Re: [PLOTS] NDVI formula using NGB and NRG images

[Liz Barry]

Hi Sharanya, 
I'm also copying the infrared list here. Please check out our wiki documentation on NDVI this at : http://publiclab.org/wiki/ndvi 

thanks!

Liz

On Thu Jan 22 2015 at 7:16:32 AM sharanya chandran <sharanya...@gmail.com> wrote:

Hi everyone,

 

I'm currently working with NDVI for NGB and NRG images.Raspberry Pi NoIR camera with a blue filter was used to obtain the NGB images of the tree canopy during daytime,the image acquisition was performed from the terrace of a three storeyed building.I calculated the NDVI of these images but still the values ranged around 0.0091 to 0.101. Inspite of abundant vegetation the values were'nt upto the mark.I have doubt in the formula for computing NDVI,the formula which I used is NDVI=(NIR-B)/(NIR+B).

Using another formula i.e summing up the reflective channels,(NIR and Green) and subtracting the absorptive channel (Blue).i.e.,NDVI_new=((NIR+G)-B)/((NIR+G)+B) ,will this formula be reliable to be used in the case of NDVI estimation for NGB images?.It would be great if somebody helps me out with a clear explanation about  NDVI calculation(formula) for each NGB and NRG images.

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[Chris Fastie]

Sharanya,

The formulas you are using are correct for a blue filtered camera. NIR is captured in the red channel, so either the blue or green channels can be used for visible light. The blue channel will probably have a higher proportion of visible to NIR light, so it should work the best.

You should not expect to get NDVI results that are scaled correctly unless your process is calibrated. We have been tweaking the process by doing a custom white balance on the camera to force the NIR channel to be a few times brighter than the visible channel. That can sometimes produce NDVI values in the correct range for properly exposed photos.

We have found that a red filter works better than a blue filter in some CMOS cameras. I don't know why that is, but some CMOS cameras simply don't work with blue filters. I don't recall any results of red or blue filters being used on a Raspberry Pi camera (but there might be some). I don't know whether custom white balance has been implemented on the raspberry Pi camera.

Chris

[Liz Barry]

HI Juan, 

I see that no one got back to you so i'm bumping this back up. Thank you for persevering!

Can you post the images in your zip file on publiclab.org/post? That will make them easier for more people to see. 

Let me know if you have any problems posting

Yours, 

Liz

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On Fri, Feb 27, 2015 at 1:41 PM, juan guillermo luna gomez <juanguiller...@gmail.com> wrote:

HI,

Do you have experience in this topic? Do I have some attached images in crops of coffee, I'd like your input on the matter?

[Liz Barry]

Hi folks,
Juan Guillermo Luna Gomez is still hoping to hear back from folks on the interpretation of his images. Any infraredders who find themselves with a free minute to respond would be greatly appreciated!!!

[uekk...@gmail.com]

hello

I am trying to calculate NDVI using NoIR camera with Raspberry Pi , I want to know can NoIR camera with Raspberry Pi produce NDVI with out using blue filter or not and what is the benefits of using blue filter, how it help for capturing NIR band and what are the band captured with NoIR camera without blue filter and using blue filter? I will be very much help full for me

[Chris Fastie]

There is a Public Lab wiki page that explains how single and dual camera NDVI systems work: https://publiclab.org/wiki/ndvi-plots-ir-kit

Chris