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Madagascar2011 - 07 - OTB radiometry processing

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  • 1. Orfeo ToolboxRadiometric corrections Stéphane MAY stephane.may@cnes.fr orfeo-toolbox.org 1
  • 2. RadiometryRadiometry = science of electromagnetic radiationsInterest for Remote Sensing Physical interpretation of signals Enhance detection Estimate physical values related to ground and / or atmospheric effects orfeo-toolbox.org 2
  • 3. Radiometry : from DN to TOA reflectanceGoal : using physical measures TOA reflectance (Top of Atmosphere) TOC reflectance (Top of Canopy)Calibration of images : DN (digital numbers) converted intoTOA reflectance Application of calibration coefficient inserted into metadata files → Luminance Normalisation with solar effect → Reflectance Example for Spot : orfeo-toolbox.org 3
  • 4. Luminance orfeo-toolbox.org 4
  • 5. LuminanceDefinition Emitted power per unit area and per unit solid angleUnit W.m-2.sr-1Numerical example: Luminance of the sky (visible): ~ 6 W.m-2.sr-1 orfeo-toolbox.org 5
  • 6. TOA reflectance orfeo-toolbox.org 6
  • 7. Radiometry : from TOA reflectance to TOC reflectanceAtmospheric correction : inversion of surface TOC reflectance Need to caracterise atmospheric conditions ➢TOMS / TOAST data : (grid : 1,25°lon x 1°lat ~ 135 x 110 km) : stratospheric Ozone amount NCEP meteo data : (grid : 2,5°lon x 2,5°lat ~ 270 x 270 km) : Atmospheric Pressuree, water vapor, wetness SeaWiFS or MODIS data : (grid : 5’ x 5’ ~ 9 x 9 km) ) : Optical aerosols thickness Cimel of aeronet network : information about aerosols, and water amount Absorption Ozone Scattering Cloud Emission Water vapor Molecules Aerosols Reflection orfeo-toolbox.org 7
  • 8. Atmospheric effectsAtmosphere Thickness ~100 km Filter descending and ascending radiationsComposition of the atmosphere: Molecules nitrogen (78%), oxygen (21%), argon, carbon oxides, ozone, water vapor (highly variable, even locally) ... Aerosols small solid or liquid particles suspended in the atmosphere. Microparticles, water particles, ice crystals, smoke ... Dimensions: mainly from 0.1 to 10 µm Abundance and type varies geographically and with time orfeo-toolbox.org 8
  • 9. Atmospheric effectsAltitude (km) EXOSPHERE800 THERMOSPHERE -90°C85 Mesopause Profil de Températures MESOSPHERE50 Stratopause 0°C STRATOSPHERE Ozone10 Tropopause -55°C TROPOSPHERE Nuages Aérosols Vapeur deau +15°C 0 Sol orfeo-toolbox.org 9
  • 10. Atmospheric effects orfeo-toolbox.org 10
  • 11. Atmospheric effectsThree main phenomena, depending of wavelength Absorption Diffusion (in the visible and NIR, up to ~ 3 µm) Emission (in the thermic IR, from ~ 3 µm) Optional : refraction, usually neglected for high elevation acquisition and not included spatial resolutions greater than 10 cmThe contribution of the signal from the ground in the measuredsignal is disturbed by filtering effects and luminance of theatmosphere Need to correct measures Need to understand the phenomena orfeo-toolbox.org 11
  • 12. Radiometry : 6S modelAtmospheric corrections : invert the TOC surface reflectance.Simulation with radiative transfer code 6S code Diffusion : code of the successive diffusion orders Absorptions : O2, CO2, H2O, O3, N2O, CH4 Spectro data at 10 cm-1« AFGL atmospheric absorption line parameters compilation » (1991) Look-Up Tables : refl. TOC -> TOA Mesure ρTOA Simulations TOA Géo d’observation Conditions atmosphériquesρTOC 0 0,01 0,02 0,03 0,8 Valeur sol interpolée orfeo-toolbox.org 12
  • 13. orfeo-toolbox.org 13
  • 14. Radiometry : 6S parametersAtmospheric parameters => parameters of radiative transfer The zenithal and azimutal solar angles that describe the solar incidence configuration (in degrees) The zenithal and azimuthal viewing angles that describe the viewing direction (in degrees) The month and the day of the acquisition The atmospheric pressure The water vapor amount, that is, the total water vapor content over vertical atmospheric column orfeo-toolbox.org 14
