GEOG3610 Remote Sensing and Image Interpretation
Spectral Signatures and Their Interpretation 1
Copyright, 1998-2013 Qiming Zhou GEOG3610 Remote Sensing and Image Interpretation
Spectral Signatures and Their InterpretationSpectral Signatures and Their Interpretation
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Spectral Signatures and Their Interpretation
EMR and earth materials interactionSpectra of earth materialsMultispectral images and their
interpretation
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EMR and earth materials interaction
When EMR from the sun reaches the earth surface, it is transmitted - transmittanceabsorbed - absorbance reflected - reflectance
The nature of how the earth materials transmit, absorb or reflect the solar EMR is called spectral signature of an object.
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Spectra of earth materials
VegetationSoil and rocksWater, ice and snowCloud, fire and smoke
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Vegetation
Contains water, cellulose (tissues and fibres), lignin (non-carbohydrate constituent of wood), nitrogen, chlorophyll (green pigments) and anthocyanin (water-soluble pigments).
Depending on how active (i.e. kinds of chlorophyll) a green vegetation is, the combination of transmittance, absorbance and reflectance vary in different bands of the spectrum.
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Leaf structureReflectance, transmittance, and
absorptance spectraLeaf maturationMesophyll arrangements (internal
structural differences)
Physiological factors
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Leaf structure
A leafs structure and its reflectance characteristics at visible and near IR wavelengths.
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Transmittance, absorbance and reflectance
Fractions of the total light incident on the upper surface of a mature orange leaf that is reflected, absorbed and transmitted.
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Absorption spectra
Absorption spectra of chlorophyll a(blue-green) and chlorophyll b (yellow-green).
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Spectral reflectance
Average spectral-response curves for six materials.
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Spectral reflectance (cont.)Below: Average spectral-response curves
for four types of vegetation
Right: Average spectral-response curves for a plant leaf as it progresses from a healthy
state through different stages of damage.
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Leaf damage;Sun and shaded leaves;Leaf water content;Leaf air spaces; andSalinity and nutrient levels.
Other factors
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Vegetation canopy
Transmittance of leaves; Amount and arrangement of leaves; Characteristics of, e.g., stalks, trunks, limbs,
etc.; Background (soil, leave litter, etc.); Solar zenith angle; Look angle; and Azimuth angle.
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Soil and rocks
The reflectance from soil and rocks is influenced by:colourmineral contents (chemical composition
or crystalline structure)structureand others
We use soil for discussion
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Field reflectance spectra
Field reflectance spectra of green
grass, dead grass, Virginia Pine, Scarlet Oak, packed bare soil and
ploughed soil with cobbles.
packed
ploughed
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Factors influencing interpretation of soils
Soil colourMineral content - depends upon the
intermolecular vibration of the molecules
Organic matter - influences soil colour and moisture
Particle size - reflectance and thermal diffusivity, and moisture.
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Reflectance of minerals
Directional hemispherical reflectance spectra in the 0.4-2.5m wavelength region and biconical reflectance spectra in the 2-25m wavelength region of two clay minerals: kaolinite and montmorillonite.
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Factors influencing interpretation of soils (cont.)
Soil texture - mainly indirect effects on, e.g., soil moisture.
Structure and surface roughness (soil aggregation) - "smoothness" of soil - have significant effects on RADAR response.
Soil emissivity - thermal emissivity: ratio of energy radiated at the surface / black body
Soil temperature - influences the interpretation of thermal imagery and time of sensing.
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Reflectance from soils
O2 and CO2 and water vapour absorption; Sun illumination varies with atmospheric
conditions and solar radiation Effects of soil structure, surface roughness,
etc. The intensity of the sun peaks at about 0.5m
falling off rapidly at shorter and longer wavelengths.
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Water, ice and snow
Watervisible transmittance is highhigh absorptance in NIR influenced by the cleanness
Snowhigh reflectance in < 1.5m low at 1.5 and 2mvery low in the thermal IR
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Reflectance of ocean water
Calculated change in bulk reflectance of ocean water with increasing concentration of phytoplankton.
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Reflectance of snow
Computed reflectance spectra of three different textures of snow (coarse, fine, and frost) for (a) the 0.3-3.0m wavelength region, (b) the 3-14m wavelength region.
a
b
Coarse
FineFrost
Coarse
Frost
Fine
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Cloud, fire and smoke Cloud
strong reflectance in visible and NIR associated with shadow can be penetrated by radar
Fire high temperature Wiens displacement law
Smoke highly visible (black or white) in visible can be penetrated by TM5 and TM7 as their
wavelength is larger than the most smoke particles.
TW
=max
W = 2,897m K
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Detecting smoke and fires
TM band 1-5 and 7 show file smoke (band 1-4) and location of the fire (band 5, 7).
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Multispectral images and their interpretation
Single image band interpretationsimilar to airphoto interpretationbeware of the spectral wavelength of
the band and the spectral signatures of the objects
Colour compositesMultispectral band statisticsMultispectral classifications
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Panchromatic and infrared photographs
Panchromatic (left) and infrared (bottom) photographs of the Goldach region, Switzerland. Note the clear separation of tree types and the differentiation between the small stream and its terrain background in the infrared photograph.
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Infrared and panchromatic photographs
(A) channel bar accretion - darker-toned areas represent the most recent deposits
(moist) that have not yet been vegetated, (B) minor channel through a channel-bar complex, (C) meander cutoff, (D) back swamp, and (E) point-bar swamp.
Infrared Panchromatic
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Single band interpretationTM1 TM2 TM3
TM4 TM5 TM7
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Colour composites
Number of composites
Example: TM 6 non-thermal bands( )!3!3
!
=
n
nN
( ) 20!36!3!6
=
=N
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Colour Infrared photos
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Colour composites (cont.)
RED GREEN BLUENature colour TM3 TM2 TM1
Colour IR TM4 TM3 TM2MSS7 MSS5 MSS4HRV3 HRV2 HRV1
Mapping bushfire AVHRR4 AVHRR3 AVHRR1TM7 TM2 TM1
Study El Nio CZCS6 CZCS2 CZCS1
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Colour composites (cont.)
TM
1 2 3
TM
2 3 4
TM
1 4 5
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Object signatures on panchromatic and infrared photographs
Object Panchromatica InfraredbSnow White WhiteClouds White WhiteSky (high oblique) Medium grey BlackClear water Dark grey BlackSilty water Light grey Medium greyDeciduous foliage Dark grey WhiteConiferous foliage Dark grey Medium greyAutumn foliage (yellow) Light grey Light greyWhite sand (dry) Light grey Light greyWhite sand (moist) Medium grey Dark greyRed sandstone (dry) Medium grey Light greyRed sandstone (moist) Medium grey Dark greySwamp Dark grey BlackAsphalt Dark grey BlackConcrete Light grey Medium grey
aAcquired with a Kodak Wratten 12 filter. bAcquired with Kodak Wratten 88A or 89B filters
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Multispectral band statistics
Histogram
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Multispectral band statistics (cont.)
Scattergram
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Summary
Different earths materials have various characteristics in reflecting solar EMR.
The reflectance pattern of an object is called its spectral signature.
Understanding spectral signatures of earths materials is essential for remote sensing image interpretation.
The ultimate goal is to guide spectral band selection and create human colour vision for proper image interpretation.
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