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 Last Post 06 Dec 2017 03:14 PM by  MariM LAI inbuilt spectral indices tool limitations  10 Replies Sort: Oldest First Most Recent First
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Yahampath Marambe

New Member

Posts:5

 11 Oct 2017 02:31 PM I am using ENVI to produce LAI map series. ENVI use the following equation to calculate LAI. (Eq1) - LAI= (3.618*EVI) - 0.118 However, the value range of EVI is -1 to +1. Then the maximum value of LAI should be 3.50 according to the equation if EVI reaches to its maximum. Typically, ENVI gives LAI values greater than that value (3.50) also. Is ENVI using a higher upper limit for EVI than the typical range (-1 to +1)? This equation also gives negative values for the LAI. I would be grateful if could describe me regarding this. Thank you!

MariM

Veteran Member

Posts:2396

 12 Oct 2017 12:56 PM It is important that your input data has been converted to reflectance (either TOA or surface) between 0-1.0. Is your input data within this range?

Yahampath Marambe

New Member

Posts:5

 14 Oct 2017 09:10 PM yes. I corrected my images with ENVI FLAASH and reflectance range between 0-1. If we just substitute numbers into LAI= (3.618*EVI) - 0.118 equation we can see maximum value for LAI can be 3.5 (for max EVI +1). However, LAI can go higher than 3.5. ENVI gives values greater than 3.5. Problem is if we use the same equation in band math, the result is different. It is limited to 3.5. I seek your guidance further. Thank you.....!!!!

Yahampath Marambe

New Member

Posts:5

 14 Oct 2017 09:12 PM Thank you for the reply, yes. I corrected my images with ENVI FLAASH and reflectance range between 0-1. If we just substitute numbers into LAI= (3.618*EVI) - 0.118 equation we can see maximum value for LAI can be 3.5 (for max EVI +1). However, LAI can go higher than 3.5. ENVI gives values greater than 3.5. Problem is if we use the same equation in band math, the result is different. It is limited to 3.5. I seek your guidance further. Thank you.....!!!!

MariM

Veteran Member

Posts:2396

Yahampath Marambe

New Member

Posts:5

 18 Oct 2017 05:22 PM The method you have given is very useful to eliminate negative values in the lower end of the LAI. However, my image is thickly vegetated. Therefore, EVI value hits +1 most of the time. Then according to the equation, my LAI value mostly limits on LAI=3.5. I can not have LAI value more than 3.5 according to the equation. We can see that if we substitute +1 in LAI= (3.618*EVI) - 0.118 equation. This only happens when I using bandmath. I got LAI more than 3.5 when using envi spectral indices tool. I am pretty sure I correctly used the bandmath. How can envi gives LAI more than 3.5, even it is not possible according to the equation. (my research needs bandmath rather than using inbuilt tools) Thank you for your continuous help.

MariM

Veteran Member

Posts:2396

 19 Oct 2017 10:36 AM Can you show me the basic stats from the input image and then the calculation of the EVI and LAI from this image?

Yahampath Marambe

New Member

Posts:5

 05 Dec 2017 10:44 PM Apology for long silent period, I research again and again but the problem remains same. I am giving here my image statistics. Landsat image Band Min Max 1 0.0161 0.6363 2 0.0081 0.6399 3 0.0141 0.7227 4 0.0077 0.7461 5 0.000 0.77360 6 0.000 1.000 7 0.000 1.000 EVI image statistics (using equation for EVI in band math) min max -0.2249 1.053 LAI image statistics (using equation for LAI in band math) min max -0.9318 3.6922 LAI image statistics (using ENVI inbuilt spectral indices tool for lai) 0.000 3.6922 Again the problem is, LAI= (3.618*EVI-0.118) if EVI= 1 (it's maximum value) LAI=3.5 (maximum LAI value is 3.5 for any case) How ENVI giving 3.6922.?? Please clarify me so that I go further in my research. Thank you

MariM

Veteran Member

Posts:2396

 06 Dec 2017 12:40 PM In the above, you show your maximum EVI is 1.053 which is over 1.0, It seems the EVI can exceed 1.0 which would mean the LAI can exceed 3.5, yes? The equation in the documentation is based on Huete. ENVI is not limiting the range of EVI to 1.0 for the LAI equation. If you have a very dense, green canopy with a high NIR and very low R and B value, you will exceed 1.0 for EVI using the equation: Using your maximum NIR value and min Red and Blue values (assuming bands 2,4,5): ENVI> print, 2.5*(.7736 - 0.0077) / ((.7736 + 6.0 * 0.0077 - 7.5 * 0.0081 +1.0)) 1.08851

Yahampath Marambe

New Member

Posts:5

 06 Dec 2017 01:47 PM Thank you very much. This is really helping. I used a number of indices and the EVI performed quite well.

MariM

Veteran Member

Posts:2396

 06 Dec 2017 03:14 PM Another thing you may check is if there is influence by cloud. In the EVI paper they state: The sudden increase in blue and red reflectances to values slightly higher than NIR reflectance in July and August 2000 is documented in the QA results as residual cloud contamination. ... These variations resulted in significant ‘noise’ that inhibited the utility of the VIs.
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