全文获取类型
收费全文 | 57534篇 |
免费 | 957篇 |
国内免费 | 568篇 |
专业分类
测绘学 | 1809篇 |
大气科学 | 4354篇 |
地球物理 | 11300篇 |
地质学 | 20764篇 |
海洋学 | 4738篇 |
天文学 | 12873篇 |
综合类 | 198篇 |
自然地理 | 3023篇 |
出版年
2021年 | 537篇 |
2020年 | 556篇 |
2019年 | 590篇 |
2018年 | 1467篇 |
2017年 | 1401篇 |
2016年 | 1730篇 |
2015年 | 1000篇 |
2014年 | 1654篇 |
2013年 | 3035篇 |
2012年 | 1810篇 |
2011年 | 2313篇 |
2010年 | 2001篇 |
2009年 | 2569篇 |
2008年 | 2205篇 |
2007年 | 2177篇 |
2006年 | 2175篇 |
2005年 | 1675篇 |
2004年 | 1567篇 |
2003年 | 1541篇 |
2002年 | 1480篇 |
2001年 | 1363篇 |
2000年 | 1332篇 |
1999年 | 1123篇 |
1998年 | 1058篇 |
1997年 | 1137篇 |
1996年 | 952篇 |
1995年 | 937篇 |
1994年 | 890篇 |
1993年 | 759篇 |
1992年 | 727篇 |
1991年 | 727篇 |
1990年 | 738篇 |
1989年 | 695篇 |
1988年 | 662篇 |
1987年 | 752篇 |
1986年 | 639篇 |
1985年 | 822篇 |
1984年 | 849篇 |
1983年 | 859篇 |
1982年 | 809篇 |
1981年 | 681篇 |
1980年 | 730篇 |
1979年 | 620篇 |
1978年 | 579篇 |
1977年 | 554篇 |
1976年 | 497篇 |
1975年 | 490篇 |
1974年 | 525篇 |
1973年 | 522篇 |
1972年 | 340篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
971.
This paper describes the collection, reduction, and analysis of 0.4–1.0 m Mars imaging spectroscopy data obtained during the 1988 and 1990 oppositions from Mauna Kea Observatory and provides a general outline for the acquisition and analysis of similar imaging spectroscopy data sets. The U.H. 2.24-m Wide Field Grism CCD Spectrograph was used to collect 13 three-dimensional image cubes covering 90% of the planet south of 50°N in the 0.4–0.8 m region (/=245 at 0.6 m) and covering 55% of the planet south of 50°N in the 0.5–1.0 m region (/=293 at 0.75 m). Spectra extracted from these image cubes reveal the detailed character of the martian near-UV to visible spectrum. Images at red wavelengths reveal the classical albedo markings at 100–500 km spatial resolution while images at blue wavelengths show little surface feature contrast and are dominated by condensate clouds/hazes and polar ice. Many of the data acquisition, reduction, and analysis steps discussed here are new or unique to imaging spectroscopy data sets. These techniques exploit the information contained within the spatial domain of data such as these, thus allowing more traditional point-spectral analysis techniques to be expanded into an imaging format. 相似文献
972.
The linearized theory for the parallel propagation of magnetoacoustic-gravity surface waves is developed for an interface of a horizontal magnetic field above a field-free medium. The media either side of the interface are taken to be isothermal. The dispersion relation is obtained for the case of a constant Alfvén speed. In the absence of gravity the interface may support one or two surface modes, determined by the relative temperatures and magnetism of the two media. The effect of gravity on the modes is examined and dispersion diagrams and eigenfunctions are given. In the usual - k
x diagnostic diagram, the domain of evanescence is shown to be divided into two distinct regions determining whether a given mode will have a decaying or growing vertical velocity component. In the absence of a magnetic field the transcendental dispersion relation may be rewritten as a polynomial. This polynomial possesses two acceptable solutions only one of which may exist in any given circumstances (depending on the ratio of the densities). If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate. The f-mode exists in a restricted band of horizontal wavenumber and only when the field-free medium is warmer than the magnetic atmosphere. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfvén speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths. 相似文献
973.
