Applying imaging spectroscopy techniques to map saline soils with ASTER images

This work presents a multi-temporal study (years 2000, 2004 and 2007) of saline soils based on multispectral Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images. Its main contribution is the evaluation of different remote sensing methodologies for mapping saline soils in areas highly affected by soil degradation. The study area is located in SE Spain (between the cities of Elche and Alicante) and is characterized by a semiarid climate and the presence of numerous saline soils and salt-flats. Imaging spectroscopy techniques such as Matched Filtering (MF) and Mixture Tuned Matched Filtering (MTMF) techniques were used for mapping saline soils. Two different approaches were applied for mapping saline soils: 1) using image based spectra of saline and non-saline training areas, and 2) using the spectrum of the halite mineral as a proxy to the spectra of saline soils, because halite concentrations are significant in the saline soils of the study area. MF and MTMF mapping techniques were applied in both methodological approaches. Accuracy assessment was evaluated with the Receiver Operating Characteristic (ROC) plot technique. A validation procedure was applied with an independent saline and non-saline validation dataset. Training and validation saline and non-saline pixels were limited to areas with low vegetation coverage to avoid spectral interferences.High ROC values were obtained for the image based approach in the training and validation stages. Large differences between ROC training vs. validation values were not found. The halite spectrum based approach retrieved lower ROC training and validation values. Furthermore, the MTMF technique achieved better results than the MF in the halite spectrum based approach. Significant differences in the mapping performance were evaluated with the ANOVA test for the time of acquisition of scenes, the methodological approach, and the mapping technique factors. The image based approach was found to be the best methodology for saline soil mapping and monitoring.