Atmospheric correction

1. APPROACHES FOR
ATMOSPHERIC
CORRECTION
NIRMAL KUMAR
AP:140

2. Atmospheric correction
To retrieve surface reflectance from RS imagery
 relationship between radiance
received at the sensor (above
atmosphere) and radiance
leaving the ground
Ls =H.ρ.T + Lp
Ls – at sensor radiance
H – total downwelling radiance
ρ – reflectance of target
T – atmospheric transmittance
Lp – atmospheric path radiance
(wavelength dependent)

3. Why do atmospheric correction?
 Physical relation of radiance to surface
property (surface normal, surface roughness, reflectance).
 Atmospheric component needs to be removed
 Image ratios (NDVI) leads to biased estimate
 Scattering increases inversely with wavelength
 The involved channels will be unequally affected
 Time difference between image acquisition and
ground truth measurements
 Comparison of RS data captured at different times
 Conditions may be different

4. Atmospheric correction methods
 Image – based methods
 Dark pixel method
 Regression method
 Empirical line method
 Radiative transfer models
 Relative correction method (PIFs)

5. Dark pixel subtraction method
Ls =H.ρ.T + Lp
 Pixel
values of low reflectance areas
near zero
 Exposure of dark colored rocks
 Deep shadows
 Clear water
 Lowest pixel values in visible and NIR are
approximation to atmospheric path
radiance
 Minimum values subtracted from image

6. Regression method
 NIR pixel values are plotted against values
in other bands
 Apply a straight line using the least square
method
 If there was no haze, the line would pass
through origin
 resulting offset is approximation for
atmospheric path radiance
 offset subtracted from image

7. Empirical line correction method
 Use target of “known”, low and high reflectance
targets in one channel e.g. non-turbid water & desert,
or dense dark vegetation & snow
 Assume radiance, L = gain * DN + offset
 Offset is assumed to be atmospheric part of signal
Radiance, L
Offset assumed to be atmospheric
path radiance
Regression line L = G*DN + O
Target DN values
DN

8. Conversion of DNs to absolute
radiance value
3 steps
• Convert DN to apparent radiance Lapp
• Convert Lapp to apparent reflectance (knowing
response of sensor)
• Convert to at-ground reflectance i.e. intrinsic surface
property by accounting for atmosphere
Use Radiative transfer models

9. Radiative transfer models
 Limited by the need to supply data about
atmospheric conditions at time of acquisition
 Mostly used with "standard atmospheres"
 Available numerical models
 􀁸 LOWTRAN 7
 􀁸 MODTRAN 4
 􀁸 ATREM
 􀁸 ATCOR
 􀁸 6S (Second Simulation of the Satellite Signal in
the solar spectrum)

10. THANK YOU