全文获取类型
收费全文 | 25323篇 |
免费 | 382篇 |
国内免费 | 263篇 |
专业分类
测绘学 | 624篇 |
大气科学 | 1933篇 |
地球物理 | 5419篇 |
地质学 | 8851篇 |
海洋学 | 2043篇 |
天文学 | 5386篇 |
综合类 | 38篇 |
自然地理 | 1674篇 |
出版年
2020年 | 136篇 |
2019年 | 130篇 |
2018年 | 291篇 |
2017年 | 267篇 |
2016年 | 416篇 |
2015年 | 300篇 |
2014年 | 423篇 |
2013年 | 1184篇 |
2012年 | 505篇 |
2011年 | 784篇 |
2010年 | 645篇 |
2009年 | 910篇 |
2008年 | 836篇 |
2007年 | 797篇 |
2006年 | 826篇 |
2005年 | 715篇 |
2004年 | 748篇 |
2003年 | 712篇 |
2002年 | 713篇 |
2001年 | 589篇 |
2000年 | 600篇 |
1999年 | 572篇 |
1998年 | 547篇 |
1997年 | 563篇 |
1996年 | 463篇 |
1995年 | 459篇 |
1994年 | 440篇 |
1993年 | 410篇 |
1992年 | 380篇 |
1991年 | 328篇 |
1990年 | 379篇 |
1989年 | 297篇 |
1988年 | 340篇 |
1987年 | 377篇 |
1986年 | 327篇 |
1985年 | 482篇 |
1984年 | 523篇 |
1983年 | 531篇 |
1982年 | 422篇 |
1981年 | 419篇 |
1980年 | 436篇 |
1979年 | 381篇 |
1978年 | 397篇 |
1977年 | 347篇 |
1976年 | 377篇 |
1975年 | 342篇 |
1974年 | 380篇 |
1973年 | 365篇 |
1972年 | 234篇 |
1971年 | 187篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
892.
E.A. Addink J.G.P.W. Clevers S.M. de Jong G.F. Epema F.D. van der Meer A.K. Skidmore W.H. Bakker 《International Journal of Applied Earth Observation and Geoinformation》2006,8(4):237-245
Objective comparison of classification performance of earth observation images, acquired at different spatial resolutions (e.g. NOAA-AVHRR, IRS-MOS, IRS-WiFS, Landsat-TM, IRS-LISS), is complicated because both class definition and training site selection are hampered by the inherent scale differences. This paper presents a new, generic method to compare the information content of such a set of images, the “Stained Glass Procedure”. It overcomes the stated problems by computing the scale-dependent, internal spectral variation in an image and by using this as an indicator for land cover information. The Stained Glass Procedure creates segments in the images and calculates the internal spectral variation in a high-spatial-resolution image for each segment. For each image from the set the average variance, weighted to area, is calculated. The Stained Glass Procedure can be used to predict the performance of sensors that are not available, yet, or to roughly determine the optimal spatial resolution for the classification of a specific area.The procedure was applied to images with pixel sizes ranging from 23 to 1100 m. Classification detail of Envisat-MERIS (300 m pixel size), not included in the image set, could be predicted accurately using the Stained Glass Procedure.The Stained Glass Procedure applies one procedure to all images, without any subjective decision during the analysis, thus offering a method to compare images with different pixel sizes in terms of classification detail that is truly objective. 相似文献
893.
894.
The purpose of this study was to evaluate the global energy production potential of woody biomass from forestry for the year
2050 using a bottom-up analysis of key factors. Woody biomass from forestry was defined as all of the aboveground woody biomass
of trees, including all products made from woody biomass. This includes the harvesting, processing and use of woody biomass.
