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We aim a better understanding of the effect of spring-time snow melt on the remotely sensed scene reflectance by using an extensive amount of optical spectral data obtained from an airborne hyperspectral campaign in Northern Finland. We investigate the behaviour of thin snow reflectance for different land cover types, such as open areas, boreal forests and treeless fells. Our results not only confirm the generally known fact that the reflectance of a melting thin snow layer is considerably lower than that of a thick snow layer, but we also present analyses of the reflectance variation over different land covers and in boreal forests as a function of canopy coverage. According to common knowledge, the highly variating reflectance spectra of partially transparent, most likely also contaminated thin snow pack weakens the performance of snow detection algorithms, in particular in the mapping of Fractional Snow Cover (FSC) during the end of the melting period. The obtained results directly support further development of the SCAmod algorithm for FSC retrieval, and can be likewise applied to develop other algorithms for optical satellite data (e.g. spectral unmixing methods), and to perform accuracy assessments for snow detection algorithms.A useful part of this work is the investigation of the competence of Normalized Difference Snow Index (NDSI) in snow detection in late spring, since it is widely used in snow mapping. We conclude, based on the spectral data analysis, that the NDSI -based snow mapping is more accurate in open areas than in forests. However, at the very end of the snow melting period the behavior of the NDSI becomes more unstable and unpredictable in non-forests with shallow snow, increasing the inaccuracy also in non-forested areas. For instance in peatbogs covered by melting snow layer (snow depth < 30 cm) the mean NDSI -0.6 was observed, having coefficient of variation as high as 70%, whereas for deeper snow packs the mean NDSI shows positive values. 相似文献
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Stephen Serjeant Steve Rawlings Mark Lacy Richard G. McMahon y Lawrence Michael Rowan-Robinson & Matt Mountain 《Monthly notices of the Royal Astronomical Society》1998,298(2):321-331
The z = 2.286 IRAS galaxy F10214 + 4724 remains one of the most luminous galaxies in the Universe, despite its gravitational lens magnification. We present optical and near-infrared spectra of F10214 + 4724, with clear evidence for three distinct components: lines of width ∼ 1000 km s−1 from a Seyfert 2 nucleus; ≲ 200 km s−1 lines which are likely to be associated with star formation; and a broad (∼ 4000 km s−1 ) C III ] 1909-Å emission line which is blueshifted by ∼ 1000 km s−1 with respect to the Seyfert 2 lines. Our study of the Seyfert 2 component leads to several new results. (i) From the double-peaked structure in the Lyα line, and the lack of Lyβ, we argue that the Lyα photons have emerged through a neutral column of N H ∼ 2.5 × 1025 m−2 , possibly located within the AGN narrow-line region, as proposed for several high-redshift radio galaxies. (ii) The resonant O VI 1032, 1036-Å doublet (previously identified as Lyβ) is in an optically thick (1:1) ratio. At face value this implies an extreme density ( n e ∼ 1017 m−3 ) more typical of broad-line region clouds. However, we attribute this instead to the damping wings of Lyβ from the resonant absorption. (iii) A tentative detection of He II 1086 suggests little extinction in the rest frame ultraviolet. 相似文献
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基于MODIS数据的东北地区积雪覆盖率估算 总被引：1，自引：1，他引：0
东北地区是我国三大积雪区之一,森林覆盖面积占总面积的40%左右。受森林冠层的影响,当前的MODIS雪盖产品（V6）提供的积雪覆盖率标准模型对东北地区积雪覆盖率估算结果存在明显的低估现象。基于此,采用分区建模的方式：在森林地区,计算归一化差值林地积雪指数（NDFSI）,建立像元积雪覆盖率（FSC）与NDFSI及NDVI之间的线性关系;在非森林地区,采用MOD10A1 V6提供的归一化差值积雪指数（NDSI）,建立像元积雪覆盖率（FSC）与NDSI及NDVI之间的线性关系。采用Landsat 8 OLI数据提取的积雪覆盖率（FSC）对分区建模的估算结果与标准模型的估算结果进行对比,发现进行估算的过程中均方根误差和平均绝对误差这两项指标的数值相对于标准模型有了大幅下降,这一结果在林区有更显著的表现。计算得到的决定系数R2,在本文模型也有提高。以T1林区影像为例,本文模型的均方根误差和平均绝对误差分别为0.246、0.055,而标准模型的两项指标则分别为0.420、0.348。本文模型和标准模型的决定系数分别为0.675、0.641。 相似文献
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我国在中西太平洋(Western and Central Pacific Ocean,WCPO)金枪鱼围网渔业作业的方式主要分为捕捞自由鱼群(free swimming school,FSC)和人工集鱼群(fish aggregation devices,FAD),镰状真鲨(Carcharhinus falciformis)是这两种作业方式中主要的兼捕鲨鱼。本研究根据2018—2019年我国围网渔业渔船在中西太平洋区域(15°S—15°N,140°E—180°E)共20个作业航次记录的渔捞日志,对FSC和FAD两种状态的捕捞作业下兼捕到的镰状真鲨渔获率、生存状态等进行比较分析,结果表明:(1) FAD作业相比FSC作业兼捕到的镰状真鲨分布范围更广,两种作业方式兼捕到的镰状真鲨单位捕捞努力量渔获(catch per unit effort,CPUE)主要分布区域均集中在基里巴斯和瑙鲁专属经济区;(2) FAD和FSC兼捕镰状真鲨CPUE出现较高月份均在5—9月,FAD兼捕镰状真鲨CPUE显著高于FSC (P 0.05);(3) FSC和FAD兼捕镰状真鲨的死亡率范围分别为0.00%～92.86%和20.26%～76.71%,经检验两种作业方式下镰状真鲨死亡率间无显著性差异(P 0.05)。研究结果有助于了解不同作业方式对鲨鱼兼捕的影响,以实现提高目标鱼种渔获率、降低鲨鱼兼捕率的目的。 相似文献
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In this new globalised era there is an increasing demand in the developed world for environmental certification (Forestry Stewardship Council) of furniture and timber products. The imposition of this requirement has major implications for producers in developing countries. This paper examines this issue from a value chain perspective, exploring how certification requirements are driven through the links of the global furniture value chain. It does so by focusing on the dynamic underlying the spread of FSC certification through the furniture and timber industry in South Africa, and investigates the role of the various organisational ‘drivers’ of the system. In doing so it explores how developing country firms relate to these opportunities and pressures, drawing out the logistical implications, the costs and benefits, as well as the future for FSC certification. It concludes with a discussion of the role of buyers, agents, certification agencies and manufacturers in driving FSC certification through the wood products value chain in South Africa. 相似文献
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