Reflections in bumpy terrain: implications of canopy surface variations for the radiation balance of vegetation

Data from an optical imaging sensor and a small-footprint lidar were used to examine the relation between canopy reflectance and outer surface complexity in forest stands in the southern Cascades of Washington state. Albedo was estimated from the Airborne Visible Infrared/Imaging Spectrometer; canopy surface variation (termed "rugosity") was estimated from small-footprint lidar; and stand ages were obtained through U.S. Forest Service records and global information system coverages. Results showed that albedo from Douglas-fir/western hemlock stands decreased, and variation in the outer surface of the canopy increased with age. Estimates of rugosity increased most rapidly in young stands and then more slowly after about 150 years. Albedo declined by 10% across the age sequence, suggesting that older stands of this forest type enjoy a substantial advantage in energy input. The results highlight the impacts of land-cover change on local energy balance and climate.     

Retrieval of remotely sensed LAI using Landsat ETM+ data and ground measurements of solar radiation and vegetation structure: Implication of leaf inclination angle

A time series of leaf area index (LAI) of a managed birch forest in Germany (near Dresden) has been developed based on 16-day normalized difference vegetation index (NDVI) data from the Landsat ETM+ sensor at 30 m resolution. The Landsat ETM+ LAI was retrieved using a modified physical radiative transfer (RTM) model which establishes a relationship between LAI, fractional vegetation cover (fC), and given patterns of surface reflectance, view-illumination conditions and optical properties of vegetation. In situ measurements of photosynthetically active radiation (PAR) and vegetation structure parameters using hemispherical photography (HSP) served for calibration of model parameters, while data from litter collection at the study site provided the ground-based estimates of LAI for validation of modelling results. Influence of view-illumination conditions on optical properties of canopy was simulated by a view angle geometry model incorporating the solar zenith angle and the sensor viewing angle. Effects of intra-annual and inter-annual variability of structural properties of the canopy on the light extinction coefficient were simulated by implementing variability of the leaf inclination angle (LIA), which was confirmed in the study site. The results revealed good compatibility of the produced Landsat ETM+ LAI data set with the litter-estimated LAI. The results also showed high sensitivity of the LAI retrieval algorithm to variability of structural properties of the canopy: the implementation of LIA dynamics into the LAI retrieval algorithm significantly improved the model accuracy.     

针叶林冠层热点及多角度无人机遥感观测及特征分析

多角度遥感观测是研究植被冠层BRDF(Bidirectional Reflectance Distribution Function)特性的重要手段,但目前对森林冠层进行连续间隔采样的多角度遥感观测及数据较少,热点方向的观测尤为缺乏.本研究基于无人机多角度高光谱成像系统,在主平面上对针叶林冠层以等角度连续间隔采样进行多...     

基于真实白杨林地场景冠层辐射特性的计算机模拟

叶面积指数（LAI）和叶倾角分布（LAD）是决定植被冠层结构的重要参数。在计算机模拟植被冠层,两个参数是植被三维真实结构生成的重要控制因子。本论文中,结合计算机图像学理论,基于实验的地面实测结构参数数据利用可改写的扩展L-system方法生成草以及白杨树的真实三维场景。RGM（A radiosity-graphics combined model）模型是基于辐射度方法的计算机模拟模型,利用此模型来计算生成的三维场景可见光及近红外波段的冠层辐射特性,如冠层波谱以及方向反射特性等。在本研究中,模拟了两种不同下垫面的白杨林地：（1）下垫面只有土壤的白杨树场景;（2）下垫面包括土壤和草的白杨树。在特定的场景组分光学特性下,模拟得到两种情况的主平面冠层BRF（bi-di-rectional reflectance factor）,并对两者的差异进行了分析。可以看出,下垫面对冠层BRF的影响不可忽视。但是,由于白杨林地结构的复杂,大尺度的场景中必须由成千上万个面元组成,因此辐射度方法不能模拟大尺度的真实结构场景。为了拓展辐射度方法应用范围,根据白杨树树冠的特点,将其抽象为椭球体,从而减少场景组成面元个数,满足了辐射度方法的要求。并结合几何光学模型的思想,在对椭球体面元赋值加入了间隙率;并考虑了整个树冠的承照面以及阴影面的差异,模拟大尺度林地冠层BRF,且与GOMS模型结果符合的很好。通过以上研究,可以看出计算机模拟为遥感研究获取多角度数据信息提供了一种很好的手段。     

