Crowd-Sourcing Approach ofBuilding Ground Truth Database forGlobal Urban Area MappingHiroyuki Miyazaki1, Koki Iwao2, and Ryosuke Shibasaki11Center for Spatial Information Science, The University of Tokyo2National Institute of Advanced Industrial Science and Technology
Needs on Global Urban Area Maps• Satellite-based urban area map enables– Monitoring without dependence on administrative district– comparing urban forming internationally– Disaster prevention & hazard assessment of broad areasMonitoring urban expansionAngel et al. (2005)Grid-based population estimationBhaduri et al. (2002)Consistent definition and representationof urban area over countries and regions.Consistent geographical unit acrosscountries and over time.
Sufficient ground truth data?• IGBP Land Cover Validation Confidence Sites(Muchoney et al., 1999)– # of urban sites: 44 / 966• Global Land Cover Ground Truth database (Tateishi,2002)– # of urban sites: 3 / 333 (Asia)• Degree Confluence Project (Iwao et al., 2006)– # of urban sites: 11 / 749 (Eurasia)• Too scarce for mapping urban area globally
Campaigns of visual interpretations?• To complete validating 55412 cities (> 1000 pop.) by visualinterpretations with achievable jobs for a person,– have enough manpower and software licenses?– associate thousands of file names and job assignments?– send thousands of reference images to the operators?– organize hundreds of spreadsheets from the operators?– assures the data not to be leaked from the operators?– frequent joins and leaves are flexibly possible?Conveniences are varying by the person and by the time.
Crowd sourcing• A method to create & collect massive data by anundefined large group of people or community, the“crowd”, over the Internet.OpenStreetMap: tracing roads by GPSand visual interpretation of photosGeo-wiki: validatingdisagreements of global landcover maps using Google EarthDegree Confluence Project:posting ground information atthe integer lat-lon grids.
Web-based Geographical Information Systems• Web Map Service (WMS)– The web service for generating and transferringmap images through WWW.– Standardized by Open Geospatial Consortium**an international organization of standardization of geographical data• Web Feature Service (WFS)– The web service for generating and transferringmap vector data through WWW.– Standardized by Open Geospatial Consortium• Expensive costs for the new software? No!
Power of FOSS4G• FOSS4G: Free and Open Source Software for Geospatial• MapServer– A server software of WMS• TinyOWS– A server software of WFS• PostgreSQL & PostGIS– Relational database management system with geospatialdata extension• Geographical Data Abstraction Library (GDAL)– A library for interoperability of file formats– Users don’t need to mind GeoTiff, Erdas, flat binary andothers.
System design & implementationSatelliteimage archive(Thousands ofGeoTiffs)WebMappingService(MapServer)UserinterfaceReference imagesof requested extentRequest forreference imageswith authenticationWebFeatureService(TinyOWS)Interpreted groundinformation Ground truthdatabase(PostgreSQL &PostGIS)CatalogindexRecord withgeometryWWWDisplayInterpretCrowd of the worldSecurely protectedfrom the interpretersand the InternetOther mapserviceOperator
An interface for trained GIS usersASTER satelliteimage archiveRequest of thereference imageReference imagefor the extentPostinginterpreted datawith geometryGround truthdatabaseQuantum GIS: a free and open-source GIS software, whichenables intuitive geospatial operation like ArcGIS and othercommercial GIS software.Google Maps window Delineate urbanand non-urbanboundary
Web-browser interface for non-expertsASTER GlobalUrban Area MapGroundassessmentdatabaseDEMOCoarse-resolutionurban area maps(urban and built-upclass of MCD12Q1)Requestsfor maps &referenceimagesGoogle Maps Maps to be assessedOverview Zoom-in AssessmentsPosting theassessment
Features of the system• Securely restricted data access: administrator may easilyrestrict accesses to the reference images by ways ofauthentication commonly used within WWW. Frequent leave-and-joins are flexibly durable.• On-the-fly processing: no need for image composition,merging & extracting for a specific extent, and convertingimage format. MapServer will do those for every request ofreference images.• Already organized: no need to manually re-organize thedatabase. TinyOWS and PostgreSQL records the posts withsystematic order.• Extensibility: easily customizable interface for specific cases.• Interoperability: friendly with other web mapping services(e.g. overlaying with Google Maps, Microsoft Bing, andOpenStreetMap)
Conclusion• Proposed a method of building ground truthdatabase with a crowd sourcing approach.• Developed a crowd-sourcing system for visualinterpretations over the Internet.• Implemented the system with WMS & WFS ofFOSS4G (Free & Open Source Software forGeospatial).
Future works• Creating & collecting ground data of urbanarea mapping by the system (under operation)• Scientific evaluation of the developed data(accuracy, precision, and etc.)• Investigating psychological factors of themotivations of the “crowd”– Financial rewards, competitive game design, etc.