atmospheric correction

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Atmospheric correction


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relationship between radiance received at the sensor (above atmosphere) and radiance leaving the ground Ls =H..T + LpTo retrieve surface reflectance from RS imagery

Ls at sensor radianceH total downwelling radiance reflectance of targetT atmospheric transmittanceLp atmospheric path radiance (wavelength dependent)

2Why do atmospheric correction?Physical relation of radiance to surface property (surface normal, surface roughness, reflectance).Atmospheric component needs to be removedImage ratios (NDVI) leads to biased estimateScattering increases inversely with wavelengthThe involved channels will be unequally affectedTime difference between image acquisition and ground truth measurementsComparison of RS data captured at different timesConditions may be different3Atmospheric correction methodsImage based methodsDark pixel methodRegression methodEmpirical line methodRadiative transfer modelsRelative correction method (PIFs)4Dark pixel subtraction method Ls =H..T + LpPixel values of low reflectance areas near zeroExposure of dark colored rocksDeep shadowsClear waterLowest pixel values in visible and NIR are approximation to atmospheric path radianceMinimum values subtracted from image

5Regression methodNIR pixel values are plotted against values in other bandsApply a straight line using the least square methodIf there was no haze, the line would pass through originresulting offset is approximation for atmospheric path radianceoffset subtracted from image

6Empirical line correction methodUse target of known, low and high reflectance targets in one channel e.g. non-turbid water & desert, or dense dark vegetation & snowAssume radiance, L = gain * DN + offsetOffset is assumed to be atmospheric part of signal

Target DN valuesDNRadiance, LRegression line L = G*DN + OOffset assumed to be atmospheric path radiance7Conversion of DNs to absolute radiance value3 stepsConvert DN to apparent radiance LappConvert Lapp to apparent reflectance (knowing response of sensor)Convert to at-ground reflectance i.e. intrinsic surface property by accounting for atmosphereUse Radiative transfer models8Radiative transfer modelsLimited by the need to supply data about atmospheric conditions at time of acquisitionMostly used with "standard atmospheres"Available numerical models LOWTRAN 7 MODTRAN 4 ATREM ATCOR 6S (Second Simulation of the Satellite Signal in the solar spectrum)9THANK YOU10


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