航空多光谱图像地形影响校正的模拟反射率方法

开展强起伏地形等复杂条件下的多光谱数据处理实用方法研究,是使中高山地区多光谱定量化解释成果水平取得明显提高的重要途径之一。本文根据地形因素对航空多光谱图像地质解释影响的特点、多光谱仪的成像方式,应用简化的太阳辐射能量平衡原理,研究了如何从航空多光谱图像像元的亮度变化中消除地形影响,模拟生成反射率图像的新方法。新方法综合考虑了地表太阳辐照度的变化、地物的光谱反射率、反照率和热辐射信息,与目前常规的处理技术(如比值法和剩余对数法) 相比,消除了地形坡度和坡向变化对图像解释的影响,保留了地物反照率信息,为多光谱定量化解释提供了更合理有效的反射率图像,对超光谱数据的处理和解释均有潜在的应用价值。     

利用BRDF原型和单方向反射率数据估算地表反照率

地表反照率是影响地表能量收支平衡的决定性参数之一,精确反演地表反照率需要考虑地表各向异性反射特征。本文尝试以双向反射分布函数BRDF原型为地表各向异性反射的先验知识,通过单方向反射率反演地表反照率。首先根据地面实测及MODIS多角度反射率数据对反演方法进行分析和精度评价,然后借助MODIS BRDF产品统计出研究区的主导BRDF原型,并联合环境一号卫星(HJ-1)单方向反射率数据反演30 m地表反照率,最终将结果与地表实测数据进行比较。结果表明:BRDF原型对BRDF的变化进行了约束,且能够适用于几十米尺度的遥感数据反照率的反演;不同级别的各向异性反射特征的分布是不均一的,借助于主导BRDF原型能够使大部分样本的地表反照率满足精度要求;利用研究区MODIS BRDF产品统计得到的主导BRDF原型为先验知识,通过HJ-1数据反演得到的地表反照率与地表实测反照率有较高的一致性,而朗伯假定条件下的反照率高于实测结果。本文算法简单高效,可为产生全国范围的中高分辨卫星反照率产品提供有价值的算法参考。     

A forestry GIS-based study on evaluating the potential of imaging spectroscopy in mapping forest land fertility

Fertility, or the availability of nutrients and water, controls forest productivity. It affects its carbon sequestration, and thus the forest's effect on climate, as well as its commercial value. Although the availability of nutrients cannot be measured directly using remote sensing methods, fertility alters several vegetation traits detectable from the reflectance spectra of the forest stand, including its pigment content and water stress. However, forest reflectance is also influenced by other factors, such as species composition and stand age. Here, we present a case study demonstrating how data obtained using imaging spectroscopy is correlated with site fertility. The study was carried out in Hyytiälä, Finland, in the southern boreal forest zone. We used a database of state-owned forest stands including basic forestry variables and a site fertility index. To test the suitability of imaging spectroscopy with different spatial and spectral resolutions for site fertility mapping, we performed two airborne acquisitions using different sensor configurations. First, the sensor was flown at a high altitude with high spectral resolution resulting in a pixel size in the order of a tree crown. Next, the same area was flown to provide reflectance data with sub-meter spatial resolution. However, to maintain usable signal-to-noise ratios, several spectral channels inside the sensor were combined, thus reducing spectral resolution. We correlated a number of narrowband vegetation indices (describing canopy biochemical composition, structure, and photosynthetic activity) on site fertility. Overall, site fertility had a significant influence on the vegetation indices but the strength of the correlation depended on dominant species. We found that high spatial resolution data calculated from the spectra of sunlit parts of tree crowns had the strongest correlation with site fertility.     

