measuring the greenness indexswac/docs/mod15/adi_greenness_activity.pdf · vegetation index is a...
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MeasuringtheGreennessIndex
UsingPicturePostandAnalyzingDigitalImagesto
measureseasonalchangesinvegetation
Introduction
Avegetationindexisasinglenumberthatmeasuresvegetationvigororbiomass.Vegetationindicesarecalculatedfromdifferentcombinationsofspectralbandsderivedfromdigitalimages,oftencollectedbysatellites.Theyallowustostudytheseasonalityofvegetationgrowth,measurevegetationhealth,andassesswhetheranareahasgreenvegetationornot.
Satellitesoftenuseinfraredandredbandstoderiveinformationaboutvegetation,butdigitalcamerastypicallycollectonlyred,greenandbluebands.Bycalculatingavegetationindex(inthiscase,the‘greennessindex’)fromthered,green,andbluebandsofadigitalimage,wecantracksimilarseasonaltrendsinvegetationthataremeasuredbysatellites.Highergreennessvaluesrelatetoincreasedpresenceofchlorophyllinthevegetationbeingmonitored.
ForthisactivitywehaveselectedfourimagesfromthePicturePostwebsite(http://picturepost.unh.edu)thatweretakenfrom2008to2009atMenotomyRocksPark,inArlington,MA.Eachpictureisfromadifferentseasonoftheyear.UsingtheAnalyzingDigitalImagestool,youwillbeabletocalculateasimplevegetationindexfromeachpicturetoobservethechangeingreennessovertime.
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InitialObservations
Takealookateachofthefourpicturesprovided.Whatdoyounoticeabouttheimages?Howdoyouthinktheywillchangeingreennessovertime?Whichfeaturesinthepicturewillchangethemost?Theleast?
CalculatingtheGreennessIndexusingAnalyzingDigitalImages
1. OpentheAnalyzingDigitalImagessoftware.
2. Clickonthe buttononthebottom‐leftcornerofthescreen.Inthe‘open’window,choosethepictureyouwouldliketoopen.
3. Awindowwillpopupaskingyouto“selectmethodtocalibratethepixelsize.”
Choose
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4. Yourimageshouldnowappearonthescreen.Under‘SelectSpatialTool’dropdownmenu,choosethe‘RectangleTool.’
5. Thistoolallowsyoutoselectasmallsectionofthepicture.Clickonthepictureand
holdthemousedownwhiledraggingitacrossthescreentoselecttherectangle.Chooseonefeatureinthepicturetofocuson(somethingthatyouthinkwillchangein‘greenness’overtime)andrecordwhatfeatureyouchoseonyourworksheet.Placetherectangleoverthatfeature,youdon'tneedtoencompassthewholefeatureintherectangle;justasmallsectionofitisenough!Onceyouhaveyourrectangle,thescreenshouldlooklikethis:
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6. RecordtheXandYstartandstoppositionsforyourrectangleinthetablebelow:
X YStartPoint
StopPoint
7. Atthebottomleftcornerofthescreenthereisachartofcolorintensities.Recordthered,green,andbluecolorintensityinyourdatatable.
8. Withoutmovingtherectangleorclickingonthepictureagain,gotoFile→OpenPicture.Ifyouaccidentallymovetherectangle,youcanusetheXandYpointsyourecordedabovetorecreateit.
9. Chooseanewpicturefromadifferentseasonandclick‘Open.’
10. Awindowwillpopupaskingifyouwouldliketokeepthesamesettings.YES!Click
YournewimagewillnowappearwiththeSAMErectangleyoualreadycreated.
11. Again,recordthered,green,andbluecolorintensitiesinyourdatatable.
12. Repeatsteps8through11fortheremainingtwopictures.
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MeasuringGreennessIndexActivityWorksheet
1.Whatfeatureoftheimagedidyouchoosetomeasure?Whydidyouchoosethatfeature?
