Post on 27-Jan-2015
description
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Orfeo Toolbox Radiometric corrections
Orfeo Toolbox Radiometric corrections
Stéphane MAY
stephane.may@cnes.fr
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Radiometry = science of electromagnetic radiations
Interest for Remote Sensing
Physical interpretation of signals
Enhance detection
Estimate physical values related to ground and / or
atmospheric effects
Radiometry Radiometry
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Goal : using physical measures
TOA reflectance (Top of Atmosphere)
TOC reflectance (Top of Canopy)
Calibration of images : DN (digital numbers) converted into TOA reflectance
Application of calibration coefficient inserted into metadata
files → Luminance
Normalisation with solar effect →
Reflectance
Example for Spot :
Radiometry : from DN to TOA reflectance Radiometry : from DN to TOA reflectance
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LuminanceLuminance
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LuminanceLuminance
Definition
Emitted power per unit area and per unit solid angle
Unit
W.m-2.sr-1
Numerical example:
Luminance of the sky (visible): ~ 6 W.m-2.sr-1
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TOA reflectanceTOA reflectance
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Atmospheric 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
Radiometry : from TOA reflectance to TOC reflectance Radiometry : from TOA reflectance to TOC reflectance
Cloud
Aerosols
Molecules
Ozone
ScatteringAbsorption
Emission Water vapor
Reflection
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Atmosphere
Thickness ~100 km
Filter descending and ascending radiations
Composition 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
Atmospheric effects Atmospheric effects
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85
50
10
Altitude (km)
800
0 Sol
Tropopause
Stratopause
TROPOSPHERE
STRATOSPHERE
THERMOSPHERE
EXOSPHERE
Ozone
Vapeurd'eau
AérosolsNuages
MESOSPHERE
Mesopause
Profil de Températures
-90°C
0°C
-55°C
+15°C
Atmospheric effects Atmospheric effects
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Atmospheric effects Atmospheric effects
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Three 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 cm
The contribution of the signal from the ground in the measured signal is disturbed by filtering effects and luminance of the atmosphere
Need to correct measures
Need to understand the phenomena
Atmospheric effectsAtmospheric effects
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Atmospheric 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
Radiometry : 6S modelRadiometry : 6S model
ρTOC
ρTOA
0 0,02 0,80,01 0,03
Mesure
Valeur sol interpolée
Simulations TOA
Géo d’observationConditions atmosphériques
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Atmospheric 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
Radiometry : 6S parametersRadiometry : 6S parameters
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Atmospheric 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.
Radiometry : 6S parametersRadiometry : 6S parameters
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Spectral 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)
Radiometry : 6S parametersRadiometry : 6S parameters
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AERONET (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
Aeronet dataAeronet data
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Aerosol optical depth data are computed for three data quality levels
Level 1.0 (unscreened)
Level 1.5 (cloud-screened)
Level 2.0 (cloud screened and quality-assured).
Extract Aerosol thickness (épaisseur) and Water amount
Download site
http://aeronet.gsfc.nasa.gov/cgi-bin/webtool_opera_v2_new
Aeronet data and OTB Aeronet data and OTB
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Mesure 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
Radiometry – Reference reflectancesRadiometry – Reference reflectances
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orfeo-toolbox.orgLongueur d’onde (µm)
EauChlorophylleABSORPTION due à :
REFLECTION due à : pigmentde la feuille
Structurecellulaire
Teneur en eau
VisibleProche
Infra-RougeMoyen Infra-Rouge
Radiometry Radiometry
Spectral signature of the chlorophyllian vegetation
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3 processing steps
Digital Number to Luminance
Luminance to TOA reflectance
TOA reflectance to TOC surface reflectance
Many parameters available in the metadata of sensors
Importance of aerosols to quantify the effects of the
atmosphere
Difficulties
Aerosol model used?
Validation of results?
Ground truth?
Radiometry processing - SynthesisRadiometry processing - Synthesis
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Optical calibration module (1/2)Optical calibration module (1/2)
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Optical calibration module (2/2)Optical calibration module (2/2)
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Menu File > Open
./img_kalideos_reunion/2009-03-21/IMAGERY.TIF
Menu 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 / quit
File > Save (Luminance, TOA, TOC, TOA-TOC files)
Repeat the Calibration process with other parameters set
Visualization > Viewer : compare results
Use case 1 : radiometric correctionUse case 1 : radiometric correction
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Use 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
Use case 1b : radiometric correctionUse case 1b : radiometric correction
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Spectral viewer (1/2)Spectral viewer (1/2)
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Spectral viewer (2/2)Spectral viewer (2/2)
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Menu File > Open
./img_kalideos_reunion/2009-03-21/IMAGERY.TIF
Menu Viewer > Spectral viewer
Left click : select your area
Right click : select pixel
Spectral angle menu > Select one curve ID
➢Click on Compute button
File > Save (the Spectral angle file). Use Float data type
Create several reference spectral angle images : roads, houses, sea, clouds, vegetation, bare soil
Use case 2 : Spectral viewerUse case 2 : Spectral viewer
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Thank you for your attention !
Monteverdi Monteverdi