Bianchi type-II, VIII, and IX models have been investigated in scalar-tensor theories developed by Saez (1985), Saez and Ballester (1985), and Lau and Prokhovnik (1986) (under the assumption of a certain relationship between the cosmological term and the scalar field ). The dynamical behaviour of these models has been studied. 相似文献
974.
The H analysis of the development of the strong impulsive and faint gradual phase of the June 26, 1983 flare indicates the following: (1) The flare originated from two microprominences on the southeast border of NOAA 4227. Several similar events are summarized in Table II. (2) The main flare structure was a flare cone, which consisted of a bright surge-like stream, elevated above two flare ribbons (located in the cone's base). The flare cone had a height of about 40 × 103 km and lasted 4 min in H. The upper part of the cone was terminated by a very fine loop, which was bent to the west, where later a chromospheric brightening occurred at the footpoint of a flaring arch. A 300 keV burst and radio spikes were observed during the maximum flare phase. (3) The flaring arch system, with its apex at a height of about 48 × 103 km, formed the skeleton for the coronal helmet structure (Figure 7(c)). The velocity of the plasma moving along the flaring arch was between 3500 km s–1} and 6900 km s–1} during the first brightening (14:07 UT). 相似文献
975.
Guadalupian (Middle Permian) Conodont Faunas at Shangsi Section, Northeast Sichuan Province 总被引:1,自引:0,他引:1
This study recovered the Guadalupian conodont faunas from Shangsi (上寺) Section in Northeast Sichuan (四川). Four genera and nine species were identified, and three conodont zones were recognized and established; they include Jinogondolella nankingensis Zone, J. aserrata Zone, and J. postserrata Zone. The Roadian and Wordian boundary is set in the interval 5 m from the top of Bed 86 by the first appearance of the conodont J. aserrata. The Wordian and Capitanian boundary is set in the interval 2.3 m from the top of the Bed 95 by the first appearance of J. postserrata. Most of the specimens demonstrated low color alteration index (CAI) as 1.5-3, indicating that most part of the Maokou (茅口) Formation may have the suitable thermal conditions for the formation of oil source rocks. 相似文献
976.
R.P. Gupta D.P. Kanungo M.K. Arora S. Sarkar 《International Journal of Applied Earth Observation and Geoinformation》2008
Evaluation of maps generated from different conceptual models or data processing approaches at spatial level has importance in many geoenvironmental applications. This paper addresses the spatial comparison of different landslide susceptibility zonation (LSZ) raster maps of the same area derived from various procedures. 相似文献
977.
U. K. Shanwad V. C. Patil H. H. Gowda G. S. Dasog 《Journal of the Indian Society of Remote Sensing》2008,36(4):375-386
The current study was taken up to investigate the utility of remote sensing and GIS tools for evaluation of Integrated Wasteland
Development Programme (IWDP) implemented during 1997–2001 in Katangidda Nala watershed, Chincholi taluk, Gulbarga district,
Karnataka. The study was carried out using IRS 1C, LISS III data of December 11, 1997 (pre-treatment) and November 15, 2002
(post-treatment) covering the watershed to assess the changes in land use / land cover and biomass that have changed over
a period of five years (1997–2002). The images were classified into different land use/land cover categories using supervised
classification by maximum likelihood algorithm. They were also classified into different biomass levels using Normalized Difference
Vegetation Index (NDVI) approach. The results indicated that the area under agriculture crops and forest land were increased
by 671 ha (5.7%) and 1,414 ha (11.94%) respectively. This is due to the fact that parts of wastelands and fallow lands were
brought into cultivation. This increase in the area may be attributed to better utilization of surface and ground waters,
adoption of soil and water conservation practices and changes in cropping pattern. The area under waste lands and fallow lands
decreased by 1,667 ha (14.07%) and 467 ha (3.94%), respectively. The vegetation vigour of the area was classified into three
classes using NDVI. Substantial increase in the area under high and low biomass levels was observed (502 ha and 19 ha respectively).