冠层特征尺度的定量计算模型与方法

冠层特征尺度是植被定量遥感的基础概念,其物理定义和数学定量表达具有重要的研究意义。首先,基于光学辐射传输角度提出的冠层特征尺度的物理定义,即水平维线性混合条件下的最小分辨率单元,建立了冠层特征尺度的数学计算模型,并引入倒置的地统计学指数模型。然后,提出了基于局部方差分析的冠层特征尺度计算方法。最后,利用森林区域高分辨率图像,对论文提出的冠层特征尺度模型进行了定量验证。结果表明,冠层特征尺度模型计算的冠层特征尺度与树林株行距存在密切联系,线性复相关系数达0.95,证明了本文方法的合理性和可行性。本文提出的冠层特征尺度模型为地表特征尺度定量计算提供了一种新方法。     

MODIS二向反射模型方差函数的研究与应用

地表二向性反射分布函数(BRDF)是表征地物反射随太阳和观测方向变化的物理量。在统计意义上,BRDF表示均值统计量,BRVF(Bidirectional Reflectance Variance Function)表示方差统计量,它们对研究地表各向异性反射特征有着重要意义。本文首先采用误差传播理论,推导出基于MODIS BRDF模型的BRVF表达形式。研究结果表明,BRVF的空间分布模式主要由几何光学核Kgeo和体散射核Kvol的一次项和二次项权重和决定。然后利用EOS地面验证核心站点(EOS Land Validation Core Sites)的MODIS BRDF产品,对BRVF空间分布模式随地表类型、光谱波段和观测角度范围进行验证。验证结果表明,基于MODIS BRDF产品的验证与理论推导有较好的一致性。BRVF空间分布模式和地表类型有关,通常在热点处有一个峰值。在大观测天顶角(60°)下,BRVF随着角度的增大而增大。BRVF在近红外波段整体上大于红波段,表明其波段依赖性。最后,将上述理论成果初步应用于69组地表测量数据的模拟中。模拟结果表明,当大角度缺少观测数据时,模型外延所引起的方向反射方差显著增大,对地表反照率的反演精度和不确定性有较大影响。其中,红波段的白天空反照率的相对误差最大可达38.26%。本研究对利用小角度观测数据进行地表反照率反演的不确定性分析有指导意义;对大角度观测数据缺失情况下,先验知识在地表反照率的反演应用可提供有意义的理论支撑。     

异质性地表反照率遥感产品真实性检验研究现状及挑战

地表反照率直接决定了地表能够吸收到的太阳辐射能量,是研究气候变化、能量平衡的一个关键参数。遥感为大尺度、连续获取地表反照率提供了一种有效的观测手段。但遥感数据本身的精度限制和反演模型的不确定性,使基于卫星数据反演的反照率产品存在误差,而这种误差的存在又会影响产品的进一步应用。正确的认识这种误差有助于提高产品的应用精度,深化其应用的深度和广度。真实性检验就是正确认识卫星反照率产品准确性、稳定性的重要手段,它是卫星产品从生产到应用的桥梁。虽然目前已经开展了大量的真实性检验工作,但即使是针对同一种卫星产品,真实性检验的结果往往并不一致。其根本原因在于验证中所采用的参考值能不能够准确地代表卫星像元尺度的地面真值。地面观测和卫星产品像元之间巨大的尺度差异以及广泛分布的地表异质性,使地面观测并不能直接作为像元尺度真值与卫星产品进行简单的对比。因此真实性检验过程并不是直接的,而是需要经过一系列严格、独立的过程得到像元尺度真值后与产品在一定的空间和时间范围内进行对比。针对目前真实性检验结果准确性和可信度不高等问题,本文尝试从地面实测数据、尺度转换、验证方式、评价方法及验证中存在的问题等几个方面来论述反照率产品的真实性检验现状及挑战。     

植被冠层日光诱导叶绿素荧光塔基自动观测方法及系统介绍

植物光合作用所提供的物质和能量是人类赖以生存的关键因素,而日光诱导叶绿素荧光SIF(Sun-Induced chlorophyll Fluorescence)是植物光合作用的副产品,与光合作用关系密切,深入研究SIF将对于更加深入理解光合作用机制有着重要的意义.目前,近地面植被冠层SIF遥感观测发展迅速,但不同SIF观...     

利用云下阴影实现陆地上空气溶胶和地表反射率的同时反演—理论方法和模拟

卫星探测信号包含大气中分子和粒子的散射贡献以及地表反射的贡献,在陆地上空二者的贡献相当,并且陆地地表反射率在时间和空间上极度不均一,因此,很难区分二者的各自贡献从而定量提取大气气溶胶和地表反射率,陆地上空气溶胶的反演也一直是一个极具挑战性的课题.而高分辨率卫星资料如TM5的可见光通道能够很好地区分云和云下阴影,如果云是不透光的,在阴影上空,卫星信号仅包含大气散射贡献和地表漫反射贡献,而在邻近的非阴影区上空,卫星探测信号还包含地表直接反射的贡献,根据这个原理,利用辐射传输模式分析了阴影区和非阴影区上空卫星探测的辐射量差别与地表反射率和大气气溶胶的关系,提出一种利用云下阴影来同时提取阴影上空大气气溶胶和地表反射率的单波长反演方案,并对气溶胶单次散射反照率,散射相函数,测量精度以及地表反射率的不均一性进行了敏感性分析.     

陆地表层覆盖变化对地表反照率影响的四国对比

陆表覆盖变化影响地表特征从而改变地表能量平衡是理解人类活动对全球气候变化影响的关键环节。选择国际气候谈判主要国家的美国、印度和巴西作为中国的对比国,对比分析不同国别、不同气候带典型陆表覆盖类型的地表反照率时空差异,进而模拟开垦和城市化等陆表覆盖变化对反照率的影响差异。结果表明：(1) 2000年—2015年,中国、美国的地表反照率年际变化存在明显的气候带空间分异特征,中国干旱半干旱区和美国中低纬湿润区表现出降低趋势,而中国亚热带湿润和美国高纬与中部干旱区则表现出明显的升高趋势,印度的地表反照率年际变化呈微弱下降趋势,而巴西为微弱上升趋势。(2)无雪覆盖时,耕地、林地、草地和人造地表反照率具有夏高、冬低的时间变化特征,干旱半干旱区反照率明显高于湿润区。4种类型的国别差异体现在,中国亚热带湿润区地表反照率均以上升为主,干旱半干旱区则相反;美国除耕地在干旱区呈较强的升高趋势外,其余类型基本为降低趋势;印度均表现为降低趋势;巴西则表现为略微升高趋势。(3)与无雪覆盖相比,有雪覆盖时不同陆表覆盖类型地表反照率均有所提高,林地提高幅度最小,约0.06—0.26,耕地提高最大,约为0.17—0.38,且中国林地反照率提高幅度略高于美国。(4)原陆表覆盖为林地时,开垦和城镇化均导致地表反照率升高,且干旱区升高幅度高于湿润区,湿润区的升高幅度随纬度降低而减弱;为草地时,开垦主要在巴西、印度和中、美亚热带湿润区引起地表反照率升高。而城镇化引起的反照率变化则受到原有地表覆盖、季节和气候背景影响存在较复杂的国别和气候带差异。     

