Perform optical calibration TOA/TOC (Top Of Atmosphere/Top Of Canopy). Supported sensors: QuickBird, Ikonos, WorldView2, Formosat, Spot5, Pleiades, Spot6. For other sensors the application also allows providing calibration parameters manually.
The application allows converting pixel values from DN (for Digital Numbers) to reflectance. Calibrated
values are called surface reflectivity and its values lie in the range [0, 1].
The first level is called Top Of Atmosphere (TOA) reflectivity. It takes into account the sensor gain, sensor
spectral response and the solar illuminations.
The second level is called Top Of Canopy (TOC) reflectivity. In addition to sensor gain and solar
illuminations, it takes into account the optical thickness of the atmosphere, the atmospheric pressure,
the water vapor amount, the ozone amount, as well as the composition and amount of aerosol
gasses.
It is also possible to indicate an AERONET file which contains atmospheric parameters (version 1 and
version 2 of Aeronet file are supported. Note that computing TOC reflectivity will internally compute first
TOA and then TOC reflectance.
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If the sensor is not supported by the metadata interface factory of OTB, users still have the possibility to
give the needed parameters to the application.
For TOA conversion, these parameters are :
- day and month of acquisition, or flux normalization coefficient;
- sun elevation angle;
- gains and biases, one pair of values for each band (passed by a file);
- solar illuminations, one value for each band (passed by a file).
For the conversion from DN (for Digital Numbers) to spectral radiance (or ’TOA radiance’) L, the
following formula is used :
(1) L(b) = DN(b)/gain(b)+bias(b) (in W/m2/steradians/micrometers) with b being a band ID.
These values are provided by the user thanks to a simple txt file with two lines, one for the gains and one for
the biases.
Each value must be separated with colons (:), with eventual spaces. Blank lines are not allowed. If a line
begins with the ’#’ symbol, then it is considered as comments.
Note that sometimes, the values provided by certain metadata files assume the formula L(b) =
gain(b)*DC(b)+bias(b).
In this case, be sure to provide the inverse gain values so that the application can correctly interpret
them.
In order to convert TOA radiance to TOA reflectance, the following formula is used :
(2) R(b) = (pi*L(b)*d*d) / (ESUN(b)*cos(Î)) (no dimension) where :
- L(b) is the spectral radiance for band b
- pi is the famous mathematical constant (3.14159...)
- d is the earth-sun distance (in astronomical units) and depends on the acquisition’s day and month
- ESUN(b) is the mean TOA solar irradiance (or solar illumination) in W/m2/micrometers
- Î is the solar zenith angle in degrees.
Note that the application asks for the solar elevation angle, and will perfom the conversion to the zenith
angle itself (ze. angle = 90° - el. angle).
Note also that ESUN(b) not only depends on the band b, but also on the spectral sensitivity of the sensor in
this particular band. In other words, the influence of spectral sensitivities is included within the ESUN
different values.
These values are provided by the user thanks to a txt file following the same convention as
before.
Instead of providing the date of acquisition, the user can also provide a flux normalization coefficient ’fn’.
The formula used instead will be the following :
(3) R(b) = (pi*L(b)) / (ESUN(b)*fn*fn*cos(Î))
Whatever the formula used (2 or 3), the user should pay attention to the interpretation of the parameters he
will provide to the application, by taking into account the original formula that the metadata files
assumes.
Below, we give two examples of txt files containing information about gains/biases and solar illuminations
:
- gainbias.txt :
# Gain values for each band. Each value must be separated with colons (:), with eventual spaces. Blank lines
not allowed.
10.4416 : 9.529 : 8.5175 : 14.0063
# Bias values for each band.
0.0 : 0.0 : 0.0 : 0.0
- solarillumination.txt :
# Solar illumination values in watt/m2/micron (’micron’ means actually ’for each band’).
# Each value must be separated with colons (:), with eventual spaces. Blank lines not allowed.
1540.494123 : 1826.087443 : 1982.671954 : 1094.747446
Finally, the ’Logs’ tab provides usefull messages that can help the user in knowing the process different
status.
This section describes in details the parameters available for this application. Table 4.40, page 501 presents a summary of these parameters and the parameters keys to be used in command-line and programming languages. Application key is OpticalCalibration.
Parameter key | Parameter type |
Parameter description |
in | Input image |
Input |
out | Output image |
Output |
ram | Int |
Available RAM (Mb) |
level | Choices |
Calibration Level |
level toa | Choice |
Image to Top Of Atmosphere reflectance |
level toatoim | Choice |
TOA reflectance to Image |
level toc | Choice |
Image to Top Of Canopy reflectance (atmospheric corrections) |
milli | Boolean |
Convert to milli reflectance |
clamp | Boolean |
Clamp of reflectivity values between [0, 100] |
acqui | Group |
Acquisition parameters |
acqui.minute | Int |
Minute |
acqui.hour | Int |
Hour |
acqui.day | Int |
Day |
acqui.month | Int |
Month |
acqui.year | Int |
Year |
acqui.fluxnormcoeff | Float |
Flux Normalization |
acqui.sun | Group |
Sun angles |
acqui.sun.elev | Float |
Sun elevation angle (°) |
acqui.sun.azim | Float |
Sun azimuth angle (°) |
acqui.view | Group |
Viewing angles |
|
||
acqui.view.elev | Float |
Viewing elevation angle (°) |
acqui.view.azim | Float |
Viewing azimuth angle (°) |
acqui.gainbias | Input File name |
Gains — biases |
acqui.solarilluminations | Input File name |
Solar illuminations |
atmo | Group |
Atmospheric parameters (for TOC) |
atmo.aerosol | Choices |
Aerosol Model |
atmo.aerosol noaersol | Choice |
No Aerosol Model |
atmo.aerosol continental | Choice |
Continental |
atmo.aerosol maritime | Choice |
Maritime |
atmo.aerosol urban | Choice |
Urban |
atmo.aerosol desertic | Choice |
Desertic |
atmo.oz | Float |
Ozone Amount |
atmo.wa | Float |
Water Vapor Amount |
atmo.pressure | Float |
Atmospheric Pressure |
atmo.opt | Float |
Aerosol Optical Thickness |
atmo.aeronet | Input File name |
Aeronet File |
atmo.rsr | Input File name |
Relative Spectral Response File |
atmo.radius | Int |
Window radius (adjacency effects) |
atmo.pixsize | Float |
Pixel size (in km) |
inxml | XML input parameters file |
Load otb application from xml file |
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outxml | XML output parameters file |
Save otb application to xml file |
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|
Input Input image filename (values in DN)
Output Output calibrated image filename
Available RAM (Mb) Available memory for processing (in MB)
Calibration Level Available choices are:
Convert to milli reflectance
Flag to use milli-reflectance instead of reflectance.
This allows saving the image with integer pixel type (in the range [0, 1000] instead of floating point in the
range [0, 1]. In order to do that, use this option and set the output pixel type (-out filename double for
example)
Clamp of reflectivity values between [0, 100] Clamping in the range [0, 100]. It can be useful to preserve area with specular reflectance.
Acquisition parameters This group allows setting the parameters related to the acquisition conditions.
Atmospheric parameters (for TOC) This group allows setting the atmospheric parameters.
Available choices are:
Load otb application from xml file Load otb application from xml file
Save otb application to xml file Save otb application to xml file
To run this example in command-line, use the following:
To run this example from Python, use the following code snippet:
None
This application has been written by OTB-Team.
These additional ressources can be useful for further information: