全文获取类型
收费全文 | 941篇 |
免费 | 93篇 |
国内免费 | 73篇 |
专业分类
测绘学 | 341篇 |
大气科学 | 68篇 |
地球物理 | 147篇 |
地质学 | 186篇 |
海洋学 | 76篇 |
天文学 | 7篇 |
综合类 | 79篇 |
自然地理 | 203篇 |
出版年
2024年 | 1篇 |
2023年 | 9篇 |
2022年 | 22篇 |
2021年 | 48篇 |
2020年 | 57篇 |
2019年 | 38篇 |
2018年 | 37篇 |
2017年 | 53篇 |
2016年 | 53篇 |
2015年 | 46篇 |
2014年 | 56篇 |
2013年 | 69篇 |
2012年 | 61篇 |
2011年 | 70篇 |
2010年 | 39篇 |
2009年 | 56篇 |
2008年 | 65篇 |
2007年 | 39篇 |
2006年 | 54篇 |
2005年 | 45篇 |
2004年 | 31篇 |
2003年 | 29篇 |
2002年 | 21篇 |
2001年 | 20篇 |
2000年 | 18篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 13篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有1107条查询结果,搜索用时 31 毫秒
991.
992.
在哈萨克斯坦东北部的阿尔泰山南坡,位于森林上限的西伯利亚落叶松的树轮宽度对生长季初期温度敏感,且在近年来气候变暖的背景下对温度的响应较为稳定,可以作为该区域温度变化的良好替代材料.利用森林上限区的树轮资料,从树轮宽度中提取初夏温度信息,建立了树轮宽度年表与卡通卡拉盖气象站6月平均温度的转换方程,重建了这一区域310年来的初夏温度变化历史,重建方程的方差解释量达到42.7%.由于校准期较长,利用独立检验方法对重建方程进行检验,各项检验的参数表明重建方程是稳定可靠的.重建序列与相邻的中国阿勒泰地区西部和阿尔泰山北坡树轮反映的温度变化序列的冷暖阶段是一致的,其中19世纪的温度波动较为明显,持续时间最长的冷期(1842-1871年)和暖期(1872-1906年)都出现在这个阶段.重建温度序列存在11a左右的周期,与太阳活动的周期一致. 相似文献
993.
海洋新菌的分类与鉴定方法 总被引:2,自引:0,他引:2
细菌的分类鉴定与细菌的研究和应用有着同等重要的地位。目前从自然界中可以培养的细菌总量不到1%,大多数海洋细菌的分类鉴定仍需建立在纯培养基础上。本文对目前可培养海洋细菌的分类鉴定方法做了系统的综述,详述了系统发生树的构建,汇总了生理生化分类特征,概述了主要的遗传学分析方法,包括16S rDNA序列分析,G+C含量测定,DNA-DNA杂交,MLST(多位点序列分型,Multi Locus Sequence Typing)以及核酸指纹图谱等,并指出每种方法适用的范围以及应用时需要注意的事项。另外,本文还提出了新菌分类鉴定的一般步骤。文章最后对目前海洋细菌分类鉴定的发展趋势进行了总结和展望。 相似文献
994.
995.
This study scrutinises the use of terrestrial laser scanning (TLS) to measure diameter at breast height (DBH) and tree height at individual tree species level. LiDAR point cloud scans are collected from uniformly defined control points. The result of processed TLS data demonstrates the precise measurements of tree height and DBH by comparing it with field data (DBH, tree height, tree species and location). The average tree height and DBH obtained through TLS measurements were 9.44?m and 43.30?cm, respectively. A linear equation between TLS derived parameters and field measured values were established, which gave the coefficient of determination (r2) of 0.79 and 0.96 for tree height and DBH, respectively. Further, these parameters were used to calculate above ground biomass (AGB) for individual tree species by considering a non-destructive approach. The total AGB and carbon stock from 80 different trees are computed to be 49.601 and 22.320?tonnes, respectively. 相似文献
996.
With the emergence of very high spatial and spectral resolution data set, the resolution gap that existed between remote-sensing data set and aerial photographs has decreased. The decrease in resolution gap has allowed accurate discrimination of different tree species. In this study, discrimination of indigenous tree species (n?=?5) was carried out using ground based hyperspectral data resampled to QuickBird bands and the actual QuickBird imagery for the area around Palapye, Botswana. The purpose of the study was to compare the accuracies of resampled hyperspectral data (resampled to QuickBird sensors) with the actual image (QuickBird image) in discriminating between the indigenous tree species. We performed Random Forest (RF) using canopy reflectance taking from ground-based hyperspectral sensor and the reflectance delineated regions of the tree species. The overall accuracies for classifying the five tree species was 79.86 and 88.78% for both the resampled and actual image, respectively. We observed that resampled data set can be upscale to actual image with the same or even greater level of accuracy. We therefore conclude that high spectral and spatial resolution data set has substantial potential for tree species discrimination in savannah environments. 相似文献
997.
空间数据索引对于空间数据管理和分析具有重要作用,与数据处理速度和效率密切相关。随着人类社会进入大数据时代,遥感矢量数据具备了大数据的特点,传统的空间索引构建方法无法有效满足矢量大数据处理的需求,为此引入云计算环境来进行空间索引构建,目的是提高空间索引构建速度。首先,对常见的空间索引及现阶段云计算环境下基于MapReduce模型的空间数据管理进行了综述;然后以云计算环境下R树索引构建为例,对云计算环境下空间索引的构建进行了研究;最后,通过实验验证,证明了云计算环境下空间索引构建的速度与单机索引构建相比,加速比为28~40倍;在相同云环境下,网格索引构建速度最快,R+索引构建速度相对较慢。 相似文献
998.
999.
GUPTA Pratibh BALAJI Raju PARANI M CHANDRA T S SHUKLA P KUMAR Anil BANDOPADHYAY Rajib 《海洋学报(英文版)》2015,34(8):73-82
Fifty-seven bacteria were isolated from Southern Ocean(Indian sector) water samples which were collected from different latitude and longitude of the ocean. All the isolates were able to grow at 4°C, 20°C, 37°C and tolerable Na Cl concentration up to 13.5%(w/v). 29 out of 57 isolates were identified using 16 S rDNA amplification and the sequences were submitted to National Center for Biotechnology Information(NCBI). All the isolates were classified by using Ribosomal Database Project(RDP) and found that isolates belongs to Proteobacteria and Bacteriodes. The average G+C content was 56.4%. The isolates were screened for the presence of extracellular enzymes, viz. amylase, catalase, urease, esterase, lipase and protease. The disc diffusion method is used to screen antibiotic production by the isolates against four pathogenic bacteria, viz. Salmonella typhimurium(NCIM 2501),Staphylococcus aureus(NCIM 2122), Bacillus subtilis(NCIM 2193), and Pseudomonas aeruginosa(NCIM 2036).Nine out of 29 were found to be antibiotic producer. 相似文献
1000.
An application of contingent valuation and decision tree analysis to water quality improvements 总被引:1,自引:0,他引:1
This paper applies contingent valuation and decision tree analysis to investigate public preferences for water quality improvements, and in particular reduced eutrophication. Such preferences are important given that the development of EU water quality legislation is imposing significant costs on European economies. Results are reported of a survey undertaken of residents of Arhus County, Denmark for water quality improvements in the Randers Fjord. Results demonstrate strong public support for reduced eutrophication and identify key determinants of such support. 相似文献