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VOLUME1:AREASANDDENSITIES21
CHAPTER3
UnderstandingandMeasuring
UrbanExpansion
THECLASSIFICATIONOFSATELLITEIMAGERY
ThemapsoftheurbanextentofcitiesintheglobalsamplewerecreatedusingLandsatimagerythat
hasbeenavailablesincetheearly1970swithimprovedqualityovertime.FortheAtlas,weusedcloud-
freeimagesfromLandsat5(1984),Landsat6(1993),Landsat7(1999)andLandsat8(2013)satellites.
Theimagesareavailableevery16daysinscenesof185-by-185kilometerseachwithatypicalpixelsize
of30-by-30meters.Theseimageshaveseveralspectralbandsthatcanbeusedtoidentifyimpervious
surfacesroughlycorrespondingtobuilt-upareas,aswellaswatersurfaces.Thismakesitpossibleto
classifythembyhuman-assistedalgorithmsintothreeclasseswithahighdegreeofaccuracy:built-up,
openspace,andwater.PotereandhiscolleaguestestedanearlierclassificationofLandsatimageryofa
subsetofcitiesintheglobalsamplebyourresearchteambycomparingittoGoogleEarthimageryin
thousandsofrandomlyselectedlocations.Theyconcludedthat
[t]heuser’saccuracyfortheurbanclasswasquitehigh,indicatingthataportionofthe
Landsat-basedsitethatislabeled“urban”willalsoappearasurbanizedinthehigh-resolution
22ATLASOFURBANEXPANSION-THE2016EDITION
imagery91%ofthetime.Theproducer’saccuracyforurbanareasisslightlylower,indicating
thaturbanizedareaswithinoursamplewerecorrectlyidentified89.3%ofthetime.Forthis
assessment,boththeuser’sandproducer’saccuracieswereimportantbecausewewantedtobe
certainthatthe…mapcollectionwasneithermissingurbanland(urbanomissionerrors)nor
mislabelingnonurbanareasasurbanland(urbancommissionerro
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