class 8: radiometric corrections sensor corrections atmospheric corrections conversion from dn to...
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Class 8: Radiometric Corrections
Sensor CorrectionsAtmospheric Corrections
Conversion from DN to reflectanceBRDF Corrections
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.
Two types of corrections:Geometric Corrections (class 5)Radiometric Corrections (class 8)
Radiometric Correction
Attempts to correct data may themselves introduce errors
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 differentfor each band, and in theory for each pixel
Campbell 10.4
Radiometric Corrections1. Correction for detector errors• Line drop• Destriping2. Atmospheric corrections• Histogram adjustment• Atmospheric radiative transfer models3. Conversion from DN to radiance4. Conversion from radiance to reflectance5. BRDF corrections
Sensor correctionsLine Dropout
43 47 51 5740 46 50 54
38 40 42 50
0 0 0 0
Solution: Mean from aboveand below pixels
43 47 51 5740 46 50 54
38 40 42 50
39 43 46 52
Or use other spectral band
Images: Lillesand-KieferCampbell 10.4
Atmospheric Corrections
1) Histogram adjustment• Clear sky• Hazy sky
2) Physical Models
Campbell 10.4
Simple Atmospheric Corrections – Histogram Adjustment
Clear Atmosphere
Small atmosphericcontribution to
brightness
Brightness values
Darkest values near zero
Narrow range of brightness values
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 tothe 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)
Campbell 10.4
Simple Atmospheric Corrections – Histogram Adjustment
Hazy Atmosphere
Added brightnessof atmosphere
Brightness values
Darkest values far from zero
Wide range of brightness values
In this case, the minimumvalue is higher, and the
histogram shape has changed
Campbell 10.4
Atmospheric Correction Models
Physical models simulate the physical process of scattering at the level of individual particles and molecules
Complex models that need many meteorological data as input.
The data may not always be available
LOWTRAN 7MODTRANCAM5S, 6S
Absorption by gasesscattering by aerosols
Campbell 10.4
Second Simulation of the Satellite Signal in the Solar Spectrum: 6S
Atmospheric Correction Models
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)
Atmospheric Correction Models
AS
AS
Ko
nza
pra
irie
ref
lect
ance
sp
ectr
um
ASAS band central wavelength (nm)
6S corrected reflectance
Top of atmosphere reflectance
Vermote et al., 1997
From DN to Radiance to Reflectance
1 G=(-3.58E-05)*D+1.376 0.4863 1959.22 G=(-2.10E-05)*D+0.737 0.5706 1827.43 G=(-1.04E-05)*D+0.932 0.6607 1550.04 G=(-3.20E-06)*D+1.075 0.8382 1040.85 G=(-2.64E-05)*D+7.329 1.677 220.757 G=(-3.81E-04)*D+16.02 2.223 74.960
Source:CCRS Web site
LANDSAT TM Spectral Band
Calibration GainCoefficient
(counts/(W/m2/sr/m))
CharacteristicWavelength
(m)
SolarIrradiance
(W/m2/m)
Radiance = (DN - Offset)/Gain
Reflectance = Radiance/Incident Solar IrradianceIncident Solar Irradiance=Solar Irradiance *cos(SZA)
D = days since launch
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.
Linear Region y = a.x + b (DN = gain*Radiance + offset)
Non-Linear Region
Saturation
Offset b
Input Value x (radiance)
Source:CCRS Web site
y (D
N)
ptot LET
L
ET
LL ptot
Ltot= radiance measured by the sensor= reflectance of the targetE = irradiance on the targetT = transmissivity of the atmosphereLp= path radiance (radiance due to the atmosphere)
Atmospheric Corrections
L & K 7.2
Atmospheric Corrections
L & K 7.2
E = ----------------E0 coss
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
Bidirectional Reflectance Distribution Function (BRDF) Correction
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.
Structures like trees cast shadows that change the amount of light that reaches a sensor depending on its
view zenith angle
SensorView Zenith Angle (VZA)
Solar Zenith Angle(SZA)