Pre-processing of Landsat-8 OLI/TIRS data stage consists
of those operations that prepare data for subsequent analysis that attempts to
correct or compensate for systematic errors. The digital images are subjected
to several corrections such as radiometric and atmospheric. Landsat-8 data were
converted to surface reflectance by
top-of-atmosphere (TOA)
method Using Envi, which is recommended for calibration in
mineralogical mapping, as it does not require prior Knowledge of samples
collected in the field.
Thermal atmospheric correction was performed on
TIR bands with a normalized pixel regression method [1].
The 90-m resolution TIR
bands were re-sampled to correspond to 30-m spatial dimensions for some image
processing applications. Nearest neighbor re-sampling Was used
to preserve the original pixel values in the re-sampled images.
Radiometric correction is done to reduce or correct errors
in the digital numbers of images. The process improves the interpretability and
quality of remote sensed data. Radiometric calibration and correction are
particularly important when comparing data sets over a multiple time periods.
The energy that sensors onboard aircrafts or satellites record can differ from
the actual energy emitted or reflected from a surface on the ground. This is
due to the sun's azimuth and elevation and atmospheric conditions that can
influence the observed energy. Therefore, in order to obtain the real ground
irradiance or reflectance, radiometric errors must be corrected for. The value
recorded for a given pixel includes not only the reflected or emitted radiation
from the surface, but also the radiation scattered and emitted by the
atmosphere. In most cases were are interested in the actual surface values. To
achieve these values radiometric calibration and correction must be applied.
Calibrated A sensor records the intensity of
the electromagnetic radiation for each pixel as a digital number (DN). These
digital numbers can be converted to more meaningful real world units like
radiance, reflectance or brightness temperature. Sensor specific information is
needed to carry out this calibration. In the case of Landsat data, the metadata
file contains this information. Most image processing software packages have
radiometric calibration tools. In ENVI some Landsat data can be converted
directly to reflectance, with out needing to first calculate radiance. The Radiometric calibration has
been done by Envi.
Atmospheric correction is the process of removing the
effects of the atmosphere to produce surface reflectance values. Atmospheric
correction can significantly improve the interpretability and use of an image.
Ideally this process requires knowledge of the atmospheric conditions and
aerosol properties at the time the image was acquired. The data had been corrected by
FLAASH Module in ENVI.
