GIS TUTORIAL 4: Introduction to Google Earth Engine
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1 GIS TUTORIAL 4: Introduction to Google Earth Engine In this tutorial you will receive a brief introduction to Google Earth Engine and learn how to edit and run simple scripts to produce Sentinel-2 and Landsat-5 satellite imagery. 1 Getting Started 1.1 Google Earth Engine introduction Google Earth Engine (GEE) should run from any modern browser – you do not need to install extra software. Chrome is a good choice as it is also created by Google. To use GEE, you need to sign in with your Google account here: https://code.earthengine.google.com/. If you have not already signed up you will need to do so here: https://earthengine.google.com/signup/. • It will take a few days for Google to approve your access to GEE, so please do this promptly if you have not already done so. Once you have signed in you will see the following screen when you visit the main website.
Transcript of GIS TUTORIAL 4: Introduction to Google Earth Engine
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GISTUTORIAL4:IntroductiontoGoogleEarthEngineInthistutorialyouwillreceiveabriefintroductiontoGoogleEarthEngineandlearnhowtoeditandrunsimplescriptstoproduceSentinel-2andLandsat-5satelliteimagery.
1 GettingStarted
1.1 GoogleEarthEngineintroduction
GoogleEarthEngine(GEE)shouldrunfromanymodernbrowser–youdonotneedtoinstallextrasoftware.ChromeisagoodchoiceasitisalsocreatedbyGoogle.
TouseGEE,youneedtosigninwithyourGoogleaccounthere:
https://code.earthengine.google.com/.
Ifyouhavenotalreadysignedupyouwillneedtodosohere:https://earthengine.google.com/signup/.
• ItwilltakeafewdaysforGoogletoapproveyouraccesstoGEE,sopleasedothispromptlyifyouhavenotalreadydoneso.Onceyouhavesignedinyouwillseethefollowingscreenwhenyouvisitthemainwebsite.
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1.2 GoogleEarthEngineinterface
Unlike QGIS which is controlled through a graphical user interface, GEE relies on scripts –programming code that gives the platform its instructions. The interface is made up of thefollowingwindowsandcontrolswhichwillbereferredtorepeatedlythroughoutthesetutorials.
TheConsolewindowhastwoothertabs,theInspectorandTaskstabswhichneedtobeclickedontobeaccessed.ThesameistruefortheScripts/Docs/Assetswindow,butwewon’tbeusingtheseasmuch.
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2 EditingandRunningASimpleSentinel-2Script(videotutorialplaylist)
WearegoingtobeusingasimplescripttoviewanddownloadsomeSentinel-2imagery.• Sentinel-2isasatelliteownedandoperatedbytheEuropeanSpaceAgencythathasbeen
collectingdatasinceJune2015.• Unlikealotofsatelliteimagery,Sentinel-2isfreetodownloadanduse.• Ithasaten-metreresolutionandthirteendifferentbands.ItisverysimilartoLandsat
whichwehaveusedbefore,buttheESAversioninsteadofNASA.Youcanfindoutaboutthesatellitehere.
Thescripthasalreadybeenwrittenforus.Whatwearegoingtodointhenextstepsisaccessit,editittoourownspecifications,andthenrunit.
• Click here to open the simplest version of the script in your web browserhttps://code.earthengine.google.com/e02fefa580c9e7d1a0af6e96f5baccf5
First,havealookatthetextintheCodeEditor.
Itmaylookcomplicated,butbasicallyeachlineisaninstructiontotellthescripttodosomething:
• Thegreentextisnotestoexplainwhateachpartofthescriptdoes:o Line3:importstheSentinel-2collectiono Line6:definestheSentinel-2bandsweareinterestedino Line 9: creates a single image by taking themiddle value of all the available
Sentinel-2imageryforeachpixelo Line12:tellsGEEhowtodisplayourimageo Line15:tellsGEEtodisplayito Line18:automaticallymovestheMapViewtothecentreoftheredrectangle.
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Aswework throughthe tutorial,wewillencounter longerversionsof thescriptswhichhavemoreinstructionstorefineourresultsfurther.
