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I am trying to calculate an NDSI (Normalized Difference Snow Index) using Landsat 8 OLI level 1 imagery atmospherically corrected in ERDAS ATCOR using raster calculator in ArcMap. NDSI is used to to detect the presence of snow cover and is calculated by using the following equation:

(band 3 - band 6)/(band 3 + band 6)

When I calculate index on the raw imagery I get a suitable index that discriminates snow cover from other land cover and ranges from -1 to 1. However, if I try to do same index on the atmospherically corrected imagery I get a range from 1189 to -1135 and a very weird output image.

I noticed that the imagery in the raw imagery had a pixel type and depth of 16 bit unsigned integer compared to the corrected imagery which had a pixel type and depth of 16 bit signed integer. So I'm assuming this is the case. Somehow in the process of atmospheric correction in ERDAS the imagery was converted from 16 bit unsigned to 16 bit signed.

A snip of the NDSI created using the 16 bit signed imagery

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