The projection was performed by comparing the future demand with the future supply of wood, based on existing databases, scenarios,
and outlook studies. Specific attention was paid to the impact of the underlying factors that determine this potential and
to the gaps and uncertainties in our current knowledge. Key variables included the demand for industrial roundwood and woodfuel,
the plantation establishment rates, and the various theoretical, technical, economical, and ecological limitations related
to the supply of wood from forests. Forests, as defined in this study, exclude forest plantations. Key uncertainties were
the supply of wood from trees outside forests, the future rates of deforestation, the consumption of woodfuel, and the theoretical,
technical, economical, or ecological wood production potentials of the forests. Based on a medium demand and medium plantation
scenario, the global theoretical potential of the surplus wood supply (i.e., after the demand for woodfuel and industrial roundwood is met) in 2050 was calculated
to be 6.1 Gm3 (71 EJ) and the technical potential to be 5.5 Gm3 (64 EJ). In practice, economical considerations further reduced the surplus wood supply from forests to 1.3 Gm3 year−1 (15 EJ year−1). When ecological criteria were also included, the demand for woodfuel and industrial roundwood exceeded the supply by 0.7 Gm3 year−1 (8 EJ year−1). The bioenergy potential from logging and processing residues and waste was estimated to be equivalent to 2.4 Gm3 year−1 (28 EJ year−1) wood, based on a medium demand scenario. These results indicate that forests can, in theory, become a major source of bioenergy,
and that the use of this bioenergy can, in theory, be realized without endangering the supply of industrial roundwood and
woodfuel and without further deforestation. Regional shortages in the supply of industrial roundwood and woodfuel can, however,
occur in some regions, e.g., South Asia and the Middle East and North Africa. 相似文献
895.
Recent observations of externally illuminated young stellar objects in the Orion Nebula show that they have a variety of morphologies. In particular, long thin tails which terminate in points have been observed. Mechanisms will be considered here to understand the formation and properties of these tails. 相似文献
896.
Differences in trace metal concentrations among fluvial morphologic units and implications for sampling 总被引:2,自引:0,他引:2
This study examines the segregation of trace metals within and between fluvial morphologic units in sand-sized and finer
bed sediments in a cobble bed stream. The types of fluvial morphologic units sampled are low gradient riffles, high gradient
riffles, glides, eddy drop zones, lateral scour pools, attached bars, and detached bars. Three to nine samples were collected
from ten of each type of morphologic unit. All 12 metals show significantly different concentrations between some morphologic
units in sediments smaller than 2 mm. Eddy drop zones and attached bars consistently have the highest metal concentrations,
while low gradient riffles, high gradient riffles, and glides typically have the lowest concentrations. Metals showing the
greatest between-unit variability are Al, Cr, Fe, Cu, and Ti, followed by Co, Mn, and Pb, while Mg, Mn, V, and Zn display
relatively few differences between units. Lateral and longitudinal variations of metals within units are not significant,
and there was no consistent, predictable variation in metal concentrations with distance downstream. Results indicate that
metal studies in other gravel- and cobble-bed streams should include a reconnaissance survey to determine variations between
morphologic units, stratify sampling by morphologic unit, and analyze spatial autocorrelation to determine sample spacing.
Received: October 1997 · Accepted: 23 March 1998 相似文献
897.
898.
A. R. Breen P. J. Moran C. A. Varley W. P. Wilkinson P. J. S. Williams W. A. Coles A. Lecinski J. Markkanen 《Annales Geophysicae》1998,16(10):1265-1282
Co-rotating interaction regions (CIRs) between fast and slow streams of plasma are a prominent feature of the solar wind. Measurements of interplanetary scintillation (IPS) using the three widely separated antennas of the EISCAT facility have been used to detect the compression regions at the leading edges of interaction regions and to determine the location and velocity of the structure. Observations show that interaction regions have developed as close to the Sun as 25–30 solar radii, a result supported by theoretical modelling which shows that the conditions needed for CIRs to develop exist inside 30 solar radii. 相似文献
899.
W.S. Fyfe 《Gondwana Research》1998,1(3-4):315-318
The most basic solid earth resources for use by man include construction materials, energy resources, metals, ceramics, soils. With the exception of construction materials, the formation of economic mineral deposits and soils involves the thermal or gravity driven flow of various types of geofluids. The search for mineral deposits thus involves the understanding of the exact nature of fluids, their mass and flow patterns. Given the giant thermal disturbances associated with the formation and destruction of Gondwana, it must be no surprise that these regions contain very significant mineral resources on a large scale. 相似文献
900.