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Using ENVI 5.3. I have done a radiometric calibration(radiance) and flaash atmospheric correction then a seamless mosaic and subsetted data from ROI. I then calculated the NDVI. My results in May 2017 were correct but I have a problem in May 2018.

Some results are out of range (-1,1). I used (float(b5)-float(b4))/(float(b5)+float(b4)).

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    It seems likely the detail of your implementation matters here. Can you say how you performed the operation? Please edit your question to show what software you used, the exact code you used, and the actual results you saw.
    – BradHards
    Commented Jun 16, 2019 at 7:20
  • i use ENVI 5.3. i do radiometric calibration(radiance) and flaash atmospheric correction then seamless mosaic and subset data from ROI. then calculate NDVI. my resulte in may 2017 were correct but i have problem in may 2018.
    – soroush
    Commented Jun 16, 2019 at 7:51
  • What are the ranges of b4 and b5 in the original Landsat image? Is there for example any negative value? Commented Jun 16, 2019 at 8:58
  • the range of b4 and b5 in original landsat images are positive. b4 in one of them is between 0 and 58074 and b5 is between 0 and 60018. in other image all bands are in range (0,65535)
    – soroush
    Commented Jun 16, 2019 at 19:16
  • Your band values seem really high. Are you using reflectivity values? gisgeography.com/ndvi-normalized-difference-vegetation-index Commented Jun 17, 2019 at 15:27

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Can't hurt to check your order of operations. ENVI is very picky.

((float(b5))-(float(b4)))/((float(b5))+(float(b4)))

Also, spot check a couple of pixels manually to see how the math is working out. Is it correct for any of them?

Check the 2018 bands and make sure you are running the band math on the correct layers. Also, check the values on the year giving you problems. Maybe you need to redo your corrections on that file OR there is bad data.

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