2.1 Geometrytoolsanddefiningyourareaofinterest(videotutorial)
Whenwefirstopeneachversionofthescript,themapwillautomaticallyzoomtoanareaofOmanandsomeSentinel-2imageryforthatareawillstarttoload.
• So,thefirstthingwewilldoischangethistoourownareaofinterest.• Click“Geometryimports”intheGeometryToolsandturnonthe“geometry”polygon.
• ClickonthepolygonandclickDelete.
• NavigateinthemaptoyourownareaofinterestandusetheRectangletooltodefineanewarea.
• OnceyouhavedrawnyourrectanglepresstheStopDrawing(Hand)button.
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• NowclickRunintheCodeEditor.
• TurnofftheredrectangleintheGeometryImportsmenuandwaitfortheimagerytoload
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• YoucannowzoominandexploretheSentinel-2imageryofyourarea.
2.2 EditingtheSentinel-2bandsintheCodeEditor(videotutorial)
Bydefault,thescriptisprogrammedtocreatea“truecolourcomposite”–animagewithcoloursthatwewouldrecognisewiththenakedeye.
Wearenowgoing tochange theSentinel-2bands tomakedifferent colourcompositeswhichhighlightcertainfeaturesinthesatelliteimagery,suchasgeology,urbanareasorvegetation.Thisisveryusefulwhenweareassessingchangetolandscapeovertimeandexaminingdisturbancesandthreatstoarchaeologicalsites.
• Clickheretoopenthenextversionofthescripthttps://code.earthengine.google.com/42eee8ffa6ee2e80e400cdd2c9b58028
o ThiswillreturnustoourdefaultexampleinOman.• Findthe“BANDS”sectionofthescriptintheCodeEditor.• Changethered“B”numbersin“S2_bands”tomatchoneofthebandcombinationslisted
inthenotesandclickRun.
• Tryadifferentbandcombinationtocompare.• TurnonanddeletetheredrectangleinGeometryTools.• RepeatthestepsinSection3.1andredrawarectangleinyourownareaofinterestand
tryrunningthescriptforsomedifferentbandcombinations.
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2.3 FilteringtheSentinel-2collectionbydate(videotutorial)
Inourearlierscripts,wehavecreatedamosaicusingtheentireSentinel-2collection,whichcontainshundredsofimagesfrom2015onwards.GEEallowsustodefineadaterangethatweareinterestedinandfiltertheimagerytoonlythosedates.Thisisincrediblyusefulifweareanalysingchangetolandscapesorarchaeologicalsitesovertimeandwanttocomparedifferenttimeperiods.
• Clickheretoopenthenextversionofthescripthttps://code.earthengine.google.com/d750f44cc671e657b2812c42b59c55e1
• Findthe“DATE”sectionofthescriptintheCodeEditor
Thetwodatesinredtextdefinethestartandendofthefilterrange.
• Thesedatescanbechanged,butitisimportanttonotethatSentinel-2imagerywasonlyproducedfromJune2015,sothisistheearliestdatethatcanbeselected.
• Changetheyearto2016onbothdatesandclickRun.• Expand“ImageCollection”intheConsoleWindow.
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Ifyouexpandthe‘features’section,itwilltellyouthedetailsofeachimageinthecollectionbasedonthedatesthatyouhavegiven.
• RepeatthestepsinSection3.1todefineyourownareaofinterestandtrychangingthedateranges.
• Seeifyoucanspotanychangesinyourareaovertime.
2.4 ExportingSentinel-2imagerytoQGIS(videotutorial)
HavingnowusedallthedifferentpartsofthissimpleSentinel-2script,wearenowgoingtolearnhowtoexporttheimageryintoQGIS.
• Clickheretoopenthefinalversionofthescripthttps://code.earthengine.google.com/d1e21be1c93d44d27376027173b14075
• SetyourareaofinterestfollowingthestepsinSection3.1.• ChooseabandcombinationthatyouarehappywithasoutlinedinSection3.2.• SetthedatefiltertoausefulrangeasdescribedinSection3.3.• Runthescriptandmakesureyouarehappywiththefinalimage.• ScrolldowntothelastsectionofthescriptintheCodeEditor.
