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出版日期: 2016-09-25
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DOI: 10.11834/jrs.20166250
2016 | Volumn20 | Number 5





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论文
地表覆盖遥感产品更新完善的研究动向
expand article info 陈军1 , 张俊1 , 张委伟1,2 , 彭舒1
1. 国家基础地理信息中心, 北京 100830
2. 苏州科技大学 环境科学与工程学院, 江苏 苏州 215009

摘要

近年来,多尺度地表覆盖遥感产品的不断涌现,为环境变化研究、地球系统模拟、地理国(世)情监测和可持续发展规划等提供了重要科学数据。为更好地满足广大用户日益增长的应用需求,应对地表覆盖遥感产品进行持续更新完善,保持其时效性、增强时序性、丰富多样性。针对大面积地表覆盖遥感产品更新完善所面临的主要问题,介绍和评述了国内外有关研究动向,包括影像与众源信息相结合的更新、数据类型细化与完善、地表覆盖真实性验证,并作了简要展望。

关键词

地表覆盖 , 遥感数据产品 , 更新完善 , 变化检测 , 众源信息

Continous updating and refinement of land cover data product
expand article info CHEN Jun1 , ZHANG Jun1 , ZHANG Weiwei1,2 , PENG Shu1
1.National Geomatics Center of China, Beijing 100830, China
2.Suzhou University of Science and Technology, Suzhou 215006, China

Abstract

In the past years, the land cover community has strived to develop and supply more datasets at different spatial scales (e.g., regional, national, or global) with increasing spatial-temporal resolutions and improved classification accuracy. Although these data products have been widely applied in climate change studies, environmental monitoring, sustainable development, and many other societal benefit areas, the user communities constantly propose new demands, such as additional land cover classes, up-to-date time series, and consistency among different datasets. Therefore, the continuous updating and content refinement of land cover data products have become key objectives of the land cover community. The updating and refinement of land cover data products differ from their original creation. Change detection with remotely sensed imagery is a major approach for updating a large area land cover, and the rapidly increasing crowdsourcing information provides another valuable resource. However, as a technical challenge is that no existing change detection algorithm can be applied to all kinds of imageries and geographic regions because of the extremely complex spectral heterogeneity of land cover classes. Therefore, an efficient change detection approach with the consideration of the existing land cover datasets needs to be developed. One cuttingedge issue is to integrate the imagery-based change intensity measurement with prior knowledge represented by existing land cover datasets. Change detection for time series imagery is moving from the comparison of two neighboring points to global trend analysis. The coupling of SAR and infrared images with multispectral images needs to be explored from several aspects, such as relative radiometric correction, spectral matching, and temporal-spatial data fusion. Another key challenge comes from the rational utilization of crowdsourcing information for updating and refining land cover. Crowdsourcing information may differ in terms of data contents, position accuracy, spatial-temporal resolution, and uncertainty, and hence, previous studies have aimed to develop appropriate methods and techniques for evaluating volunteered data quality, discovering useful information from deep web, extracting incremental changes, and integrating multi-source datasets. The increasing amount of freely accessible remote sensing data has increased the data intensiveness of the generating future land cover data products. Specific tools and systems must be designed and developed to support the updating and refining of large area land cover. One of these tools is an online land cover updating system that allows users to execute web-based land cover change detection and processing in an open web environment. The key issues in using this tool include domain-knowledge-based change detection service modeling and dynamic service composition. Data Cube is another system that has a flexible classification concept, but this tool is still under investigation. Nevertheless, this tool is expected to facilitate the on-demand extraction of land cover classes with deep learning and other data mining algorithms.

