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.

InitialObservations

Takealookateachofthefourpicturesprovided.Whatdoyounoticeabouttheimages?Howdoyouthinktheywillchangeingreennessovertime?Whichfeaturesinthepicturewillchangethemost?Theleast?

CalculatingtheGreennessIndexusingAnalyzingDigitalImages

1. OpentheAnalyzingDigitalImagessoftware.

2. Clickonthe buttononthebottom‐leftcornerofthescreen.Inthe‘open’window,choosethepictureyouwouldliketoopen.

3. Awindowwillpopupaskingyouto“selectmethodtocalibratethepixelsize.”

Choose

4. Yourimageshouldnowappearonthescreen.Under‘SelectSpatialTool’dropdownmenu,choosethe‘RectangleTool.’

5. Thistoolallowsyoutoselectasmallsectionofthepicture.Clickonthepictureand

holdthemousedownwhiledraggingitacrossthescreentoselecttherectangle.Chooseonefeatureinthepicturetofocuson(somethingthatyouthinkwillchangein‘greenness’overtime)andrecordwhatfeatureyouchoseonyourworksheet.Placetherectangleoverthatfeature,youdon'tneedtoencompassthewholefeatureintherectangle;justasmallsectionofitisenough!Onceyouhaveyourrectangle,thescreenshouldlooklikethis:

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.

MeasuringGreennessIndex­ActivityWorksheet

1.Whatfeatureoftheimagedidyouchoosetomeasure?Whydidyouchoosethatfeature?

GreennessIndexDataTable

PictureDate

RedIntensity

GreenIntensity

BlueIntensity

GreennessIndex

2.Onceyouhaverecordedthered,green,andblueintensitiesforallpictures,usethespacebelowtocalculatethegreennessindexforeachdate,andrecordthevaluesinyourtable.Greennessindexiscalculatedbyusingtheequationbelow:

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

DiscussionQuestions:

Firstly,takealookatthelineyougraphedthatrepresentstheGreennessIndex.Whatpatternsdoyouseeingreennessovertime?Isthiswhatyouexpectedfromyourinitialobservations?

NowlookatthelineyougraphedthatrepresentsNDVI.DoesitfollowthesamepatternastheGreennessIndex?Whyorwhynot?

Whydoyouthinkitisimportanttocalculatevegetationindices?Howcouldthistypeofinformationbemoreusefulthansimplylookingatthepictures?

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.