Thissectioncontrolstheexportsettings:
• image:tellsthescriptwhichimagetoexport(S2_mosaic),• description:specifiesthenameoftheexportjob(Sentinel_2),• scale:specifiestheresolutionoftheimage(10m),• maxPixels:definesthemaximumnumberofpixelsperimage(1e13=
10000000000000),• region:specifiestheareatobeexported(geometryindicatesthatitshouldexportthe
areadefinedbytheredrectangle).
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Unlessyouareexportingaverylargeimageandwantalowerresolution(e.g.100or1000m),youcanleavethesesettingsastheyare.
• IntheConsolewindowclicktheorangeTaskstab.• Clicktheblue‘Run’buttonnextto“Sentinel_2”.• Type“GEE”in“Drivefolder”.• GiveyourSentinel-2imageaname(includingthegeneralareaandbandcombinationis
agoodidea)andclickRun.
• WaituntiltheexportcompletesintheTaskwindowo Whileitisworkingitwillshowacogandhowlongagothetaskstarted,o Whenitiscompleteitwillturnblueandshowatickmarko Thismaytakeafewminutesdependingonthesizeoftheimage
• IfitistakingalongtimeclicktheRefreshbutton,orthe[?]thatappearsnexttothesmallcog.
• OpenanewtabandgotoGoogleDrivehttps://drive.google.com/drive/u/0/my-drive• FindandopenyournewGEEfolder.
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• FindyourSentinel-2image,rightclickitandselectDownload.• Whenthedownloadhascompleted,movetheimageintoyourGISfolderandcreatea
newfolderforit(e.g.GEE).
• OpenQGISandaddyourSentinel-2imageusingthe‘OpenDataSourceManager’buttonontheToolbarandchoosing‘Raster’.
o NavigatetoyoursavedSentinel-2imagefileandclickAdd.
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• ReturntotheGEEwindowtosaveyourcopyofthescriptsoyoucanrevisititlater.• ClicktheblackarownexttotheSavebuttonandselect“SaveAs”.• GiveyourscriptanameandclickSave.
YoucannowaccessthisscriptinyourScriptswindow.
PRACTICE:createanddownloadaSentinel-2compositeforyourareaofinterestandincorporateitintoamapinQGIS.
3 WorkingwithSentinel-2andLandsat-5Data(videotutorialplaylist)
Sentinel-2imageryisexcellentforexaminingthemostrecentalterationstothelandscape,buttheimageryonlygoesbacktoJune2015.Ifwewanttocompareimageryoveralongertimeperiod,weneedtoincludeotherimagery.
TheLandsat-5satellitecollectedverysimilardatatoSentinel-2between1984and2012.Thisistheperfectdatasetforexaminingchangestothelandscapeoveralongperiodoftime.OurnextscriptcombinesbothSentinel-2andLandsat-5datatoprovideacomparisonbetweenmodernandhistoricalimagery.
3.1 TheLayersPanelanddefiningyourareaofinterest(videotutorial)
First,wearegoingtoopenthescript,explorethetwodifferentlayersanddefineanewareaofinterest.
• Clickheretoopenthenewscript:https://code.earthengine.google.com/1132354830d01200e6e064cafd4c4f95
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• HoveryourmouseovertheLayersPanel(ontheupperrightsideoftheMapView).
NoticethattwolayersarenowbeingdisplayedintheMapViewWindow–theSentinel-2imageaboveandtheLandsat-5below.
• TurnofftheSentinel-2imagebyclickingonthecheckbesideitandzoomintotheimagerytocomparethetwodatasets.
• DeletetheoldredrectangleanddefineanewareaofinterestwiththeGeometryToolsfollowingtheinstructionsinSection3.1.
o Comparethetwodifferenttypesofimageryforyourarea.
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3.2 FilteringtheSentinel-2andLandsat-5collectionsbydate(videotutorial)
Togetthemostoutofthisscriptwewanttobeabletosetthedatesoftheimagerytofollowourownparameters.