Key words

land cover , remote sensed data product , updating , change detection , crowdsourcing

1 引言

近几年,对地观测、信息处理等技术飞速发展,地表覆盖遥感信息提取能力逐步增强,多尺度地表覆盖数据产品不断涌现(Ban等,2015Grekousis等,2015)。美国、中国、加拿大、巴西、印度、澳大利亚等国研制了覆盖全国的多分辨率地表覆盖数据产品(Deng和Liu,2012Jin等,2013Lymburner等,2013De Campos Macedo等,2013NRSC,2014Olthof等,2015)。大洲尺度的地表覆盖数据产品有欧盟环境署(EEA)研制的100—250 m分辨率Corine数据集(Büttner等,2004),北美250 m分辨率NALCD数据集(Latifovic等,2012)、南美及加勒比海500 m分辨率(Blanco等,2013)以及南美2010年30 m分辨率数据集(Giri和Long,2014)。

在全球尺度上,全球地表覆盖数据产品的空间分辨率已从原来的300—1000 m提高到了30 m (Giri等,2013Gong等,2013),最具代表性的数据成果是美国马里兰大学研制的2000年—2012年全球30 m森林覆盖及增减数据集(Hansen等,2013)和中国研制的GlobeLand30(Ban等,2015)。其中GlobeLand30覆盖全球陆域(www.globeland30.org),含水体、人造覆盖、耕地、森林等10个一级类,有2000年、2010年两期数据(陈军等,2014Chen等,2014a2015),被联合国有关机构评价为世界上首套高分辨率全要素全球地表覆盖数据产品(O’Connor等,2015)和全球性的重要基础数据(UN-GGIM,2015)。2014年9月23日,中国政府领导人将其作为献给世界气候峰会的礼物,捐赠给联合国使用,并向国际社会开放共享,被称为中国“对国际社会的又一重要贡献”(冉有华和李新,2015)。截止到2016年4月底,已有来自118个国家和近百个国际及区域组织的6000多名用户使用此数据,广泛应用于气候变化、防灾减灾、生态环境监测、城镇化研究、农业等诸多领域,应用效益正逐步显现。

虽然地表覆盖数据产品的空间分辨率、分类精度大幅提升,种类不断丰富,但依然难以完全满足广大用户日益增长的应用需求。究其原因,一是地表覆盖作为地球表面物质类型和自然属性的综合体,受到自然环境和人类活动的双重影响,表现出很强的动态性。这对其数据产品的时效性与时序性提出了很高要求(Grekousis等,2015Broxton等,2014)。一些用户既需要现势性强的地表覆盖数据产品,又希望获得长时间系列的数据产品。例如,新近开始实施的联合国2030可持续发展议程要进行约230个关键指标的监测与评估,迫切需要2015年及之后、之前(如1970年代、80年代和90年代)的地表覆盖数据产品(Hák等,2016)。二是不同用户的需求往往千差万别,任何一种单一的地表覆盖数据产品都难以满足所有用户的需求(Mora等,2014)。因此,深入研究地表覆盖数据产品更新与完善的基本问题与关键技术,保持其现势性,增强时序性,细化信息类型,丰富产品种类,已成为国内外遥感与地理信息界的一项重要任务。

当前大区域地表覆盖遥感数据产品的持续更新完善已提上日程,主要热点包括影像与众源相结合的更新、数据类型细化与完善、地表覆盖真实性验证等3个方面。

2 影像与众源信息相结合的更新

从技术的角度看,地理空间数据更新不同于初始建库,其特点是发现与测定变化,对数据库(或版本)进行修订,并保持其时态现势性和维护逻辑一致性(陈军等,2010)。地表覆盖做为一种重要的地理空间数据,其更新亦是从原有数据产品出发,综合利用遥感影像和众源信息等现势信息源,发现和测定地表覆盖的变化区域与属性,更新生成新一期数据产品,并满足现势性和连续性要求(张委伟等,2016)。

2.1 基于影像的变化检测

多年来,学者们研究提出了许多基于遥感影像的变化检测模型或算法(Chen等,2011Tewkesbury等,2015Gómez等,2016),但每种算法均存在着一定的局限性,没有一种算法能够完全满足大范围地表覆盖变化检测提出的高可信要求(Lu等,2004Hansen和Loveland,2012)。首先是难以有效解决随机干扰和季相差异带来的伪变化问题。尤其是在大范围变化检测时,地物光谱、图像景观极为复杂,算法的通用性较低。其次是变化检测单元设定较为困难。虽然基于对象的变化检测优于像元变化检测,但如何有效地选择和确定多幅影像的分割尺度与方式一直存在争议,独立分割或叠置分割均存在不足之处(Chen等,2012aHussain等,2013)。当前,人们仍在研究新的变化检测算法,如将变化检测从光谱量值空间转化到斜率空间,利用光谱斜率差异进行变化检测(Chen等,2013);利用机器学习进行变化类型训练,用于自动选择合适变化检测的影像特征(Li等,2015);将MODIS时序数据与Landsat相融合,改正或修正季相差异对变化检测的影响(Verbesselt等,2010Rao等,2015);或利用时间序列影像提供的趋势性信息,以减小时相差异带来的伪变化,提高变化检测的可靠性。其中基于时间序列影像的变化检测是将基于两点观测的变化判断,上升为全局趋势性渐变和随机突变的判断(Zhu和Woodcock,2014)。目前这方面的研究多集中在面向植被/森林类型的连续性变化检测,如植被变化跟踪(Huang等,2010)、森林扰动连续检测(Zhu等,2012)等。也有研究者尝试将其用于评估城市扩张面积(Sexton等,2013)、湿地变化(Chen等,2014b)和农业用地损失(Pandey和Seto,2015)。

鉴于更新是以原有地表覆盖数据为基础进行的,能否充分利用地表覆盖的先验知识,对于提高变化检测效率和更新处理水平至关重要。因此,需要归纳凝练地表覆盖的先验知识,如同期数据的空间一致性和更新前后的时间连续性,发展知识驱动的变化检测算法、顾及时空关系的数据一致性检查方法等。有专家曾以原有地表覆盖数据作为训练样本来源,通过变化向量分析(CVA)方法处理分类后验概率,驱动变化检测过程,提出了后验概率空间变化向量分析(CVAPS) (Chen等,2012b)。美国USGS是以前一期地表覆盖作为本底,通过CVA或综合变化检测发现变化区域,利用决策树分类确定类别信息,不变区域的类别信息与前一期保持一致,从而提高更新处理的效率(Xian等,2009Jin等,2013)。

2.2 基于众源信息的变化发现

OpenStreetMap、Geo-Wiki等众源地理数据的大量出现(Goodchild,2007单杰等,2014),为地表覆盖变化发现与更新提供了新的参考信息来源(Fritz等,2009Foody和Boyd,2013)。但这类参考信息的数据内容、表达形式、时空分辨率、数据精度及地域分布复杂多样,需加强对多源异构众源信息的质量评价与整合研究,以从中筛选出可信度高、地域分布互补的有用数据,整合成表达尺度合适的增量信息,或直接从中提取增量信息(Salk等,2016赵肄江和周晓光,2015Zhou等,2015Bordogna等,2016)。

与此同时,还应该吸收众包技术的最新研究进展,设计志愿者参与的众包模式,研发适用的众源信息采集手段,以方便广大志愿者和专业人员通过互联网、IPAD、手机等,在线提供地表覆盖变化信息(冯剑红等,2015McCartney等,2015)。近期的相关研究包括面向在线标报的空间型微博(Estima和Painho,2013Xing等,2015)、面向数据验证的空间社交媒体信息(Ferster和Coops,2016)等。其中,带有位置信息和时间戳的空间社交媒体数据(如相片和CHECKIN数据)为地表覆盖数据验证提供了一种新的补充手段。另一项相关研究是深层网络的信息发现技术研究,如利用名字、规则及关键词匹配,搜索和发现隐藏于深层脚本的地表覆盖Web地图服务,以获得可资利用的地表覆盖参考资料(Hou等,2016)。

2.3 在线变化检测与更新工具

大范围地表覆盖更新面临的一个实际问题是,如何因地制宜地选择最佳变化检测算法,灵活地设定检测流程。以往人们主要是借助于桌面图像处理系统,通过选择、试验与比对,选定变化检测算法和确定处理流程。这依赖于对所选用遥感影像资料特性、变化检测算法适用性等认识,以及在变化检测、后续处理等方面的实际经验,往往耗时费力,难以适应大范围地表覆盖变化检测与更新的工程化要求,尤其是难以支持跨区域的协同更新。当前的一个重要发展方向是,借鉴服务计算(service-oriented computing)和模型服务(model web)的思路(Foster,2005Nativi等,2013),将地表覆盖变化检测与更新处理的有关计算模型、处理算法转换为Web服务,提供基于知识的在线变化检测与更新工具(陈军等,2013)。为此,需梳理变化检测算法与影像之间的多级关系,归纳凝练出相应的领域知识,发布为基于Web Service的服务模型,形成基于领域知识的在线变化检测与更新系统,以方便用户优选变化检测算法、构建处理流程,提高基于多源影像的大范围变化检测与更新的效率。

