全文获取类型
收费全文 | 96909篇 |
免费 | 1544篇 |
国内免费 | 1150篇 |
专业分类
测绘学 | 2408篇 |
大气科学 | 7142篇 |
地球物理 | 19507篇 |
地质学 | 34041篇 |
海洋学 | 8481篇 |
天文学 | 21760篇 |
综合类 | 305篇 |
自然地理 | 5959篇 |
出版年
2021年 | 839篇 |
2020年 | 962篇 |
2019年 | 1003篇 |
2018年 | 2298篇 |
2017年 | 2138篇 |
2016年 | 2827篇 |
2015年 | 1735篇 |
2014年 | 2708篇 |
2013年 | 5114篇 |
2012年 | 2941篇 |
2011年 | 4044篇 |
2010年 | 3363篇 |
2009年 | 4601篇 |
2008年 | 4208篇 |
2007年 | 4004篇 |
2006年 | 3746篇 |
2005年 | 3184篇 |
2004年 | 3020篇 |
2003年 | 2870篇 |
2002年 | 2656篇 |
2001年 | 2409篇 |
2000年 | 2381篇 |
1999年 | 1991篇 |
1998年 | 2028篇 |
1997年 | 1943篇 |
1996年 | 1620篇 |
1995年 | 1571篇 |
1994年 | 1364篇 |
1993年 | 1260篇 |
1992年 | 1232篇 |
1991年 | 1134篇 |
1990年 | 1208篇 |
1989年 | 1034篇 |
1988年 | 968篇 |
1987年 | 1158篇 |
1986年 | 999篇 |
1985年 | 1254篇 |
1984年 | 1379篇 |
1983年 | 1304篇 |
1982年 | 1237篇 |
1981年 | 1084篇 |
1980年 | 1010篇 |
1979年 | 922篇 |
1978年 | 944篇 |
1977年 | 856篇 |
1976年 | 817篇 |
1975年 | 770篇 |
1974年 | 777篇 |
1973年 | 760篇 |
1972年 | 482篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
461.
We have calculated vertical velocities and their change in time from three precise levellings in Finland, with central epochs
1902, 1946, and 1986. Models with and without simultaneous determination of heights give the same results for velocities.
Evidence for velocity change in time appears at first sight strong but on closer examination turns out to be inconclusive.
We offer an alternative explanation in terms of levelling errors and estimate them. A new uplift map closely reproduces main
features of earlier maps from partial data, but does not support some details in them.
Received: 27 January 1997/ Accepted: 20 April 1998 相似文献
462.
A fast algorithm is proposed to integrate the trajectory of a low obiter perturbed by the earth's non-sphericity. The algorithm
uses a separation degree to define the low-degree and the high-degree acceleration components, the former computed rigorously,
and the latter interpolated from gridded accelerations. An FFT method is used to grid the accelerations. An optimal grid type
for the algorithm depends on the trajectory's permissible error, speed, and memory capacity. Using the non-spherical accelerations
computed from EGM96 to harmonic degree 360, orbit integrations were performed for a low orbiter at an altitude of 170 km.
For a separation degree of 50, the new algorithm, together with the predict-pseudo correct method, speeds up the integration
by 145 times compared to the conventional algorithm while keeping the errors in position and velocity below 10−4 m and 10−7 m/s for a 3-day arc.
Received: 28 July 1997 / Accepted: 1 April 1998 相似文献
463.
During a project concerned with DEM generation using SPOT stereoscopic imagery, difficulties were experienced when using Level 1B stereopairs for the task. This paper presents a mathematical solution to overcome this problem which has been implemented at the University of Glasgow. Results are included from experimental tests which were carried out over a test field in Jordan using this solution. These tests used five SPOT Level 1B stereopairs together with a single SPOT Level 1A stereopair for comparative purposes. The residual errors at the ground control points and independent check points are given and show that a satisfactory solution was achieved. 相似文献
464.
Informal settlements are a common feature of developing countries. In South Africa the improvement of living conditions in these settlements and the upgrading to formal housing types are regarded as being of central importance to the nation's development. Effective settlement improvement and upgrading activities, which we here term "management", require adequate spatial data. To date, the acquisition of spatial models of informal settlements has been based on conventional mapping techniques, and mostly on photogrammetry. Data are compiled using analogue or analytical methods. These are manual and hence require both considerable expertise and expensive equipment. Moreover, these methods are uneconomical over the often relatively small, densely populated areas covered by informal settlements and are also too expensive to employ with a regularity required to support such tasks as change detection. Alternative imaging sources and mapping techniques are therefore needed.
In this article we examine the problem of spatial information acquisition for informal settlement management from three perspectives: spatial information requirements, the role which imagery can play in satisfying these spatial information requirements, and effective imaging options. We focus on the potential of high resolution satellite imaging, small format digital aerial imagery and digital multispectral video systems for rapid mapping. We also discuss the example of automated shack extraction from aerial imagery. 相似文献
In this article we examine the problem of spatial information acquisition for informal settlement management from three perspectives: spatial information requirements, the role which imagery can play in satisfying these spatial information requirements, and effective imaging options. We focus on the potential of high resolution satellite imaging, small format digital aerial imagery and digital multispectral video systems for rapid mapping. We also discuss the example of automated shack extraction from aerial imagery. 相似文献
465.
466.
467.
Oshan Taylor M. Wolf Levi J. Sachdeva Mehak Bardin Sarah Fotheringham A. Stewart 《Journal of Geographical Systems》2022,24(3):293-324
Journal of Geographical Systems - Scale is a central concept in the geographical sciences and is an intrinsic property of many spatial systems. It also serves as an essential thread in the fabric... 相似文献
468.
469.
470.
C. Kotsakis 《Journal of Geodesy》2012,86(9):755-774
The aim of this paper is to investigate the influence of the minimum constraints (MCs) on the reference frame parameters in a free-net solution. The non-estimable part of these parameters (which is not reduced by the available data) is analysed in terms of its stability under a numerical perturbation of the constrained datum functionals. In practice, such a perturbation can be ascribed either to hidden errors in the known coordinates/velocities that participate in the MCs or to a simple change of their a priori values due to a datum switch on a different fiducial dataset. In addition, a perturbation of this type may cause a nonlinear variation to the estimable part of the reference frame parameters, since it theoretically affects the adjusted observations that are implied by the network’s nonlinear observational model. The aforementioned effects have an impact on the quality of a terrestrial reference frame (TRF) that is established via a minimum-constrained adjustment, and our study shows that they are both controlled through a characteristic matrix which is inherently linked to the MC system (the so-called TRF stability matrix). The structure of this matrix depends on the network’s spatial configuration and the ‘geometry’ of the datum constraints, while its main role is the filtering of any MC-related errors into the least-squares adjustment results. A number of examples with different types of geodetic networks are also presented to demonstrate the theoretical findings of our study. 相似文献