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)