3 基于整合与灵活分类的完善

根据不同用户的多样化需求,丰富地表覆盖数据产品的内容,是当前又一重要发展趋势。第1种做法是对原有数据产品进行类型细化,如国家基础地理信息中心等单位正对GlobeLand30的有关一级类别进行细化处理。如采用顾及形状和邻近关系的细化方法,将其水体细分为河流和湖泊;基于已有参考资料,将湿地细分为森林湿地、草本湿地、非植被湿地;通过对象化处理,将人造地表细分为城市和乡村建设用地等。第2种做法是,综合多种已有产品的优势,融合生成制图精度更高、空间一致性更好的新产品(Pérez-Hoyos等,2012Latham等,2014Yu等,2014),其特点是节省资源、效率较高。第3种做法是面对不同的用户群体,直接从影像中提取其所需的地表覆盖信息,这对信息自动化提取与智能化处理提出了新的要求。

3.1 多源数据产品整合

多源产品整合的方法大体上可分为基于数据一致性和基于回归分析的两类。前者是通过分析不同产品间的一致性,选择一致性和可靠性较好的结果进行产品整合。例如,Fritz等人(2015)基于全球、区域和国家尺度的多种地表覆盖产品以及多尺度耕地面积统计数据,建立一致性评分的融合方法,生成1 km的ⅡASA-IFPRI全球耕地制图产品。Waldner等人(20152016)提出了基于多指标分析的一致性融合方法,将地表覆盖数据的精度、置信水平和空间分辨率等作为指标进行打分,利用得分来决定数据的权重,进而对一致性区域中得分高的数据集进行融合。

后者是通过建立训练样本和数据集之间的回归关系,预测无样本区域地表覆盖类型出现的概率,从而得到整合结果。See等人(2015b)基于MODIS、GLC2000和GlobCover 3种数据,利用地理加权回归模型建立训练样本和数据集之间的回归关系,预测无样本区的地表覆盖类型,分别得到空间分辨率为300 m的两种全球地表覆盖数据Hybrid map 1和Hybrid map 2。Schepaschenko等人(2015)利用地理加权回归模型,基于多尺度地表覆盖数据和森林数据构建了全球森林覆盖分布图,该方法对数据样本的质量和数据要求较高。

3.2 灵活分类技术

2016年5月,国际对地观测组织(GEO)与联合国全球地理信息专家委员会(UN-GGIM)在荷兰鹿特丹联合召开专题会议,讨论能否研发出可业务化运行的技术系统,按照用户提出的类型、范围等要求,从影像中快速地提取用户所需的地表覆盖信息,并满足所需的精度。这取决于能否研发出所谓的灵活分类技术,具有很大的技术难度,亟待算法和技术方面的重要突破(Gong等,2016)。

就灵活分类而言,Loveland等人(2000)曾提出按“灵活地表覆盖数据库”概念,采用自下而上的等级分类系统,构建地表覆盖数据库的基本成分(光谱、物候、纹理等较为一致的集合体),由用户根据其特征,按所需的地表覆盖系统进行调整和归并。Gong等人(2016)提出了构建基于多层次信息提取和多层信息归并的地表覆盖分类系统,利用光谱、纹理、物候、植被盖度、植被高度、人类活动强度等特征,分别进行密度分割或决策树分类,建立基础性类型,根据用户需求进行归并,生成满足不同需要的地表覆盖最终产品。为此,需要融合LiDAR、Night Light等多源遥感数据和时间序列影像,发展基于大数据的深度学习、特征挖掘算法和单类分类器方法,以及特征级和分类结果级别上的融合方法。

