全文获取类型
收费全文 | 3864篇 |
免费 | 719篇 |
国内免费 | 1089篇 |
专业分类
测绘学 | 508篇 |
大气科学 | 855篇 |
地球物理 | 847篇 |
地质学 | 1928篇 |
海洋学 | 436篇 |
天文学 | 129篇 |
综合类 | 446篇 |
自然地理 | 523篇 |
出版年
2024年 | 16篇 |
2023年 | 38篇 |
2022年 | 167篇 |
2021年 | 227篇 |
2020年 | 167篇 |
2019年 | 190篇 |
2018年 | 214篇 |
2017年 | 214篇 |
2016年 | 230篇 |
2015年 | 207篇 |
2014年 | 242篇 |
2013年 | 236篇 |
2012年 | 262篇 |
2011年 | 246篇 |
2010年 | 256篇 |
2009年 | 240篇 |
2008年 | 223篇 |
2007年 | 223篇 |
2006年 | 176篇 |
2005年 | 179篇 |
2004年 | 139篇 |
2003年 | 117篇 |
2002年 | 113篇 |
2001年 | 119篇 |
2000年 | 133篇 |
1999年 | 155篇 |
1998年 | 169篇 |
1997年 | 143篇 |
1996年 | 103篇 |
1995年 | 84篇 |
1994年 | 90篇 |
1993年 | 81篇 |
1992年 | 65篇 |
1991年 | 53篇 |
1990年 | 34篇 |
1989年 | 31篇 |
1988年 | 25篇 |
1987年 | 19篇 |
1986年 | 16篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1958年 | 3篇 |
1957年 | 2篇 |
排序方式: 共有5672条查询结果,搜索用时 156 毫秒
141.
针对利用像元二分模型估算植被覆盖度的精度不高的问题,该文基于OSAVI,提出了选定模型参数(OSAVIs和OSAVIv)的方法,并将该方法应用于青海省植被覆盖度估算。该方法通过高分辨率影像在研究区内选取纯裸地和纯植被样点,并将纯裸地样点的OSAVI作为纯裸地样点像元的OSAVIs,将纯植被样点的OSAVI作为纯植被样点像元的OSAVIv,利用样点像元的OSAVIs和OSAVIv值,通过普通克里金内插法,求得研究区每个像元对应的OSAVIs和OSAVIv。经精度验证结果表明:此方法较常规的参数选取方法,RMSE由0.170降至0.156,MAE由0.137降至0.124。经进一步分析表明,此方法对边缘验证点和非边缘验证点的估算精度都有所提高,由于配准误差和周围地表漫反射的影响,边缘验证点的估算精度低于对非边缘验证点的估算精度。 相似文献
142.
为了对BDS实时精密单点定位性能进行评估,该文提出了一种适用于BDS系统的实时精密单点定位算法。采用无电离组合模型作为双频实时精密单点定位的数学模型,采用电离层残差法和Melbourne-Wübbena组合实时探测相位周跳,进而单历元实时估计坐标、模糊度等参数,实现了BDS双频实时精密单点定位算法。基于此算法,采用轨道钟差产品和采样间隔为1s的观测数据,模拟实时BDS双频精密单点定位算法,并评估其定位精度。实验结果表明:BDS双频实时定位的平面精度和三维精度均为0.2m左右。 相似文献
143.
Monthly fishery survey data of the small yellow croaker Larimichthys polyactis in the southern Yellow Sea from2003 and 2013 were employed to evaluate the variation in the resource distribution and biological characteristics(especially body length and sex ratio) in the population on a decadal scale.The results indicated that the small yellow croaker migrated from the Shawai fishing ground to the Dasha fishing ground in spring and was mainly distributed in the central and western parts of the Dasha fishing ground in April and May.Larimichthys polyactis in the Dasha fishing ground migrated eastward to offshore wintering grounds in autumn and reached the central Dasha fishing ground in October and November.The small yellow croaker entered the western waters of the Shawai fishing ground in winter.A large number of age 0+ fish occurred in the Shawai and Jiangwai fishing grounds in October of 2003 and 2013.The body lengths of the spawning stock and wintering stock in 2013 were larger than those in 2003,and the monthly sex ratios(female to male) were significantly less than 1 in both years.The monthly distribution of this fish in the southern Yellow Sea was consistent with a previous finding that "the stock migrated between the wintering grounds in the west of Jeju Island and the Lüsi spawning grounds" but tended to move more northward,with the spawning grounds extending outward.In the past decade,body length variation experienced a decline after an increase,rather than a steady decrease.The sex ratio in the single-stick stow net showed a tendency to increase over the decade,but was either less than or more than 1 depending on the fishing gear;therefore,further studies should be conducted to determine the sex structure. 相似文献
144.
δ~(18)O variations;;ice core;;outgoing longwave radiation;;meridional wind;;northeastern Tibetan Plateau 相似文献
145.
146.
Maritime-type glaciers in the eastern Nyainqêntanglha Range, located in the southeastern part of the Tibetan Plateau, are an important water source for downstream residents and ecological systems. To better understand the variability of glaciers in this region, we used the band ratio threshold(TM3/TM5 for the Landsat TM /ETM+ and TM4/TM6 for Landsat OLI) to extract glacier outlines in ~1999 and ~2013. After that, we also generated a series of glacier boundaries and monitored glacier variations in the past 40 years with the help of the Chinese Glacier Inventory data(1975) and Landsat TM, ETM+ and OLI data. The total glacier area decreased by 37.69 ± 2.84% from 1975 to 2013. The annual percentage area change(APAC) was ~1.32% a-1 and ~1.29% a-1 in the periods 1975-1999 and 1999-2013, respectively. According to the lag theory, the reaction time is probably about 10 years and we discuss the variations of temperature and precipitation between 1965 and 2011. Temperature and precipitation increased between 1965 and 2011 at a rate of 0.34°C /10 a and 15.4 mm/10 a, respectively. Extensive meteorological data show that the glacier shrinkage rate over the period may be mainly due to increasing air temperature, while the increasing precipitation partly made up for the mass loss of glacier ice resulting from increasing temperature may also lead to the low APAC between 1999 and 2013. The lag theory suggests that glacier shrinkage may accelerate in the next 10 years. Small glaciers were more sensitive to climate change, and there was a normal distribution between glacier area and elevation. Glaciers shrank in all aspects, and south aspects diminished faster than others. 相似文献
147.
148.
149.
150.