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Last Post 25 Nov 2019 07:45 AM by  MariM
Vegetation indices not falling within range
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Gregory Ross



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11 Nov 2019 09:04 AM
    Hello, we are trying to run vegetation indices on hyperspectral data and we are struggling to get the values within the index range. We are using satellite data from E0-1 Hyperion, as well as data from AVIRIS and our own data collected with an ITRES uCasi. For the E0-1 Hyperion and AVIRIS we have run the data through the pre-processing steps on the website:
    - https://www.harrisgeospatial.com/docs/HyperionVegetationAnalysisTutorial.html
    - http://www.harrisgeospati...reprocessaviris.html

    (AVIRIS and Hyperion data has been Radio corrected and atmospherically corrected and is now in surface reflectance)
    And with those datasets we can not get Cellulose absorption index to function within it's defined range. For example with the Hyperion data were getting values in the - 400, -500, -700 range. For the AVIRIS data were getting values in the -100s.

    None of the indices being used with our uCasi data are falling within the defined ranges. We tried running:
    - Modified red edge NDVI
    - NDVI
    - Photochemical Reflectance Index
    - Plant senescence reflectance index
    - Red edge NDVI
    - Structure Insensitive Pigment Index
    - Water Band index
    And while some of the resulting data falls within the ranges (mostly the normalized data), all of those indices have values that exceed the expected range which makes us think something is going wrong in the processing.
    Recently I have found some answers in the forum which suggest rescaling the values between 0-1 in float format before running the indices however, I can not find clear instructions on how to do this. When I go to view metadata and change the format to float the image goes black, so I am not certain that I have even done this step properly.

    MariM



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    25 Nov 2019 07:45 AM
    Many of the indices have very narrow band requirements as discussed here:
    https://www.harrisgeospatial.com/docs/SpectralIndices.html#Narrowba

    So the bands must fall within the range to produce an output SI. However NDVI can be calculated with wide bands.
    I would also suggest looking at the statistics of your atmospherically corrected data. If you used FLAASH or Quac to correct your data, the output is integers scaled by 10000, which saves on disk space compared to floating point reflectance that ranges from 0-1.0. You will want to convert the result back to floating point % reflectance for many of the indices to produce output in the expected range as discussed here:
    https://www.harrisgeospatial.com/docs/SpectralIndices.html#Scale

    If a significant amount of the pixels in the data cube fall outside of this range, then you may wish to investigate the input radiance cube to FLAASH/Quac to see if the radiance values look reasonable. If they are unreasonable (for example, negative values) then modelling the atmospheric correction will also be off.
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