机载WIDAS数据的Landsat卫星反照率初步验证

随着精细化监测的需求,中高空间分辨率的地表反照率产品逐渐成为气候模型的主要输入。目前,中高空间分辨率反照率产品的验证主要基于地表站点的通量塔观测数据,区域机载飞行数据的验证依然相对较少。因此,本文基于区域机载数据验证Landsat反照率产品。针对内蒙古自治区根河森林试验区所获取的机载红外广角双模式成像仪(WIDAS)多角度反射率数据,应用BRDF原型反演算法估算其反照率,分析了应用机载数据验证中高空间分辨率反照率产品的潜力。2016年内蒙古根河森林试验区机载WIDAS飞行多角度观测的可用多角度范围为25°,以前的研究表明BRDF原型反照率反演算法表现出对小观测角度的反照率反演结果的鲁棒性。因此,机载WIDAS反照率在一定程度可用于星载反照率的验证。首先,基于核驱动模型和各向异性平整指数(AFX)提取了试验区4种MODIS二向性反射分布函数(BRDF)原型;然后,将其作为先验知识应用到根河森林WIDAS机载数据的反照率反演中;最后,用WIDAS反照率和单个地面通量塔观测的反照率对Landsat卫星数据的反照率进行初步验证。验证结果表明Landsat反照率与WIDAS反照率结果较为一致,但略有低估,总体均方根误差(RMSE)约为0.02,偏差为0.0057。在多角度观测范围较小时,BRDF原型的反照率反演算法可为星载地表反照率的验证提供了一种有效的验证手段。     

Study of surface brightness from backscattered laser intensity: calibration of laser data

Systematic laboratory measurements of laser backscatter intensity are presented for brightness calibration targets, and a calibration scheme for airborne laser scanner intensity data is proposed. Thus far, the use of these data has been partly hampered by the variability of the intensity with time, and no test fields have been available for airborne reflectance calibration. Portable brightness targets (tarps), with nominal reflectances from 5% to 70%, were manufactured, and, based on these measurements, found suitable for lidar reflectance standards. Furthermore, the variability of the recorded intensity from the tarps as a function of incidence angle was low. The measurements also provide new information on the surface albedo dependence of backscattering effects: as the surface brightness increases from 5% to 70%, the hotspot brightness peak amplitudes increase by 20% to 30%, and their apparent widths reduce to a half, which implies that hotspots could be used as an albedo discriminator.     

非匀一冠层热辐射方向性3维模型构建——以玉米冠层为例

建立适用于多类型植被场景的热辐射方向性模型是进行地表热辐射方向性研究的一种手段。利用真实植株几何及生理参数的统计平均值来刻画理念植株,并给定其空间分布特征,进行不同生长期植被冠层的描述。基于冠层双向孔隙率思想构建了冠层热辐射方向性3维模型,模型继承了孔隙率模型在计算冠层热辐射方向性上的简洁优势同时以几何光学的思想考虑了冠层空间异质性对冠层热辐射方向性的影响。以玉米冠层为例,进行了不同生长期玉米冠层热辐射方向亮温的模拟,通过与实地测量数据的比对表明,本文发展的模型能够较准确地模拟不同生长期玉米植被场景的方向亮温变化规律,模拟误差主要来自理念株的刻画误差、玉米叶片形状的近似以及忽略了多次散射贡献等3个方面。模型的构建方法对稀疏植被场景、浓密植被场景、多类型植被的混合场景均可适用,不同观测几何下的植被场景4组份面积比计算结果有望应用于复杂地表条件下地表返照率的研究。     

Examining the impacts of urbanization on surface radiation using Landsat imagery

Metropolitan Beijing is facing many environmental problems such as haze and urban heat island due to the rapid urbanization. Surface shortwave, longwave, and net radiations are key components of the surface-atmosphere radiation budget. Since megacities are affected by the thermal radiation of complex landscape structures and atmospheric environments, quantitative and spatially explicit retrieval from remotely sensed data remains a challenge. We collected the surface radiation fluxes from seven fixed sites representing different land-use types to calibrate the local parameters for remotely sensed retrieval of net radiation. We proposed a remote sensing–based surface radiation retrieval method by embedding the underlying land covers and integrating the observational data. The improved method is feasible to accurately retrieve surface radiation and delineate spatial characteristics in metropolitan areas. The accuracy evaluation indicated that the difference between remotely sensed and in situ observed net radiation ranged within 0~± 40 W· m^?2. The root mean squared error of the estimated net surface radiation was 32.71 W· m^?2. The strongly spatial heterogeneity of surface radiation components in metropolitan Beijing was closely related to land-cover patterns from urban area to outskirts. We also found that the surface net radiation had a decreasing trend from 1984 to 2014, and the net radiation in the urban area was lower than that in the outskirts. According to the surface radiation budgets, urbanization resulted in the cooling effect in net radiation flux in the daytime, which was stemmed from low atmospheric transmittances from massive aerosol concentration and high surface albedo from light building materials.     

