Public Lab Wiki documentation



NDVI and NRG

7 | 29 | | #66

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Great article - I noticed that the first image shows an equation (NIR-R|G|B)/(NIR+R|G|B). The way the R,G,B are shown, the mathematical operations aren't clear, but only NIR and R are used to calculate NDVI:

NDVI=(NIR-R)/(NIR+R)

I would suggest fixing the image and also providing the correct equation as text (not just as an image).

Hi @dlebauer, good catch, thanks for pointing this out.

@cfastie, @nedhorning, @ektopyrotic, if you have any insights on why the NDVI equation in the image here depicts green and blue, please do comment. Thank you!


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NDVI uses an NIR and a Red band. Some documents have noted that instead of red you can any visible band but from my point of view that is not correct. The results might be similar but there are a few reasons red is used. One is that from a satellite, the red wavelengths are less affected by the atmosphere. The second reason is that as vegetation vigor declines the red reflectance is impacted more than the shorter visible wavelengths. In the figure below you can see that the difference in reflectance between green and dry grass is much greater in the red band (0.7 micrometers) than in the rest of the visible spectrum. There is very little difference in the blue (0.45 micrometers) band. That greater difference makes it easier to detect a decline in vegetation vigor.
DryGrassSpectra.png

I spotted another issue on that page. When you create an NRG image the NIR band does not replace the red of a true color (RGB) image. Instead, the band order becomes NIR, Red, Green.

I hope that helps.

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This is a really clear and helpful explanation, thanks @nedhorning.

And thank you for pointing out the other issue! Does this other image on the page showing the false-color bands represent NRG images accurately? I wonder if the image stating that NIR replaces red was originally in the context of single-camera multispec with a blue filter (as illustrated here). @warren, do you have any background on where this image came from?

Thanks all!

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If you want to replace the image with the questionable text "Red, Green, Blue" with one modified to include only "Red," you can find it here. I modified the image in 2012 when I used it on that wiki page.

NDVI_is_eq.jpg

One reason the original image said "Red, Green, Blue" is that the original Public Lab one-camera NDVI system used a blue filter and the blue channel was used for visible light in computation of NDVI.

Chris

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@cfastie Aha, thanks so much for the modified image! And for the context.


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