X
PrevPrev Go to previous topic
NextNext Go to next topic
Last Post 07 Apr 2010 10:22 PM by  anon
NDVI Error
 1 Replies
Sort:
You are not authorized to post a reply.
Author Messages

anon



New Member


Posts:
New Member


--
07 Apr 2010 10:22 PM
    Dear Forum, I have a problem in calculating NDVI in ENVI (4.5). I have downloaded Landsat 5 TM data from USGS site and am trying to compute NDVI using tranformation option. Firstly, I have converted, my DN value to TOA reflectance values for all the bands. The problem is , when I compute stats for band3 (Red) and Band4(NIR) separately, it gives me min and max value ranging from -0.001 to 0.36 for band 3 and -0.001 to 0.39 for band 4. That is Ok. Now when I calculate NDVI from these two bands, the NDVI should range between - 1 to +1, but it is showing min and max value of -2 to 38. Even I tried band math to compute NDVI but with the same results. It is quite strange, as i have other data and calculated NDVI with expected results. Could you please explain me the reason for this and how to correct it. Regards, PK

    MariM



    Veteran Member


    Posts:2396
    Veteran Member


    --
    08 Apr 2010 07:55 AM
    NDVI is a fairly simple equation and if you are getting the same results doing a manual calculation in band math, then go back to the R and NIR bands to see what values are in those pixels which fall outside of the range.  Link the resulting NDVI with the R and NIR bands, then look at the pixel values in each and do some single pixel calculations.  This should show you why you get the values you do.  Perhaps the outlying values are in just a few pixels or background areas.  If the out of range values occur throughout the image, then you may need to look at the original calibration.  I know there were many updates in the Landsat calibration tool for ENVI 4.7.  Also be sure you are outputting the NDVI results to floating point and not byte.
    You are not authorized to post a reply.

    Forums Products ENVI