InterpretationofO ptic alImages

InterpretingOpticalRemoteSensingImages

Four main types of information contained in an optical image are often

utilizedforimageinterpretation:

RadiometricInformation(i.e.brightness,intensity,tone),
SpectralInformation(i.e.colour,hue),
TexturalInformation,
GeometricandContextualInformation.

Theyareillustratedinthefollowingexamples.

PanchromaticImages
A panchromatic image consists of only one band. It is usually
displayed as a grey scale image, i.e. the displayed brightness of a
particular pixel is proportional to the pixel digital number which is
related to the intensity of solar radiation reflected by the targets in the
pixelanddetectedbythedetector.Thus,apanchromaticimagemaybe
similarlyinterpretedasablackandwhiteaerialphotographofthearea.
TheRadiometricInformationisthemaininformationtypeutilizedinthe
interpretation.

A panchromatic image extracted from a SPOT panchromatic scene at a ground

resolutionof 10 m. The ground coverage is about 6.5 km (width) by 5.5 km (height).
The urban area at the bottom left and a clearing near the top of the image have high
reflectedintensity,whilethevegetatedareasontherightpartoftheimagearegenerally
dark.Roadsandblocksofbuildingsintheurbanareaarevisible.Ariverflowingthrough
the vegetated area, cutting across the top right corner of the image can be seen. The
river appears bright due to sediments while the sea at the bottom edge of the image
appearsdark.

MultispectralImages
A multispectral image consists of several bands of data. For visual
display,eachbandoftheimagemaybedisplayedonebandatatimeas
a grey scale image, or in combination of three bands at a time as a
colour composite image. Interpretation of a multispectral colour
compositeimagewillrequiretheknowledgeofthespectralreflectance
signature of the targets in the scene. In this case, the spectral
informationcontentoftheimageisutilizedintheinterpretation.

The following three images show the three bands of a multispectral

imageextractedfromaSPOTmultispectralsceneatagroundresolution
of 20 m. The area covered is the same as that shown in the above
panchromatic image. Note that both the XS1 (green) and XS2 (red)
bandslookalmostidenticaltothepanchromaticimageshownabove.In
contrast, the vegetated areas now appear bright in the XS3 (near
infrared) band due to high reflectance of leaves in the near infrared
wavelength region. Several shades of grey can be identified for the
vegetated areas, corresponding to different types of vegetation. Water
mass(boththeriverandthesea)appeardarkintheXS3(nearIR)band.

SPOTXS1(greenband)
 SPOTXS2(redband)

SPOTXS3(NearIRband)

ColourCompositeImages
In displaying a colour composite image, three primary colours (red,
green and blue) are used. When these three colours are combined in
various proportions, they produce different colours in the visible
spectrum. Associating each spectral band (not necessarily a visible
band)toaseparateprimarycolourresultsinacolourcompositeimage.

Manycolourscanbeformedby
combiningthethreeprimarycolours
(Red,Green,Blue)invarious
proportions.

TrueColourComposite
If a multispectral image consists of the three visual primary colour
bands(red,green,blue),thethreebandsmaybecombinedtoproducea
"truecolour"image.Forexample,thebands3(redband),2(greenband)
and 1 (blue band) of a LANDSAT TM image or an IKONOS
multispectral image can be assigned respectively to the R, G, and B
colours for display. In this way, the colours of the resulting colour
composite image resemble closely what would be observed by the
humaneyes.
 A1mresolutiontruecolourIKONOSimage.

FalseColourComposite
The display colour assignment for any band of a multispectral image
canbedoneinanentirelyarbitrarymanner.Inthiscase,thecolourofa
target in the displayed image does not have any resemblance to its
actual colour. The resulting product is known as a false colour
compositeimage.Therearemanypossibleschemesofproducingfalse
colourcompositeimages.However,someschememaybemoresuitable
fordetectingcertainobjectsintheimage.

A very commonfalsecolour composite scheme for displaying a SPOT

multispectralimageisshownbelow:

R=XS3(NIRband)
G=XS2(redband)
B=XS1(greenband)
This false colour composite scheme allows vegetation to be detected
readily in the image. In this type of false colour composite images,
vegetationappearsindifferentshadesofreddependingonthetypesand
conditions of the vegetation, since it has a high reflectance in the NIR
band(asshowninthegraphofspectralreflectancesignature).

Clear water appears darkbluish (higher green band reflectance), while

turbid water appears cyan (higher red reflectance due to sediments)
comparedtoclearwater.Baresoils,roadsandbuildingsmayappearin
variousshadesofblue,yelloworgrey,dependingontheircomposition.

FalsecolourcompositemultispectralSPOTimage:
Red:XS3Green:XS2Blue:XS1

Another common false colour composite scheme for displaying an

opticalimagewithashortwaveinfrared(SWIR)bandisshownbelow:

R=SWIRband(SPOT4band4,LandsatTMband5)
G=NIRband(SPOT4band3,LandsatTMband4)
B=Redband(SPOT4band2,LandsatTMband3)

Anexampleofthisfalsecolourcompositedisplayisshownbelowfora
SPOT4image.
 FalsecolourcompositeofaSPOT4multispectralimageincludingtheSWIRband:
Red:SWIRbandGreen:NIRbandBlue:Redband.Inthisdisplayscheme,vegetation
appearsinshadesofgreen.Baresoilsandclearcutareasappearpurplishormagenta.
Thepatchofbrightredareaontheleftisthelocationofactivefires.
Asmokeplumeoriginatingfromtheactivefiresiteappearsfaintbluishincolour.

FalsecolourcompositeofaSPOT4multispectralimagewithoutdisplayingtheSWIR
band:
Red:NIRbandGreen:RedbandBlue:Greenband.Vegetationappearsinshadesof
red.
 red.
Thesmokeplumeappearsbrightbluishwhite.

NaturalColourComposite
For optical images lacking one or more of the three visual primary
colour bands (i.e. red, green and blue), the spectral bands (some of
whichmaynotbeinthevisibleregion)maybecombinedinsuchaway
that the appearance of the displayed image resembles a visible colour
photograph,i.e.vegetationingreen,waterinblue,soilinbrownorgrey,
etc.Manypeoplerefertothiscompositeasa"truecolour" composite.
However,thistermismisleadingsinceinmanyinstancesthecoloursare
only simulated to look similar to the "true" colours of the targets. The
term"naturalcolour"ispreferred.

TheSPOTHRVmultispectralsensordoesnothaveablueband.The
three bands, XS1, XS2 and XS3 correspond to the green, red, and NIR
bandsrespectively.Butareasonablygoodnaturalcolourcompositecan
beproducedbythefollowingcombinationofthespectralbands:

R=XS2
G=(3XS1+XS3)/4
B=(3XS1XS3)/4

whereR,GandBarethedisplaycolourchannels.
 NaturalcolourcompositemultispectralSPOTimage:
Red:XS2Green:0.75XS2+0.25XS3Blue:0.75XS20.25XS3

VegetationIndices
Different bands of a multispectral image may be combined to
accentuatethevegetatedareas.Onesuchcombinationistheratioofthe
nearinfrared band to the red band. This ratio is known as the Ratio
VegetationIndex(RVI)

RVI=NIR/Red

Since vegetation has high NIR reflectance but low red reflectance,
vegetatedareaswillhavehigherRVIvaluescomparedtononvegetated
aeras. Another commonly used vegetation index is the Normalised
DifferenceVegetationIndex(NDVI)computedby

NDVI=(NIRRed)/(NIR+Red)

NormalisedDifferenceVegetationIndex(NDVI)derivedfromtheaboveSPOTimage

IntheNDVImapshownabove,thebrightareasarevegetatedwhilethe
nonvegetated areas (buildings, clearings, river, sea) are generally dark.
Note that the trees lining the roads are clearly visible as grey linear
featuresagainstthedarkbackground.

The NDVI band may also be combined with other bands of the
multispectral image to form a colour composite image which helps to
discriminate different types of vegetation. One such example is shown
below.Inthisimage,thedisplaycolourassignmentis:

R=XS3(NearIRband)
G=(XS3XS2)/(XS3+XS2)(NDVIband)
B=XS1(greenband)
 NDVIColourCompositeoftheSPOTimage:Red:XS3Green:NDVIBlue:XS1.

At least three types of vegetation can be discriminated in this colour

composite image: green, bright yellow and golden yellow areas. The
greenareasconsistofdensetreeswithclosedcanopy.Thebrightyellow
areas are covered with shrubs or less dense trees. The golden yellow
areas are covered with grass. The non vegetated areas appear in dark
blueandmagenta.

TexturalInformation
Textureisanimportantaidinvisualimageinterpretation,especiallyfor
high spatial resolution imagery. An example is shown below. It is also
possibletocharacterizethetexturalfeaturesnumerically,andalgorithms
for computeraided automatic descrimination of different textures in an
imageareavailable.

This is an IKONOS 1m resolution

pansharpenedcolorimageofanoil
palm plantation. The image is 300
m across. Even though the general
colour is green throughout, three
distinct land cover types can be
 identified from the image texture.
The triangular patch at the bottom
leftcorneristheoilpalmplantation
with matured palm trees. Individual
treescanbeseen.The predominant
textureistheregularpatternformed
by the tree crowns. Near to the top
of the image, the trees are closer
together, and the tree canopies
merge together, forming another
distinctive textural pattern. This
area is probably inhibated by
shrubs or abandoned trees with tall
undergrowthsandshrubsinbetween
thetrees.Atthebottomrightcorner,
colour is more homogeneous,
indicating that it is probably an
openfieldwithshortgrass.

GeometricandContextureInformation
Using geometric and contextual features for image interpretation
requires some apriori information about the area of interest. The
"interpretational keys" commonly employed are: shape, size, pattern,
location,andassociationwithotherfamiliarfeatures.

Contextualandgeometricinformationplaysanimportantroleintheinterpretationof
veryhighresolutionimagery.Familiarfeaturesvisibleintheimage,suchasthebuildings,
roadsidetrees,roadsandvehicles,makeinterpretationoftheimagestraightforward.

ThisisanIKONOSimageofacontainerport,evidencedbythepresenceofships,
cranes,andregularrowsofrectangularcontainers.Theportisprobablynotoperatingat
itsmaximumcapacity,asemptyspacescanbeseeninbetweenthecontainers.
ThisSPOTimageshowsanoilpalmplantationadjacenttoaloggedoverforestinRiau,
Sumatra.Theimageareais8.6kmby6.4km.Therectangulargridpatternseenhereis
amaincharacteristicoflargescaleoilpalmplantationsinthisregion.

ThisSPOTimageshowslandclearingbeingcarriedoutinaloggedoverforest.The
darkred
regionsaretheremainingforests.Trackscanbeseenintrudingintotheforests,
implicatingsome
loggingactivitiesintheforests.Theloggingtracksarealsoseenintheclearedareas
(darkgreenishareas).Itisobviousthatthelandclearingactivitiesarecarriedoutwith
theaidoffires.
Asmokeplumecanbeseenemanatingfromasiteofactivefires.
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Pleasesendcomments/enquiries/suggestionsaboutthistutorialtoDr.S. Copyright
C.Liewatscliew@nus.edu.sg CRISP,2001