NDVI and NRG
ndvi

NDVI stands for "Normalized Difference Vegetation Index". NRG stands for "Near-infrared / Red / Green". NDVI and NRG are both ways to visualize the amounts of infrared and other wavelengths of light reflected from vegetation. Because both these methods compare ratios of blue and red light absorbed versus green and IR light reflected, they can be used to evaluate the health of vegetation. It's a snapshot of how much photosynthesis is happening. This is helpful in assessing vegetative health or stress. (Read more here: https://www.agronomy.org/publications/jeq/articles/36/3/832) ## Do-It-Yourself These techniques for vegetation analysis were developed for satellite imagery, but at Public Lab, we've been working a lot on capturing infrared imagery using our DIY [near-infrared camera](/wiki/near-infrared-camera) setup, and combining it with visible bands to produce NDVI images such as the one above. ## What these images mean What exactly are these images we're trying to make? What do they tell us about vegetation, and why? These diagrams should help to understand what it is we're doing and why these are good ways to analyze plant life. ## The NDVI equation [![NDVI_is_eq.jpg](/i/44723)](/i/44723) **NDVI = (Near Infrared - Red)/(Near Infrared + Red)** NDVI is a ratio which tries to emphasize photosynthesis while filtering out sun glare. The above equation is run for every pixel, using source data from an infrared photo and a visible light photo, like this pair: [![5390895115_c9d4d38fec_o.jpg](https://publiclab.org/system/images/photos/000/021/771/large/5390895115_c9d4d38fec_o.jpg)](https://publiclab.org/system/images/photos/000/021/771/original/5390895115_c9d4d38fec_o.jpg) The result can be false-colored to make the high-photosynthesis areas more clear, and used to examine where plants are and how healthy they are. [![PetVISNDVIcomp.png](https://publiclab.org/system/images/photos/000/021/770/large/PetVISNDVIcomp.png)](https://publiclab.org/system/images/photos/000/021/770/original/PetVISNDVIcomp.png) _Figure above: Normal color photo (right) and normalized difference vegetation index (NDVI) image (left). NDVI image was derived from two color channels in a single photo taken with a camera modified with a special infrared filter. Note that tree trunks, brown grass, and rocks have very low NDVI values because they are not photosynthetic. Healthy plants typically have NDVI values between 0.1 and 0.9. -- @cfastie_ ### Activities Here are a range of activities you can do to produce and interpret your own NDVI imagery, whether downloaded from a satellite imagery provider or [collected yourself using a DIY technique](/wiki/multispectral-imaging) [activities:ndvi] **** ![IMG_0511-split.png](https://i.publiclab.org/system/images/photos/000/000/279/medium/IMG_0511-split.png) ![infrared-combination.png](https://i.publiclab.org/system/images/photos/000/000/278/medium/infrared-combination.png) Most DIY converted cameras today (those from Public Lab) use RGN instead of NRG, so the blue channel represents infrared instead of the red channel. That looks like this: [![rgn-split.png](/i/45468)](/i/45468?s=o) **** ## NRG imagery Some people are also interested in producing NRG imagery (like the below image), where `Near-Infrared, Red, and Green` are used to compose a picture instead of the usual `Red, Green, and Blue`. [![5415783775_502f79ac8c_o.png](/i/25064)](/i/25064) This diagram explains the swapping, which allows us to 'see' infrared as if it were a normal color: [![5396083368_40528d3da2_o.png](/i/25063)](/i/25063) **In NRG images, the deeper and clearer the red color, the denser and healthier the vegetation (more or less).** ### Questions [questions:ndvi] ### Other examples of DIY NDVI imaging From around the internet: Begin watching at 2 minutes to see the resulting imagery: *This topic is part of the [Grassroots Mapping Curriculum](/wiki/mapping-curriculum) series.* **** [![5416397210_5e3be40cf5_o.png](/i/25066)](/i/25066) [![5412520298_93873f36d0_o.png](/i/25065)](/i/25065) ...


Author Comment Last activity Moderation
nedhorning "Yep - that's it " | Read more » over 9 years ago
cfastie "Ned, So like this. We need to make a different file of reflectances for each filter because the filters we were using were very narrow band filt..." | Read more » over 9 years ago
nedhorning "Chris - I'll email you the master list with all of the reflectance values for each target since they aren't really useful without the images. You n..." | Read more » over 9 years ago
cfastie "Cool. Ned, do you have a reflectance csv file for all of the calibration targets in the test photos from iFarm (the lead image)? And a key for whic..." | Read more » over 9 years ago
RQ-05 "I will start with the experimentation on the exposure. Thanks. I will keep you posted. RQ-05 " | Read more » over 9 years ago
cfastie "Altering the white balance settings could have made made that photo more turquoise. Some experimentation is called for. Changing the exposure (so t..." | Read more » over 9 years ago
RQ-05 "Wow.....really great stuff! So, I need to get the Mobius settings adjusted to a more tourquis appearence. Would that be in the white balance or an ..." | Read more » over 9 years ago
cooperbreeden "David, I'm wanting to explore using this technology down in Nashville TN to monitor algae blooms. It's still completely brand new to me but I'd lov..." | Read more » over 9 years ago
cfastie "Hi RQ-05, Using Infragrammar in the Infragram sandbox is a good way to experiment. I usually use Ned Horning's Fiji plugin to produce NDVI images. ..." | Read more » over 9 years ago
nedhorning "The image for this note with the blue color was probably a JPEG image white balanced using red paper. The raw image will have much higher values in..." | Read more » over 9 years ago
agustin_castellano "Thanks Ned. I have a red filter (600nm) and the filter color is red and the pictures that i take with this filter are red why you have a similar fi..." | Read more » over 9 years ago
nedhorning "Hi Agustin - I have a feeling the problem with with the calibration targets you are using. The targets I sued for this note were scanned by a labor..." | Read more » over 9 years ago
agustin_castellano "Hi Ned me again, could be that the analisys that you did it was over the image without the red filter? because when i read the article i understand..." | Read more » over 9 years ago
RQ-05 "Chris, Here is the original of the two that are already processed. Is it possible to only use the PL Sandbox to get usable images, or do I need ano..." | Read more » over 9 years ago
agustin_castellano "Hi ned! I would like to ask you how do you setup the camera? I have a canon power shot a490 with red filter, i try to a similar test but with bud r..." | Read more » over 9 years ago
cfastie "It will probably be possible to give you some guidance if you attach a couple of the photos straight from the camera. Chris " | Read more » over 9 years ago
cfastie "It will probably be possible to give you some guidance if you attach a couple of the photos straight from the camera. Chris " | Read more » over 9 years ago
warren "Each ```` has to be on its own line, then it should work. Thanks, Ned! " | Read more » over 9 years ago
jmlong "Oh ok, thanks. Is there any chance I could get the raw images if they're still floating around anywhere? It would be much appreciated. Jeff " | Read more » over 9 years ago
nedhorning "I posted it on GitHub GIST: https://gist.github.com/nedhorning/f384d9afcc043741def3 " | Read more » over 9 years ago
RQ-05 "I will get a few more images for you to look at. It seems with this type of plant, that the HSV really jumps out at the true NIR reflectance. I am ..." | Read more » over 9 years ago
warren "HSV is kind of a shortcut to actually making a LUT, but they're not exactly the same thing. As I mentioned in my comment on your post today, HSV ha..." | Read more » over 9 years ago
warren "Hmm, I think I need a username too? To post formatted code, you can use four ` characters -- ```` on lines before and after your code, and it'll d..." | Read more » over 9 years ago
warren "These are some good questions. We started to abandon basic HSV because it "wraps around" -- high values and low values are both "red" -- but if you..." | Read more » over 9 years ago