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
warren "These are all NDVI except the third, normal looking one which I labelled "original". Unfortunately a bug in infragram is not allowing display of th..." | Read more » over 9 years ago
cfastie "I'm confused about which images are photos from the camera and which are NDVI images. It would also help to know what the colors mean in NDVI images. " | Read more » over 9 years ago
khufkens "Good point, I might use this approach to see if I can get better results on my PRI sensor. Things were funky and this offset in response (differenc..." | Read more » over 9 years ago
cfastie "Or without knowing the spectral response of the sensor, you can brute force the response of a dual-camera system by shooting through narrow band pa..." | Read more » over 9 years ago
khufkens "This is again a reason why you would want to know the spectral response of most imaging sensors. Without this information it will require field val..." | Read more » over 9 years ago
cfastie "White balance is important for most single-camera NDVI systems, but could be irrelevant to dual-camera systems. The goal is to capture photos with ..." | Read more » over 9 years ago
gpenzo "I need to change the scaling in my own application so I can check the result with the plugin. cfastie and khufkens thanks for help. " | Read more » over 9 years ago
cfastie "This seems to be related to an old issue posted at Github. Applying a lut to a float NDVI image from the photomonitoring plugin does not produce th..." | Read more » over 9 years ago
khufkens "I just checked the float image I created earlier this week and it seems that all negative values are clipped on my float images as well, leaving th..." | Read more » over 9 years ago
gpenzo "Hallo khufkens I checked the histogram of both float and they are not the same. PhotoMonitoringPlugin float image misses allot of information. T..." | Read more » over 9 years ago
khufkens "Also have a look at the histograms of both initial float images. You might be stretching your data somehow, leaving the majority of the pixels blac..." | Read more » over 9 years ago
gpenzo "The generated float image does not have float values in it. It is just a normal image where the plugin converts float value -1.0 to Red = 0x00 Gree..." | Read more » over 9 years ago
khufkens "Do you pull those into an application which can handle images with floats? To make sure if both images are the same just plot them in a scatter plo..." | Read more » over 9 years ago
clayton "Wouldn't you still need a spectrometer with you to calibrate the panel every time you go out into the field to account for varying lighting conditi..." | Read more » over 9 years ago
khufkens "Thanks @cfastie for reminding me of the Raleigh scattering, my equally scattered Sunday evening brain didn't quite put two and two together. The re..." | Read more » over 9 years ago
cfastie "Koen, These NDVI images look really good. There are multiple reasons that could explain why the trees in the distance have lower NDVI values with ..." | Read more » over 9 years ago
azaelb "Thanks Chris for your comments. "Part B" of these tests is to attach the red filter to the key chain camera instead of the blue one. I have two of ..." | Read more » over 9 years ago
cfastie "Hi azaelb, I was not able to get very meaningful NDVI images from a Mobius ActionCam key chain camera modified with a blue filter. A red filter wo..." | Read more » over 9 years ago
Tbtouaki "Hi Sorry for the trouble but how did you proceed to calibrate individual bands to reflectance before calculating NDVI? Did you use "dark object sub..." | Read more » over 9 years ago
nfliu "Cannot log in with my old account. Thank you very much for your reply and suggestions. I think you well explained what I am confused about. Yes, I ..." | Read more » over 9 years ago
nedhorning "Hi Nanfeng, I'm not certain why the results look the way they do but I have a feeling that it could be a result of doing pixel-by-pixel comparison..." | Read more » over 9 years ago
Nanfeng "Also, I put three figures to show the raw (i.e., bayer), jpg images and the jpg-raw DN correlation. " | Read more » over 9 years ago
Nanfeng "Hi Ned, This is a really nice work. I have a couple of questions about the conversion between the DNs of JPEG images and that of raw images. The F..." | Read more » over 9 years ago
Jimo1966 "Yes! Glue can be remove easily with Heat Gun and a Sewing Needle. Smash to bits is never an option. (or the last one) I filed down I tiny screwdriv..." | Read more » over 9 years ago