The Efficiency of Landsat TM and ETM+ Thermal Data for Extracting Soil Information in Arid Regions

A knowledge of soil surface conditions, especially desert crust, salt crust and desert varnish is usef ul for improving classif ication of remotely sensed data. Desert crust can generate high levels of ref lectance, similar to those areas with high salt concentration and non-saline soil. Therefore, soil surface crusts might bias thematic remote sensing of soils. In this study, we evaluated the ef f iciency of the Thematic Mapper (TM) and Enhanced Thematic Mapper plus (ETM+) ref lective and thermal bands in detecting crusted surfaces and soil salinity conditions. The study areas were Ardakan, Damghan, Lut Desert, Qom, and Abarkooh which are located in arid regions of Iran. To assess the Landsat TM thermal data for detecting land cover types, the following steps were taken: 1) determination of correlation coef f icients between TM wavebands, 2) assessment of the relationship between TM thermal and TM ref lective bands on land cover types, 3) assessment of the relationship between soil salinity and TM Digital Numbers (DN), 4) two dimensional Feature Space (FS) analysis of the training samples, 5) f ield sampling, 6) image classification and accuracy assessment, and 7) comparison of surface ref lectance of dif f erent soil surface types. The results show that the trend of correlation coef f icients of TM6 with ref lective bands is completely dif f erent f rom the correlation between ref lective bands. The behaviour of the thermal band on gypsiferous soils is completely dif f erent f rom that on saline soils. Moreover, with an increasing correlation between soil salinity and ref lective bands, the correlation between soil salinity and the thermal band decreases. In image classification,the thermal band improved the separability of the crusted and gravely classes. Therefore the TM/ETM+ regions of the electro-magnetic spectrum have complementary capabilities for spectral separability of gravely and crusted surfaces. In general, selection of the TM/ETM+ thermal band combination is an important step for classifying the remote sensing data and for securing class separability of gravely and crusted surfaces in arid regions. We also concluded that TM/ETM+ thermal bands may contain information complementary to the TM/ETM+ ref lective bands and therefore this combination of the TM/ETM+ thermal and ref lective bands provide a viable method for soil salinity studies in arid regions.