In remote sensing imagery, pixel values are called DN (for Digital Numbers) and can not be physically interpreted and compared: they are influenced by various factors such as the amount of light flowing trough the sensor, the gain of the detectors and the analogic to numeric converter.
Depending on the season, the light and atmospheric conditions, the position of the sun or the sensor internal parameters, these DN can drastically change for a given pixel (apart from any ground change effects). Moreover, these effects are not uniform over the spectrum: for instance aerosol amount and type has usually more impact on the blue channel.
Therefore, it is necessary to calibrate the pixel values before any physical interpretation is made out of them. In particular, this processing is mandatory before any comparison of pixel spectrum between several images (from the same sensor), and to train a classifier without dependence to the atmospheric conditions at the acquisition time.
Calibrated values are called surface reflectivity, which is a ratio denoting the fraction of light that is reflected by the underlying surface in the given spectral range. As such, its values lie in the range [0,1]. For convenience, images are often stored in thousandth of reflectivity, so that they can be encoded with an integer type. Two levels of calibration are usually distinguished:
This transformation can be done either with OTB Applications or with Monteverdi. Sensor-related parameters such as gain, date, spectral sensitivity and sensor position are seamlessly read from the image metadata. Atmospheric parameters can be tuned by the user. Supported sensors are :
The OpticalCalibration application allows to perform optical calibration. The mandatory parameters are the input and output images. All other parameters are optional. By default the level of calibration is set to TOA (Top Of Atmosphere). The output images are expressed in thousandth of reflectivity using a 16 bits unsigned integer type.
A basic TOA calibration task can be performed with the following command :
A basic TOC calibration task can be performed with the following command :
These transformations can also be done in Monteverdi.
The 6S model needs atmospheric parameters to be able to compute radiative terms to estimate the atmospheric contributions on the input signal. Default parameters are available in the module. For atmospheric parameters, it is possible to indicate AERONET file. The AERONET (AErosol RObotic NETwork) program is a federation of ground-based remote sensing aerosol networks established by NASA and PHOTONS (Univ. of Lille 1, CNES, and CNRS-INSU) and is greatly expanded by collaborators from national agencies, institutes, universities, individual scientists, and partners. The program provides accessible public domain database of aerosol optical, mircrophysical and radiative properties.
The module produces four outputs: