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Seeing through the clouds and trees:Using Radar Systems to map rainforest health

February 2001

Collecting information for mappingtropical rainforests is often impededby the cloudy, and frequently smokeyconditions common in these areas.This restricts the use of traditionalaerial photography and satellitebased imaging systems for monitor-ing these environments. There is asolution to this problem. RainforestCRC researchers are collaboratingwith NASA and local managementagencies to develop effective newmapping techniques.

The new mapping approach relies onusing a specific type of radar (radio-detection and ranging) system toproduce images of the forest and thesurface of the earth. SyntheticAperture Radars (SAR) can produceimages containing information abouttopography, vegetation height anddensity. Like flash cameras, they senda non-harmful pulse of energy fromthe sensor to the ground, after whichthey measure the strength of thereturn signal.

The return radar signal is used toestimate the distance of the groundfrom the sensor and the characteris-tics of the surface from which it hasreflected. This signal can passthrough cloud and smoke, enablingit to be used to map tropical forestenvironments. Different types of SARimages provide information oncanopy characteristics, volume ordensity of wood, biomass and char-acteristics of the forest floor.

Rainforest CRC researchers are nowdetermining the type of informationon forest structure and conditionwhich can be mapped from imagingradar, and designing procedurescapable of using these data for moni-toring rainforests in the Wet Tropicsand elsewhere in the world.

Why use SAR’s for rainforestanalysis ?Remote sensing instruments cancollect data over relatively largeareas in a relatively short time period.This enables field data from inten-sively sampled sites to be comparedwith image data sets and used toestimate the biological and physicalproperties of these areas. They canalso provide data for remote orotherwise inaccessible areas.

A great advantage of radar imagingsystems is that they measure theinteraction of different types ofenergy with the surface of the earth.Just as we see separate colours likeblue, green and red, imaging radarsystems measure the response indifferent types of energy, as well as

their polarization (orientation ofelectric and magnetic wave of theenergy). One type of radar C-band(old military code for the names ofthese systems) only interacts with thecanopy of the forest and providesinformation on surface roughness.Another type of energy, L-band, isable to be transmitted through thecanopy of forests and interacts withtrunks and forest floor features.

NASA JPL DC-8 aircraft flies the route from Port Douglas to Cooktown collecting SAR

image data as ground teams below compile vegetation and ground survey data

(Source: NASA JPL)

(Remote Sensing Series - I)

Prior to and following the AirSARflight, very high spatial resolutionimages were collected usingmultispectral systems operated bythe University of Queensland, and ahyperspectral instrument operatedby CSIRO’s Earth ObservationCentre. These datasets will provideessential calibration data for theradar and will also be examinedusing information collected duringthe Space Shuttle’s historic globalradar topography mapping missionin February 2000 to produce an accu-rate digital elevation model for theEarth.

Where to from here?The AirSAR image data are currentlybeing processed at the Jet PropulsionLaboratory, NASA. Once these dataare returned, the area will be mappedinto rainforest, mangroves, pastureand crops. Detailed analyses will thenbe applied to relate field measuredstructural properties of vegetation tothe radar image to produce maps of

Hence, the radar image can be usedto provide a variety of informationabout the rainforests. Combined, thedifferent radar types provide infor-mation about tree height, canopydensity, vegetation structure anddensity.

A Global Rainforest MappingMission being undertaken by Japan’sNational Space Agency, NASA andthe European Joint Research Centrefor Remote Sensing is also usingimaging radars. The focus of thiswork however, is mapping the extentof rainforests, whereas the focus ofthe Rainforest CRC project is to mapthe structure and condition to helpdetermine and monitor the health ofthe forest.

Collecting the DataOn August 31, 2000 NASA’s AirborneSAR instrument was flown along aten kilometre wide strip from PortDouglas to Cooktown. At the sametime research staff from the Univer-sity of Queensland and CSIRO Land& Water collected ground survey andvegetation data in rainforests andmangroves to calibrate the imagedata set. Vegetation structural infor-mation included tree height, canopydepth, trunk diameter at breast height(dbh), tree location and as far aspossible species. The purpose of theflight was to investigate the relation-ship between the radar images andrainforest structure and condition. Aprevious NASA AirSAR mission in1996 collected images over the samearea which, together with field dataand the 2000 images, will be used toexamine changes in the rainforestover time.

vegetation biomass and density, andthe changes which have occurredsince the 1996 image.

These data will also be incorporatedwith other satellite and airbornedatasets and the methods developedwill be tested and refined beforebeing delivered as operational moni-toring tools. The combined data setwill enable the structure andcondition of the rainforest to becharacterized and a methodology formapping these various parameterscan then be developed for general usein Australia’s Wet Tropics andelsewhere in the world.

The ground team setting up calibration

equipment prior to the AirSAR flight

For more information:Michael Stanford & Stuart PhinnBiophysical Remote Sensing GroupThe University of QueenslandBrisbane QLD 4072 AustraliaPhone: (07) 3365 6517Email: michaels@mailbox.uq.edu.auORCatherine Ticehurst and Alex HeldCSIRO Land and WaterGPO Box 1666 Canberra ACT 2601Phone: (02)6246 5815 Email:catherine.ticehurst@cbr.clw.csiro.au

Related Web Sites:

NASA imaging radar homepage:http://southport.jpl.nasa.gov/NASA AirSAR homepage withdata from the 2000 mission:http://airsar.jpl.nasa.gov/Access Remote Sensing data forAustralia - Australian Centre forRemote Sensing:http://www.auslig.gov.au/acres/index.htm