rs ch2 spectral signatures · dr.ir. jan clevers centre for geo-information dept. environmental...
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SPECTRAL SIGNATURES
dr.ir. Jan Clevers
Centre for Geo-InformationDept. Environmental Sciences
Wageningen UR
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Subjects Spectral Signatures
• Spectral signature (reflection)
• Temporal signature
• Vegetation indices
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Reflective Optical Window
200µµµµm0.3 0.6 1.0 5.0 10 50 10m100 1mm 1cm 1m
200µµµµm0.3 0.6 1.0 5.0 10 50 10m100 1mm 1cm 1m
multispectral scanners
optical
lidar
photography
human eye
mic
row
aves
microwave radiometry
blocking effect of atmosphere
radar
atm
osp
her
ictr
ansm
itta
nce
UV
blue
gree
nre
d
NIR
MIR
MIR
TIR TIR
thermal scanners
VIS
Wavelength
reflective
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100
10-1
10-2
10-3
10-4
0.2 0.5 2 101.0 5 20
radiation (Wcm-2 µm-1)
wavelength (µm)
REFLECTION EMISSION
emitted
thermal radiation
reflected
solar radiation
Source of EM radiation
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curves for an “average”object on earth
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Reflection is the process of sending back EM radiation byan object
Reflectance factor [0,1]-coefficient [0,1]-percentage [0%,100%]
1) ρ(λ) = Lr(λ) / Li(λ)= reflected divided by incoming
radiation per wavelength λor wavelength interval ∆λ
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Reflection
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Hand-held radiometer
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2) ρobj(λ) = Lobj(λ) / Lref(λ) * ρref(λ)
Reflectance factor: the radiance of an object in a certain direction under certainconditions of irradiation, relative to the radiance of an (idealwhite) diffuse surface in the same direction and the sameirradiation conditions.
3) Atmospheric correctionmodel
Reflection
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Reference targets
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λwavelength (µm)
red middle-
infrared
near-
infraredblue
gree
n
dry soil
water
wet soil
vegetation
reflectance (%)
SPOT HRG Panchromatic
Pan: IKONOS, Quickbird, GeoEye, WorldView
Meteosat
NOAA AVHRR
Landsat TM
40
20
00.8
60
0.4 1.2 1.6 2.0 2.4
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SPOT HRG multispectral
Multispectral: IKONOS, Quickbird, GeoEye, WorldView
Spectral band positionsTIRS
TIRS
TIRS
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Spectral Signatures: Soil
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0.8%
4.7%
8.3%
12.9%16.9%20.2%
Newtona silt loam
0
20
40
60
1.51.0 2.0 2.50.5
Wavelength (µm)
Example influence of soil moisture content:R
efle
ctan
ce (
%)
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absorptance
(water)abso
rpta
nce
(chl
orop
hyll)
transmittance
reflectance
VIS NIR MIR
100
0
20
40
60
80
2.82.42.01.61.20.80.4
Wavelength (µm)
%
Spectral Signatures: Leaf
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Individual leaf
VIS → chlorophyll (absorptance)
NIR → cell walls - air transitions (reflectance)
MIR → water (absorptance)
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Spectral information of a leaf
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Spectral Signatures: Leaf
citrustomatosorghumcotton
Wavelength (µm)
0.5 1.0 1.5 2.0 2.5
20
40
60
0
Plant type
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Ref
lect
ance
(%
)
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Ref
lect
ance
(%
)
(1)
(2)
100
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
20
40
60
80
0
Wavelength (µm)
Disease infection
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Spectral Signatures: Vegetation
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1.0
VIS veg. ≈ 1/2 VIS leaf
NIR veg. > NIR leaf
dry soil
wet soil
0.4 0.5 0.6 0.7 0.8 0.9
50
40
30
20
10
0
Reflectance (%)
Wavelength (µm)
barley
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60
50
40
30
20
10
0 50 100 150days after sowing
NIR
redgreen
Reflectance (%)
bare soil
Temporal signatures: Barley Crop
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Field trial with barley
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Single-engine aircraft for 70-mm photography
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Mounts and cameras for 70-mm photography
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Microlight aircraft for 70-mm photography
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Camera mounts for the microlight
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Spectral bands for multispectral photography
1.0
50
40
30
20
10
0
Reflectance (%)
Wavelength (µm)
0.4 0.5 0.6 0.7 0.8 0.9
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Green recording, 27 May
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Red recording, 27 May
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NIR recording, 27 May
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True colour recording, 29 July
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False colour recording, 29 July
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Reference targets
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Vegetation Indices:
simple band combinations in order to- strenghten the spectral contribution of vegetation- eliminate the disturbing influence of the
atmosphere, soil, etc.
Example:
• simple ratio: NIR/Red
• “Normalized difference vegetation index”:NDVI = (NIR-Red) / (NIR+Red)
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NOAA-NDVI image of Africa
March June
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Barley crop: Red, NIR
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Barley crop:
WDVI
50 150100
Ref
lect
ance
(%
)
50
40
30
20
10
0
wet
wet
dry
days after sowing50 150100
days after sowing
WD
VI (
%) 50
0
10
40
30
20 wet
wet
dry
dry
dry
Temporal signatures
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LAI = -1/α • ln(1 - WDVI / WDVIω)
LAI
WDVI
6040200
2
4
6
8
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Relationship WDVI - LAI
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NDVI = (NIR - Red) / (NIR + Red)
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LAI
10
8
6
4
2
86420
NDVI
Relationship NDVI - LAI
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LAI
25 50 75 100 125
8
6
4
2
0
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NIR / Red
Relationship NIR/Red - LAI
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LAI R NIR NIR-R NIR/R NDVI
wet 0 10 10 0 1 0dry 0 20 20 0 1 0
2 2 40 38 20 0.9054 2 50 48 25 0.9238 2 60 58 30 0.935
colour 8 1 60 59 60 0.967
NDVI = (NIR - R) / (NIR + R)
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Example vegetation indices