NDSI - Normalized Difference Soil Index

NDSI Normalized Difference Soil Index is a usefulu index to identify areas where soils are dominant, and to assess soil quality degradation. A normalized difference index is typically applied to highlight the difference between the strongest and weakest spectral responses of a target land cover. NDVI, as an example, was developed to highlight the difference of the spectral responses of vegetation at the red and near infrared bands. Unfortunately, such characteristically strong or weak spectral responses do not exist for soil due to its complex physical and chemical compositions, as well as regional differences. , Deng et al. (2015) found out that the index derived using bands 7 and 2 combination serves as a better alternative as it can adequately enhance the soil information and greatly suppress the Impervious surface area and vegetation values. Normalized difference soil index (NDSI) formulated using TM bands 7 (SWIR) and 2 (NIR narrow) where Band 7 and Band 2 represent the reflectance of TM band 7 (middle infrared) and band 2 (green) respectively.

Hence, bands 7 and 2 were chosen as the best-fit model to compute the NDSI. The formulation of NDSI is expressed 

Generic Formula (R is the reflectance at the wavelengths (nm) denoted by the subscripts):

Formula by Sentinel-2 bands: 

References:

Deng, Y., Wu, C., Li, M., & Chen, R. (2015). RNDSI: A ratio normalized difference soil index for remote sensing of urban/suburban environments. International Journal of Applied Earth Observation and Geoinformation, 39, 40-48.