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 . 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.