lect-06

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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