Youshouldnoticethatthisnewscriptisverysimilartothepreviousone,exceptthatitnowcontainstwoparts–onefortheLandsat-5imageryandonefortheSentinel-2imagery.Thebasiccommandsarealmostidenticalforbothdatasets,andtothepreviousscriptthatwelookedat.
Togetthewidestpossiblecomparisonwearegoingtochangethedatesto1984and2020fortheLandsat-5andSentinel-2imageryrespectively.
• Online7ofthescript,intheLandsatsection,changebothyearsto1984.• Online35ofthescript,intheSentinel-2section,changebothyearsto2020.• Runthescript.• Tryenteringdifferentdatesandlookforevidenceofchangebetweenthetwoimages.
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3.3 CreatingSentinel-2andLandsat-5colourcomposites(videotutorial)
Wealsowanttobeabletocreateourowncolourcompositeswithdifferentbandcombinations,butwemustbecarefultomakesurethattheSentinel-2andLandsat-5versionsmatch.
• HavealookattheSentinel-2andLandsat-5bandsintheCodeEditorandthetwosetsofimageryintheLayerPanel.
EventhoughbothimagesareTrueColourComposites,thebandcombinationsareslightlydifferent.ThisisbecausetheinstrumentsonboardtheSentinel-2andLandsat-5satellitesarenotidentical–differentbandsfocusondifferentpartsoftheelectromagneticspectrum.
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Thismeansthatcaremustbetakenwhenprogrammingthebandsintothescript–ifyouusethesamenumbersforeachimage,thenyouwillendupwithverydifferentlookingcolourcomposites.
Thefollowingtableprovidesequivalentbandcombinationsforbothdatasets.Italsoprovidesguidanceonpossibleapplicationsforeachbandcombination.
Composite Sentinel-2 Landsat-5 Applications natural colour 4/3/2 3/2/1 general - recognisable to the naked eye natural-like 12/8/3 7/4/2 general - recognisable but clearer than natural colour colour infrared 8/4/3 4/3/2 agricultural areas (bright red) false colour (agriculture) 11/8/2 5/4/1 agricultural areas (bright green) false colour (urban) 12/11/4 7/5/3 urbanisation false colour (land/water) 8/11/4 4/5/3 coastal and wetland areas false colour (geology 1) 12/11/2 7/5/1 desertification, mining and industry false colour (geology 2) 11/3/2 5/2/1 desertification, mining and industry
• SettheSentinel-2andLandsat-5bandstoproduceacolourinfraredcomposite.• RunthescriptandcomparetheSentinel-2andLandsat-5images.
• Ifyouhaven’talready,changetheareaofinteresttoyourarea• Changethebandcombinationstoacombinationthatisrelevanttoyourareaandrunthe
script.
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3.4 ExportingSentinel-2andLandsat-5imagery(videotutorial)
Once you are happywith the area, date and band combinations used in your Landsat-5 andSentinel-2imagerybothcanbeexported.
• Makesureyouarehappywiththeareaofinterest,dateandbandcombinationsthatyouhavechosenfollowingthestepsintheprevioussectionsandrunyourscript.
• ClickontheTasksWindow.• Click“Run”nextto“Landsat-5”.• Type“GEE”in“DriveFolder”.• GiveitanappropriatenameincludingtheareaandbandcombinationandclickRun.• RepeatthisforyourSentinel-2image–bothcanrunatthesametime.
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OncecompletedyoucandownloadthemfromGoogleDrivetoopeninQGISusingthestepsexplainedearlier.
Thereisalsoashortcutforgettingtotheimageeasily.
• HoveryourmouseoveracompletedimageintheTaskswindowandclickthe[?]button
• Click“OpeninDrive”
• Right-clickyourimagesandclick“Download”todownloadthem.• Whencompleted,movethemfromyourDownloadsfoldertoyourGIS/GEEfolder.• OpenandbrowseyourimagesinQGIS.• Don’tforgettosaveyourscriptifyouwanttorevisititinthefuture(the“Save”arrow>
“SaveAs”).
ACTIVITY:createoneormoremapsinQGISusingfalsecolourcompositesfromGEEtoillustratechangesinthelandscapethathavedisturbedorthatthreatenarchaeologicalsitesinyourareaofinterest.