Investigating relationships between Landsat-7 ETM data and spatial segregation of LULC types under shifting agriculture in southern Cameroon

This paper investigates statistical relationships between land use/land cover (LULC), Landsat-7 ETM⁺ imagery and landscape mosaic structure in southern Cameroon where the conversion of tropical rain forest to shifting cultivation leads to dynamic processes, acting on the spatial aggregation of various LULC types. A Global Positioning System (GPS) was used in the field to identify a total of 171 shifting cultivation patches representing eight LULC types in two sub-areas. Because of the lack of a cloud-free image for the date of field sampling, the ETM⁺ imagery was acquired 2 months after field survey, during which it was assumed that no significant changes in LULC occurred (all dry season). Per pixel correlations were developed between spectral reflectance data, vegetation indices and LULC. As an exploratory study, several statistical methods (analysis of variance, means separations (Tukey HSD), principal component analysis (PCA), geo-statistical analysis, image classification and landscape metrics) were applied on point data and sensor images for evaluating the spatial variability within the landscape. Most variables explained 30–72% of LULC variation in the whole dataset. Those variables with high information content of LULC (infrared bands 4, 5, 7 and derived indices and PC1) also showed long ranges (6 km) spatial dependence as compared to those varying only within 1 km range. The results of these statistical analyses suggested the need to group some LULC types and the application of the Maximum Likelihood Classifier (MLC) for supervised classification provided a LULC map with the highest accuracy (81%) after consolidation of perennial LULC types, such as bush fallow, forest fallow and cocoa plantations. Landscape metrics computed from this map showed a high level of patch diversity and connectivity within the landscape and provided input data that can further be used to simulate predictive maps as substitute to cloud-covered sensor imageries. Landsat-7 ETM⁺ imagery proved to be useful in discriminating (with about 80% accuracy) the most dynamic LULC types such cropped plots and young fallow patches (shifting every season) and the extension front of the agricultural landscape.     

山地叶面积指数反演理论、方法与研究进展

叶面积指数LAI（Leaf Area Index）是表征叶片疏密程度和冠层结构特征的重要植被参数,在气候变化、作物生长模型以及碳、水循环研究中发挥着重要作用。遥感是获取区域及全球尺度LAI的一个重要手段,当前LAI产品主要基于遥感数据反演得到,但是多数LAI产品算法并未考虑地形特征的影响,导致山地LAI遥感反演精度不确定性大。提高山地LAI遥感反演精度亟需考虑地形因子对冠层反射率的影响,其中山地冠层反射率模型和遥感数据地形校正是提升山地LAI遥感反演精度的关键。本文围绕山地LAI遥感反演理论与方法,综合分析了国内外山地冠层反射率模型和地形校正模型的研究进展,总结了目前山地LAI遥感反演存在的问题,并讨论了未来研究的发展趋势。     

基于高光谱遥感数据提取森林结构参数的研究

以位于三峡库区的龙门河森林自然保护区为研究区,综合利用线性光谱混合模型和几何光学模型,基于高光谱遥感数据提取森林结构参数是本文研究的重点。在研究区地面调查数据的基础上,通过高光谱数据和混合光谱分解法,获得反演几何光学模型所需的四分量参数,根据背景光照分量与森林植被冠层各参数间的关系,反演得到森林冠层郁闭度及平均冠幅的定量分布图,并利用37个野外实测样本进行结果验证。