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You are advised to first calibrate surface reflectance of the image. Then, you should check the minimum and maximum reflectance values in each band. If your minimum values are showing negative values just replace them with zero (0). Use this formula in Envi Band math>>>
(b1 le 0)*0+(b1 ge 10000)*1+(b1 gt 0 and b1 lt10000)*float(b1)/10000
where b1 is band number.
for NDWI calculation, you can write your formula in Envi band math like...
(((float(bi)-float(bj))/(float(bi)+float(bj))))
where bi and bj are bands that are use for calculating NDWI. After all these steps, you can get the NDWI ranging from -1 to 1.
Cheers!!
Rajesh
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