  • 15. Radiometry : 6S parametersAtmospheric parameters => parameters of radiative transfer The ozone amount that is the Stratospheric ozone layer content; The aerosol model that is the kind of particles (no aerosol, continental, maritime, urban, desertic) The aerosol optical thickness at 550 nm that is the is the Radiative impact of aerosol for the reference wavelength 550 nm The filter function that is the values of the filter function for one spectral band, from λinf to λsup by step of 2.5 nm. One filter function by channel is required. This last parameter are read in text files, the other one are directly given to the class. orfeo-toolbox.org 15
  • 16. Radiometry : 6S parametersSpectral sensitivity file Data provided in OTB-Data (not with Monteverdi) ➢OTB-Data/Input/Radiometry/ For each band with a step of 0.25 nm, the spectral band sensitivity of the instrument ➢Input file : values in the Atmospheric parameters window ➢No input file : iso sensitivity (1 everywhere) or default file automatically loaded (>OTB V3.10) orfeo-toolbox.org 16
  • 17. Aeronet dataAERONET (AErosol RObotic NETwork) This program is a federation of ground-based remote sensing aerosol networks established by NASA and LOA-PHOTONS (CNRS) and is greatly expanded by collaborators orfeo-toolbox.org 17
  • 18. Aeronet data and OTBAerosol optical depth data are computed for three data qualitylevels Level 1.0 (unscreened) Level 1.5 (cloud-screened) Level 2.0 (cloud screened and quality-assured).Extract Aerosol thickness (épaisseur) and Water amountDownload site http://aeronet.gsfc.nasa.gov/cgi-bin/webtool_opera_v2_new orfeo-toolbox.org 18
  • 19. Radiometry – Reference reflectancesMesure of the reflectance of a source Principle : use a source and measure reflected luminance ➢ Direct measure : known source ➢ Indirect measure : use of a known reference reflectance surface (spectralon, or BaSO4) Measures in laboratory ➢ Artificial source : directional, mobile (lamp, laser) ➢ Sensor (radiometer or spectro-radiometer) : mobile Direct measure or indirect measure orfeo-toolbox.org 19
  • 20. RadiometrySpectral signature of the chlorophyllian vegetation Proche Visible Moyen Infra-Rouge Infra-RougeREFLECTION due à : pigment Structure Teneur en eau de la feuille cellulaireABSORPTION due à : Chlorophylle Eau Longueur d’onde (µm) orfeo-toolbox.org 20
  • 21. Radiometry processing - Synthesis3 processing steps Digital Number to Luminance Luminance to TOA reflectance TOA reflectance to TOC surface reflectanceMany parameters available in the metadata of sensors Importance of aerosols to quantify the effects of the atmosphereDifficulties Aerosol model used? Validation of results? Ground truth? orfeo-toolbox.org 21
  • 22. Optical calibration module (1/2) orfeo-toolbox.org 22
  • 23. Optical calibration module (2/2) orfeo-toolbox.org 23
  • 24. Use case 1 : radiometric correctionMenu File > Open ./img_kalideos_reunion/2009-03-21/IMAGERY.TIFMenu Calibration > Optical calibration Load Aeronet file ➢ ./aeronet/070101_091231_REUNION_ST_DENIS.lev20 Load spectral sensity file ➢ ./OTB-Data/Input/Radiometry/SPOT5/HRG1/rep6S.dat Change correction parameters Change Radiative terms Save / quitFile > Save (Luminance, TOA, TOC, TOA-TOC files)Repeat the Calibration process with other parameters setVisualization > Viewer : compare results orfeo-toolbox.org 24
  • 25. Use case 1b : radiometric correctionUse the commande line tool : OtbOpticalCalibration-cli -h otbOpticalCalibration-cli -in IMAGERY.TIF -out img_toc.tif -level TOC -AerosolModel 0 –OzoneAmount 0 --AtmosphericPressure 1030 --AerosolOptical 0.0329 –WaterVaporAmount 4.226 -aeronet file.lev20 -srs rep6S.dat orfeo-toolbox.org 25
  • 26. Spectral viewer (1/2) orfeo-toolbox.org 26
  • 27. Spectral viewer (2/2) orfeo-toolbox.org 27
  • 28. Use case 2 : Spectral viewerMenu File > Open ./img_kalideos_reunion/2009-03-21/IMAGERY.TIFMenu Viewer > Spectral viewer Left click : select your area Right click : select pixel Spectral angle menu > Select one curve ID ➢ Click on Compute buttonFile > Save (the Spectral angle file). Use Float data typeCreate several reference spectral angle images : roads, houses, sea,clouds, vegetation, bare soil orfeo-toolbox.org 28
  • 29. MonteverdiThank you for your attention ! orfeo-toolbox.org 29