Class%208:%20Radiometric%20Corrections

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Class 8 Radiometric Corrections
Sensor Corrections Atmospheric Corrections Convers
ion from DN to reflectance BRDF Corrections
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Two types of corrections Geometric Corrections
(class 5) Radiometric Corrections (class 8)
Remote sensing images are contaminated by various
radiative processes. The need to correct them
varies with the applications and sensors used.
Every time two images need to be combined (e.g.,
in a mosaic) or compared, the corrections become
obviously important.
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Radiometric Correction
Correction is made on the brightness (gray level)
values of the image.
Source of errors to be corrected
atmospheric degradation sensor
malfunctions Illumination-view geometry
Corrections are usually different for each band,
and in theory for each pixel
Attempts to correct data may themselves introduce
errors
Campbell 10.4
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Radiometric Corrections

• Correction for detector errors
• Line drop
• Destriping
• 2. Atmospheric corrections
• Histogram adjustment
• Atmospheric radiative transfer models
• 3. Conversion from DN to radiance
• 4. Conversion from radiance to reflectance
• 5. BRDF corrections

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Sensor corrections
Line Dropout
Mean from above and below pixels
Solution
Or use other spectral band
Images Lillesand-Kiefer Campbell 10.4
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Sensor corrections
Striping
Local averaging
Normalization
Images Lillesand-Kiefer Campbell 10.4
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Atmospheric Corrections

• Histogram adjustment
• Clear sky
• Hazy sky
• 2) Physical Models

Campbell 10.4
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Simple Atmospheric Corrections Histogram
Adjustment
Clear Atmosphere
Cloud shadowed region and water bodies have very
low reflectance in infrared bands. This should
give a peak near zero on the histogram. The
shifted peak is due to the low reflectance
regions with atmospheric scattering. A
correction can be obtain by removing this value
from all pixels. This method is called the
Histogram Minimum Method (HMM)
Narrow range of brightness values
Small atmospheric contribution to brightness
Brightness values

• Darkest values near zero

Campbell 10.4
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Simple Atmospheric Corrections Histogram
Adjustment
Hazy Atmosphere
Wide range of brightness values
In this case, the minimum value is higher, and
the histogram shape has changed
Added brightness of atmosphere
Brightness values
Darkest values far from zero
Campbell 10.4
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Atmospheric Correction Models
Physical models simulate the physical process of
scattering at the level of individual particles
and molecules
Absorption by gases scattering by aerosols
LOWTRAN 7 MODTRAN CAM5S, 6S
Complex models that need many meteorological
data as input. The data may not always be
available
Campbell 10.4
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Atmospheric Correction Models
Second Simulation of the Satellite Signal in the
Solar Spectrum 6S
Input file example (Saskatchewan study site
Landsat imagery) 7
(landsat TM) 9 02 17.14 -105.22 53.85
(month,day,hour,long,lat) 2
(mid lat summer) 1
(continental) 30
(visibility, km) -0.59
(TARGET ALTITUDE IN KM) -1000
(SATELLITE CASE) 29
(Landsat band 1) 0
(HOMOGENEOUS CASE) 0
(NO BRDF effect) 1
(uniform target vegetation) -2.0
(no atm. correction)
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Atmospheric Correction Models
6S corrected reflectance
Top of atmosphere reflectance
ASAS Konza prairie reflectance spectrum
ASAS band central wavelength (nm)
Vermote et al., 1997
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From DN to Radiance to Reflectance
Calibration Gain Coefficient (counts/(W/m2/sr/mm))
Characteristic Wavelength (mm)
Solar Irradiance (W/m2/mm)
LANDSAT TM Spectral Band
1 G(-3.58E-05)D1.376 0.4863 1959.2 2
G(-2.10E-05)D0.737 0.5706 1827.4 3
G(-1.04E-05)D0.932 0.6607 1550.0 4
G(-3.20E-06)D1.075 0.8382 1040.8 5
G(-2.64E-05)D7.329 1.677 220.75 7
G(-3.81E-04)D16.02 2.223 74.960
D days since launch
Radiance (DN - Offset)/Gain
Reflectance p.Radiance/Incident Solar
Irradiance Incident Solar IrradianceSolar
Irradiance cos(SZA)
Source CCRS Web site
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If the input signal exceeds the amount for which
the sensor was designed, the system response
will become non-linear or reach the saturation
level. This is a common occurrence in land
remote sensing systems when they image bright
clouds and/or snow cover, for example.
Saturation
Non-Linear Region
y (DN)
Linear Region y a.x b (DN gainRadiance
offset)
Offset b
Input Value x (radiance)
Source CCRS Web site
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Atmospheric Corrections
Ltot radiance measured by the sensor r
reflectance of the target E irradiance on the
target T transmissivity of the
atmosphere Lp path radiance (radiance due
to the atmosphere)
L K 7.2
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Atmospheric Corrections
E0 cos?s
E ----------------
d2
E0 solar irradiance at the mean Earth-Sun
distance ?s solar zenith angle d relative
deviation of Earth-Sun distance from the
mean distance at the time of imaging
L K 7.2
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Bidirectional Reflectance Distribution Function
(BRDF) Correction
Structures like trees cast shadows that change
the amount of light that reaches a sensor
depending on its view zenith angle
To compare pixel reflectance from different
images, or even different part of an image, the
target (pixel) reflectance must be measured under
the same view and solar geometry.
Sensor
Solar Zenith Angle (SZA)
View Zenith Angle (VZA)
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Some BRDF models
CCRS uses a modification of Roujeans model for
BRDF corrections of AVHRR data (Roujean hotspot
from 4-Scale, Chen and Cihlar, 1997) GORT (Li
and Strahler) 4-Scale (Chen and Leblanc)