GreennessIndexDataTable
PictureDate
RedIntensity
GreenIntensity
BlueIntensity
GreennessIndex
2.Onceyouhaverecordedthered,green,andblueintensitiesforallpictures,usethespacebelowtocalculatethegreennessindexforeachdate,andrecordthevaluesinyourtable.Greennessindexiscalculatedbyusingtheequationbelow:
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3.MakeagraphoftheGreennessIndexovertimeusingthetemplateprovidedbelow.
4.Manysatellitesalsomeasurevegetationindices.OneoftheseindicesistheNormalizedDifferenceVegetationIndexorNDVI,whichalsomeasuresthe‘greenness’ofanarea,butiscalculatedusingacombinationofinfraredandredbands.Inthetablebelow,wehaveprovidedtheNDVIvaluesmeasuredbytheMODIS(Moderate‐ResolutionImagingSpectroradiometer)satelliteforthesameareaanddatesthatthesePicturePostimagesweretaken.UsingtheseNDVIvalues(andadifferentcolorpen!),plotNDVIforeachdateonthegraphabove.
Date NDVIApril2008 0.46June2008 0.64October2008 0.50January2009 0.53
0.0
0.2
0.4
0.6
0.8
1.0
Season
Gre
en
ne
ss I
nd
ex
Spring Summer Fall Winter
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DiscussionQuestions:
Firstly,takealookatthelineyougraphedthatrepresentstheGreennessIndex.Whatpatternsdoyouseeingreennessovertime?Isthiswhatyouexpectedfromyourinitialobservations?
NowlookatthelineyougraphedthatrepresentsNDVI.DoesitfollowthesamepatternastheGreennessIndex?Whyorwhynot?
Whydoyouthinkitisimportanttocalculatevegetationindices?Howcouldthistypeofinformationbemoreusefulthansimplylookingatthepictures?
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Extensions
TherearemanydifferentvariationsonthisactivitythatcanbedoneusingPicturePostandtheAnalyzingDigitalImagessoftware.Forexample,usingthesameformatabove,youcouldcomparedifferentfeaturesofoneimage(Howdoesgreennessdifferbetweenthegrassandatree?Whatabouttheroad?);oryoucouldinvestigatehowdifferentfeatureschangeovertime(willyouseethesamepatternsingreennessifyoulookatadeciduousversusaconiferoustree?),orlookatthedifferencesbetweenimagestakenindifferentpartsofthecountryordifferentecosystems.
Onceyourstudentshavecompletedthisactivity,takealookatthePicturePostAnimation(http://globecarboncycle.unh.edu/climate/ppost_new/picturepost6.html),linkedunderthePicturePostheadingoftheClimateResourceDirectorypage.Thisanimationusespicturesfromthesamesiteusedinthisactivity,butalsoincludessatellitedataofalbedo(thereflectanceofagivensurface).Usethetooltoexploretherelationshipbetweenalbedo,andgreenness,andwhatthehighandlowvaluesonthegraphslooklikeintheimages.
Implementingthistoolinyourclassroom
IfyouplanonsettingupaPicturePostinyourclassroom,thisactivityisagreatway
tomeasurechangesinthelandscapeovertime,andtomaketheconnectionbetweenwhatyouobserveinyourlocallandscapeandwhatismeasuredbysatellitedata.HaveaposterboardorexceldatatableandgraphofthegreennessindexinyourclassroomthatyourstudentscanaddtoeachtimeyoumakeanewPicturePost.FormoreinformationonimplementingPicturePostinyourclassroom,seethe‘OverviewofPicturePostLearningProgression’documentandthePicturePostwebsite(http://picturepost.unh.edu).
TheAnalyzingDigitalImagessoftwareispartoftheDigitalEarthWatch(DEW),andisavailableathttp://mvh.sr.unh.edu/software/software.htm#download.Moreresourcesarealsoavailableathttp://www.lawrencehallofscience.org/gss/rev/ip.