容。 相似文献
99.
This work presents a novel neural network‐based approach to detect structural damage. The proposed approach comprises two steps. The first step, system identification, involves using neural system identification networks (NSINs) to identify the undamaged and damaged states of a structural system. The partial derivatives of the outputs with respect to the inputs of the NSIN, which identifies the system in a certain undamaged or damaged state, have a negligible variation with different system errors. This loosely defined unique property enables these partial derivatives to quantitatively indicate system damage from the model parameters. The second step, structural damage detection, involves using the neural damage detection network (NDDN) to detect the location and extent of the structural damage. The input to the NDDN is taken as the aforementioned partial derivatives of NSIN, and the output of the NDDN identifies the damage level for each member in the structure. Moreover, SDOF and MDOF examples are presented to demonstrate the feasibility of using the proposed method for damage detection of linear structures. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
100.
This paper investigates the prediction of Class A pan evaporation using the artificial neural network (ANN) technique. The ANN back propagation algorithm has been evaluated for its applicability for predicting evaporation from minimum climatic data. Four combinations of input data were considered and the resulting values of evaporation were analysed and compared with those of existing models. The results from this study suggest that the neural computing technique could be employed successfully in modelling the evaporation process from the available climatic data set. However, an analysis of the residuals from the ANN models developed revealed that the models showed significant error in predictions during the validation, implying loss of generalization properties of ANN models unless trained carefully. The study indicated that evaporation values could be reasonably estimated using temperature data only through the ANN technique. This would be of much use in instances where data availability is limited. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献