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i have downloaded the modis MOD13Q1 from usgs glovis in hdf format. how i will convert it into original ndvi value by multiplying scale factor of 0.0001. I am using ERDAS 9.1 & ENVI 4.7.

Can you tell me the step by step procedure get the ndvi value?

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In ENVI go to "Band math" and tape: float(b1)/0.0001 look at http://www.exelisvis.fr/docs/BandMath.html , Actually there is no need for this, the reason why values ​​are multiplied by 0.0001 is only for size decreasing of files. Classification adopted by MODIS is [0.2 to 0] =Water, [0 to 0.2]=No vegetation,sand, spare vegetation ... [0.2 to 1]= Vegetation , so equivalently, you can classified as follows : [-2000 to 0]= water, [0 to 2000]= No vegetation,sand, spare vegetation ..., and [2000 to 10000]=vegetation (keeping the same original values ​​with little space in your H.D.D)

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