4 地表覆盖真实性检验

真实性验证(也称数据验证)是地表覆盖遥感数据产品研发的一个重要环节,不仅可以帮助了解数据产品的不确定性及其适用范围,还有助于分析数据产品的误差类型、来源及空间分布(吴小丹,2015)。其是根据统计学原理,在地理空间上布设若干有代表性的样本点,采集能反映地面相对真值的参考数据,利用混淆矩阵(confusion matrix)等,去计算总体精度和Kappa系数等指标,进行地表覆盖数据的精度评估(Cohen,1960Liu等,2007Olofsson等,2014Warrens,2015)。自GlobeLand30面世以来,很多同行专家对其进行了初步的精度评估,如中国专家从全球抽取了154000个样本点,得出GlobeLand30的总体分类精度为83.5%(童小华等,2016),而希腊(Manakos等,2014)、意大利(Brovelli等,2015)、德国(Arsanjani等,2016a)、伊朗(Arsanjani等,2016b)等地分类精度均在80%左右。但这些验证均是相互独立进行的,尚需按照统一的技术标准和验证方法,进行系统性验证与评估。2015年底中国与GEO秘书处合作,联合有关国家和组织,在国际上发起了全球30 m地表覆盖数据产品验证。其目的是研究提出符合30 m空间分辨率特点的验证方法,制定国际认可的验证技术规范,研发适用的在线验证工具,完成全球30 m地表覆盖数据的真实性检验。

4.1 验证方法研究

就验证方法而言,国际上曾完成了6套300 m或1000 m分辨率的精度验证以及多类型产品的精度比较,但所形成的样点总量估算、样点分配、空间布设等方面尚存在较大局限性(Herold等,2008Mora等,2014Wagner和Stehman,2015)。以往人们主要是依据专家经验设定样本总量(Congalton等,2014),按照各地类的面积进行样本量的类间分配(Stehman,2012),采用分层随机抽样或二级整群抽样等方法实现样本的空间布设(Scepan,1999Friedl等,2002Zhao等,2014)。对于大区域30 m分辨率地表覆盖数据来说,其往往表现出很强的空间异质性,而原有验证方法偏重匀质区域、忽略异质区域,异质性较强的区域难以获得较多样本量,稀少类型的样本量更是显著不足,容易导致分类精度高估(Hammond和Verbyla,1996Sweeney和Evans,2012)。

为解决这一问题,国内外对顾及地表覆盖景观异质性的抽样方法研究方兴未艾。Mayaux等人(2006)学者利用多样性指标Shannon-Weaver(SW),计算每一抽样单元的地类丰富度与均匀程度,用于确定各抽样格网的样本数量,并累加得出各验证区域的样本量,以确保异质性强的验证区域具有更多的样本。陈斐等人(2016)利用景观形状指数LSI(Landscape Shape Index),对地表覆盖空间异质性程度进行多级度量,提出了区域样本量计算、类间样本量分配、样本空间布设等的定量化方法,为实现顾及地表覆盖空间异质性的自适应样本抽样提供了新思路。有学者将不同地表覆盖类型间的边界过渡区域作为特定评估对象,发展了顾及异质性的分类精度估计方法(竞霞等,2014刘梦等,2016)。还有学者利用地统计学中的克里格方法,对不同异质性的区域分别进行分类精度评估(Park等,2016)。学者们也对地表覆盖分类结果进行不确定性预估,根据不确定性程度的高低,确定样点总量(金勇进等,2012Stehman,2012)。此外,还有学者根据地表覆盖要素的空间自相关性,进行分层样本布设,以降低样本的相关性和避免稀有样本的缺失,使样本具有较高的地类与空间代表性(童小华等,2016)。

