Atmospheric and Radiometric Corrections for Remote Sensing Data

Rüdiger Gens

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Outline

radiometric correctionstriping(partially) missing lines illumination and view angle effectssensor calibrationterrain effects

atmospheric correction

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Striping

due to non-identical detector responsedetector characteristicschanges with time / rise of temperaturefailure

various methods (sometimes used in combination)

look up tables (radiometric response measurements at different brightness levels)onboard calibrationhistogram matching (gain and offset) – line pattern

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Striping – Landsat TM

De-stripedStripingProcessed by A. Prakash, GI, UAF Processed by A. Prakash, GI, UAF

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(Partially) missing lines

errors in sampling or scanning equipmenttransmission or recording of image datareproduction of the media containing the data

two methodsinterpolation using data from adjacent scan linesinterpolation data at the same scan line from different spectral bands

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Partially missing lines - Example

Source: CCRS Remote Sensing Tutorial

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Sun angle correction

position of the sun relative to the earth changes depending on time of the day and the day of the yearin the northern hemisphere the solar elevation angle is smaller in winter than in summer Adapted from Lillesand and Kiefer

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Sun angle correction

an absolute correction involves dividing the DN-value in the image data by the sine of the solar elevation anglesize of the angle is given in the header of the image data

α=

sinDNDNcorr

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Sun Illumination

position of sunsun elevation (sun-angle)sun - earth distance

correction elevationnormalizationdivision of each pixel value by the sine of solar elevation angle for particular time and location per spectral band

correction distancesun irradiance decreases with square of distancenormalization

Adapted from Lillesand and Kiefer

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Sensor calibration

necessary to generate absolute data on physical properties

reflectancetemperatureemissivitybackscatter

values provided by data provider / agency

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Terrain effects

cause differential solar illuminationsome slopes receive more sunlight than others

magnitude of reflected radiance reaching the sensor

topographic slope and aspectbidirectional reflectance distribution function (BRDF)

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Terrain correction

Minnaert correctionfirst order correction for terrain illumination effects

Ln – normalized radianceL – measured radiancee – slope (derived from DEM)i – incidence angle of solar radiationk – Minnaert constant (estimated for each image)

kkn )icos()ecos(LL ⋅⋅= −1

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Terrain correction

shaded relief model (SRM)requires digital elevation modelgenerated with constant albedo (brightness dependent solely on topographic effects)ratio of image and SRM yields spectral radiance of ground cover (noisy)alternative

( ) aSRMDNmDN DNcorr +−⋅=

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Why do atmospheric correction?

physical relation of radiance to surface property

atmospheric component needs to be removedmultispectral data for visual analysis

scattering increases inversely with wavelength image ratios

leads to biased estimatetime difference between image acquisition and ground truth measurements

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Atmosphere and radiation

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

ps LTHL +⋅ρ⋅=

relationship between radiance received at the sensor (above atmosphere) and radiance leaving the ground

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Atmospheric correction methods

image-based methodshistogram minimum methodregression method

radiative transfer modelsempirical line method

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Histogram minimum method

histograms of pixel values in all bandspixel values of low reflectance areas near zero

exposures of dark colored rocksdeep shadowsclear water

lowest pixel values in visible and near-infrared are approximation to atmospheric path radianceminimum values subtracted from image

ps LTHL +⋅⋅= ρ

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Regression method

applicable to dark pixel areasnear-infrared pixel values are plotted against values in other bandsleast square line fit using standard regression methodsresulting offset is approximation for atmospheric path radianceoffset subtracted from image

ps LTHL +⋅⋅= ρ

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Radiative transfer models

limited by the need to supply data about atmospheric conditions at time of acquisitionmostly used with "standard atmospheres"available numerical models

LOWTRAN 7MODTRAN 4ATREMATCOR6S (Second Simulation of the Satellite Signal in the Solar Spectrum)

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ATCOR

originally developed DLR (Germany)different versions

satellite imagery – ATCOR 2 (flat terrain), ATCOR 3 (rugged terrain)airborne imagery – ATCOR 4 (flat and rugged)

various versions commercially availablestandalone version in IDLERDAS ImaginePCI Geomatics

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Empirical line method

selection of one dark and one bright targetground reflectance measurement

field radiometer

sensor radiance computed from imageslope s = cos (α) and intercept a of line joining two targetssensor radiance L

Sur

face

refle

ctan

ce R

R = s (L-a)

atmosphericradiance a

brighttarget

darktarget

α

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Haze

due to Rayleigh scatteringparticles size responsible for effect smaller than the radiation's wavelength (e.g. oxygen and nitrogen)

haze has an additive effect resulting in higher DN valuesdecreases the general contrast of the imageeffect is wavelength dependent

more pronounced in shorter wavelengths and negligible in the NIR

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CorrectedHazy

Haze – Example Indonesia

Source: C. Pohl, ITC Source: C. Pohl, ITC

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Questions