The benefit-cost analysis indicates that the use of remote sensing and GIS was 2.2 times cheaper than the conventional methods.
Thus, the repetitive coverage of the satellite data provides an excellent opportunity to monitor the land resources and evaluate
the land cover changes through comparison of images for the watershed at different periods. 相似文献
978.
A. K. Singh P. K. Singh B. Lal A. N. Singh A. Mathur 《Journal of the Indian Society of Remote Sensing》2008,36(2):183-188
A World Bank-aided project on sodic land reclamation in Uttar Pradesh is being executed by U.P. Bhumi Sudhar Nigam, Lucknow,
and Remote Sensing Applications Centre, U.P., Lucknow has the responsibility of sodic land mapping for the execution of land
reclamation programme at the cadastral level. Sodic lands are mainly concentrated in the Gangetic alluvial plains but the
problem of sodicity is particularly acute in the canal-irrigated areas. A study of the distribution pattern of sodic lands
in canal and noncanal command areas in a reclamation site (covering 60 villages out of which sodic lands were mapped in 51
villages) of Etah district in Uttar Pradesh, indicates that 18.39 per cent area of the canal command villages was barren sodic
which was 13.41 per cent of the total geographical area of the site (15417 ha), however, 11.69 per cent area was recorded
to be barren sodic in the non-canal command villages which was only 3.16 per cent of the geographical area of the site. The
results of soil chemical analysis indicate that barren sodic lands of canal command area are saline-sodic with higher concentration
of soluble salts (pH2 >8.5, EC2 >4 dSm−1), however, those of non-canal command area are sodic (pH2 >8.5, EC2 <4 dSm−1). The post-monsoon ground level in the canal-irrigated areas was in the critical and semicritical zone (< 3.0 mbgl) whereas
it was well below the semi-critical zone in the non-canal command area, which indicates that the high ground water level is
a major factor to higher the area under sodicity. 相似文献
979.
H.B. Chauhan R.M. Dwivedi 《International Journal of Applied Earth Observation and Geoinformation》2008,10(2):181-1
Coastal zone assumes importance due to high productivity of ecosystems, man-made developmental activities, natural hazards and dynamic nature of the coast. As costal ecosystems are unique and fragile, understanding the impact of developmental activities on the sustainability of the coastal zone is very important. Remote sensing, because of repetitive and synoptic nature is an ideal tool for studying this. Time series data analyses for monitoring coastal zone require different type of sensors. Present study deals with atmospheric correction of satellite data, reflectance, selection of coastal features like, mudflat, mangroves, vegetated dune, coastal water, etc. and their inter-comparison using different sensor data of RESOURCESAT sensors. Reflectance values give better separateability for various coastal features in comparison to DN values. LISS IV can be used in place of LISS III or merged (LISS III + PAN) for long-term coastal zone studies. 相似文献
980.
K. N. Prudhvi Raju Sarvajeet Kumar Kshitij Mohan Manish Kumar Pandey 《Journal of the Indian Society of Remote Sensing》2008,36(3):283-288
With growing urban expanses, one of the pre-requisites for effective governance is Urban Information Systems (UIS) with content
down to individual properties (and individuals). The basic input i.e., a map, in UIS should show individual property boundaries
showing the plan outline of all structures existing within, at a scale of 1:1000 and larger with sub-metre to centimeters
planimetric and geometric accuracy. With very high resolution remote sensing data of the order of 1m available in hand, it
is possible to prepare maps with high resolution spatial content. The present exercise demonstrates a method of preparing
a geometrically and planimetrically accurate urban cadastral map on very large scale for a small area of about 5 sq km. IKONOS
merged data with 1m resolution is used for the purpose. Mapping was done in conjunction with on-site measurements and sketches.
Guides are used to maintain shape symmetry and accuracy of buildings and other features. Working out cost of mapping per unit
area is another objective in the present exercise. For want of fully or semi-automatic methods of information extraction from
very high resolution remote sensing data, it is imperative that mapping should be carried out in conjunction with some on-site
measurements wherever necessary. 相似文献