Atmospheric correction

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Atmospheric correction

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Atmospheric correction

  1. 1. APPROACHES FOR ATMOSPHERIC CORRECTION NIRMAL KUMAR AP:140
  2. 2. Atmospheric correctionTo 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. 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. 4. Atmospheric correction methods Image – based methods  Dark pixel method  Regression method Empirical line method Radiative transfer models Relative correction method (PIFs)
  5. 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. 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. 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. 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. 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. 10. THANK YOU

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