# Lithological Mapping Research Based on Feature Selection Model of Relief-RF
## 3 Material and Methods
### Pre-Processing of Data  
(1) Sentinel-2  
The Sentinel-2A/B sensor can obtain 13 band multispectral images with 4 bands at 10 m, 6 bands at 20 m and 3 bands at 60 m. The 13 spectral bands span from the visible and near-infrared to the shortwave infrared. The downloadable Sentinel-2B data (acquired on 9 September 2018) are distributed at the L1C (TOA reflectance) level with the same projection and the same global geodetic system as Landsat-8 data. The data are only needed to be atmospherically corrected. The sen2 cor plug-in developed by the European Space Agency was used for atmospheric correction. Spectral bands 1, 9 and 10 from the Sentinel-2 data with 60 m spatial resolution were disregarded because they were designed for atmospheric correction. Six 20 m bands were resampled to a 10 m spatial resolution by the nearest-neighbour model, whereas the remaining ten bands were layer-stacked as a single file. The radiometric and atmospheric corrections of the Sentinel-2 images were used SNAP 7.0 software then the corrected Sentinel-2 images were converted to ENVI format.  
(2) Landsat-8 OLI  
Landsat-8 carries two sensors, the OLI and thermal infrared sensors. The former contains nine spectral bands, whereas the latter records thermal data with two bands, as shown in Table 2. In this study, one Landsat-8 OLI scene acquired on 5 December 2017 was selected for lithological discrimination with cloud-free cover in the study area. These data are distributed at the L1T (corrected terrain) level, with a Universal Transverse Mercator projection and a World Geodetic System WGS 84 datum. In geological research, bands 1 and 9 are not used because the first band is intended for recovering the properties of atmospheric aerosols, whereas the ninth band is specific to the study of cirrus [41]. Here, the VNIR and SWIR bands of Landsat-8 OLI were layer-stacked to a single file with a 30 m spatial resolution. Radiation correction and atmospheric corrections by FLAASH were applied to Landsat-8 OLI scene to eliminate the effects caused by the atmosphere using ENVI 5.3 software.  
Figure 3: Flowchart of lithological mapping research.  
(3) ASTER  
ASTER covers a broad range of spectral regions. It has 14 spectral bands, namely, 3 VINR bands with a 15 m spatial resolution, 6 SWIR bands with a 30 m spatial resolution and 5 TIR bands with a 90 m spatial resolution. Furthermore, another telescope is used for backward viewing in the near-infrared spectral band (Band 3B). The ASTER data employed in this study consisted of a cloud-free level 1T scene acquired on 3 October 2006. The image was pre-georeferenced to the UTM zone at a 46-north projection with the WGS-84 datum. Radiation and atmospheric corrections were applied using fast line-of-sight atmospheric analysis of spectral hypercubes (FLAASH) in the ENVI 5.3 software. Furthermore, cross-tracking correction was applied to ASTER to remove the effects of energy overspill from band 4 into bands 5 to 9 [42]. Six 30 m bands were resampled to a 15 m spatial resolution by the nearest-neighbour model, whereas the remaining fifteen bands were layer-stacked as a single file using ENVI 5.3 software.  
Then, the pre-processing images of Landsat-8 OLI and ASTER were georeferenced and co-registered using Sentinel-2 as a spatial reference and ground control points were chosen from the 30 m DEM.