4.2 验证工具研制

以往地表覆盖数据验证主要是离线、单机操作,难以支持分布式协同验证。近年来基于互联网的空间信息资源共享(如谷歌、天地图等提供的免费遥感影像资源)和在线制图工具(如Google Map Maker,OpenStreetMap)迅速发展,为研发在线验证工具提供了有利条件(Fritz等,2012Yu和Gong,2012)。美国的索诺马州立大学和波多黎各大学研制了一款基于Web的地表覆盖样本采集系统VIEW-IT,支持人机交互协同解译高分辨率影像图(Clark和Aide,2011)。奥地利国际系统研究所(ⅡASA)研发了面向地表覆盖的在线众包工具Geo-Wiki,支持自愿者开展在线标报(Fritz等,2012),但尚未提供样本量计算、样本自动布设、精度计算等功能。最近,ⅡASA和GeoVille GmbH正在联合研发地表覆盖在线验证平台LACO-WIKI (http://laco-wiki.net/),其主要支持数据上传、样本生成、产品验证和精度分析等4步简单验证流程(See等,2015a)。中国专家也在国家国际合作专项支持下,研发了地表覆盖在线验证系统(http://glcval.geo-compass.com/)。该系统是根据数据验证的主要技术流程,支持样本总量估算与类间分配、样本空间布设、专家交互检核和精度评价等在线处理,并实现了与多种在线参考资料的连通,方便专家和志愿者查找和调用参考信息、上传样本信息和在线标注错误信息等。此外,研发了面向深层网络的地理标记文本搜索方法,为从泛在网络上主动采集验证样本提供了新手段(Hou等,2015)。

4.3 样本资源库建设

样本数据集是地表覆盖数据验证的一项重要产出。以往有关的地表覆盖制图或验证项目是从各自需要出发,设计和构建样本数据集。例如,森林变化分析往往要求多时期样本,而耕地研究则需要更多专题细节(Mora等,2014),这使得所采用的标准往往不一致,所形成的样本资源库也难于共享使用(Tsendbazar等,2015)。按照标准统一、共享使用的原则,构建全球地表覆盖的验证样本资源库,是国际遥感与相关科技界关注的研究热点(Olofsson等,2012Stehman等,2012Zhao等,2014Tsendbazar等,2015)。目前Boston大学和GOFC-GOLD正致力于联合研制验证数据集(Mora等,2014),而正在实施的全球30 m地表覆盖数据产品验证也致力于形成一套全球样本数据集。

5 结语

大范围地表覆盖遥感产品研发是一个影响因素众多、技术过程复杂的科学数据工程(徐冠华等,2013),其更新完善则面临着一系列新的挑战(Hansen和Loveland,2012陈军等,2016)。今后应积极面对大范围地表覆盖更新完善这一国际趋势,充分整合多种信息资源,不断提高更新完善的自动化水平,努力打造地表覆盖大数据集,更好地服务中国全球战略和联合国2030可持续发展议程。

(1)充分整合多源信息。当前,可用于地表覆盖的遥感卫星越来越多,空间分辨率大幅提高,在轨运行周期从3.3年提升到8.6年(全色1—5 m,雷达1 m)(Belward和Skøien,2015)。其中微波遥感对于多云地区以及捕捉地表季节性变化具有重要的意义,热红外波段信息以往很少用于地表覆盖分类制图,但对于特定的地物类型识别(如冰雪、火山等)有特殊作用(Ban等,2015),应切实解决好其相对辐射校正、光谱波段匹配、时空分辨率融合等问题,为地表覆盖遥感提供丰富的影像数据源。同时,要研究解决众源信息的质量评价、数据处理等技术难题,为地表覆盖变化发现与更新提供可靠的参考信息源。

(2)提高自动化水平。更新完善是以已有地表覆盖数据产品为基础,发现变化或异同,进行检测或整合处理。这不同于传统的遥感数据分类提取,面临着地表覆盖知识驱动的变化检测、顾及地表覆盖时空关系的数据质量检核等难题。因此,应大力研究发展顾及地表覆盖先验知识的灵活分类和在线更新技术,提高训练样本选择、变化检测流程构建、变化区域类别判定、产品间一致性和可靠性评价等的自动化水平,从根本上提高更新完善的效率。

(3)打造地表覆盖大数据集。为了做好地表覆盖数据资源建设、更新完善与服务应用,应在GEO和UN-GGIM等国际政府间组织的支持下,加大国际合作力度,积极推动设立相关国际科学计划,统一标准,共同设计、共同发展、共同操作,发挥不同国家、不同组织的优势,建立合作长效机制,最大限度地整合各种可用资源,构建全球地表覆盖信息集成服务平台CoGland(ISPRS,2015),不断产出多尺度、多分辨率、多类型地表覆盖数据产品,推动地表覆盖大数据集的逐步形成与广泛应用。

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