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How to read VARI and TGI indexes

Can you please explain how to read the VARI and TGI indexes? One more thing once i upload the RGB photos and go to the index generator do I have to sort out any of the formulas for these indexes or it is all automatic by selecting whichever index i need?

As far as i am aware the higher the green the higher the plant health and the higher the red the lower the health of the plants. I am trying to figure out how to read those two like the NDVI is -1 for dead and +1 for excellent health. Sorry totally new to this and I have a potential customer who wants to see the benefits of a service like that… (obviously multispectral will be much better than an RGB camera but it’s a start). Thanks in advance.

Hi @lepabrena

VARI and TGI would be read the same way as NDVI or any other indices. A higher value would mean healthier vegetation, lower values would mean stressed vegetation. I would recommend you to always look at the histogram and match the colors with the values. This is because there are several color scales. All the indices in the index generator can be generated, only the indices made from the R, G, B bands will be present when there are RGB images. However, if it is not selected or enabled, you will need to enable it.

@momtanu.chakraborty

Would the VARI and TGI values be the same as for NDVI -1 to +1 ? If they are meant to be -1 to +1, why in my case they do not go anywhere near -1 or +1?

I have done a test out in the field and my readings are

-0.10 to +0.21 (VARI) field 2
-0.12 to +0.17 (VARI) field 1

-0.02 to 0.12 (TGI) field 2
-0.03 to 0.12 (TGI) field 1

The info I got from your website:

VARI - RGB index for leaf coverage. This index is used to estimate the fraction of vegetation in an image with low sensitivity to atmospheric effects.

VARI - It is a measure of “how green” an image is (from a different website)

TGI - RGB index for chlorophyll sensitivity. TGI index relies on reflectance values at visible wavelengths. It is a fairly good proxy for chlorophyll content in areas of high leaf cover

To my understanding, VARI just shows how “green” an image is and obviously the greener it is the higher the possibility plants are healthy. On the other hand, TGI is not as accurate as NDVI and is a substitute for NDVI to measure chlorophyll content in plants. I may be wrong and this is what I am trying to understand. Am i right?

I have selected two different fields for the test to check if problems will be shown through the VARI and TGI indexes. Field 1 was less “green” and slightly yellowish compared to field 2, which was in perfect health. Both fields have been chosen by my potential customer who suggested that Field 1 definitely had some kind of a problem as opposed to Field 2. Customer said both fields are in good health but field 1 was in inferior health compared to field 2.

Can you please let me know how interpret the data I had from this test from both fields? Would you mind telling me which of the two - VARI or TGI - is better to assess crop health and why? I reckon both have to be used simultaneously as they would probably give different insight. If necessary I can send you the VARI and TGI images from both fields so you can give me better understanding of the indexes.

Thank you for your help.

Hi,

NDVI: It is useful to determine the density of green on a patch of land. This index combines information available in the red and near-infrared band into a single representative value and goes from -1 to 1. To understand this index we need to check the wavelengths of the visible and near-infrared sunlight (“colors”). Why? because when the light of the sun strikes vegetation, these wavelengths are absorbed or reflected. It is important to know that pigment of leaves, chlorophyll, absorbs visible light (red region of the spectrum) which goes from 0.4-0.7um for use in photosynthesis. The cell structure of the leaves has different behavior and reflects the near-infrared light. So, the more leaves a plant has, which we can say are healthier reflect more in the NIR but less in the red, and the other unhealthy vegetation reflects more red than NIR. I find this video helpful to understand how this works, and this text.

VARI for leaf coverage Areas with abundant leaf coverage indicate that the biosphere is actively using hydrosphere for plant growth which is good. Therefore the higher the value the better.

TGI for chlorophyll sensitivity Chlorophyll is a pigment that gives plants their green color, and it helps plants create their own food through photosynthesis. Therefore higher values mean healthier, than lower values.

VARI AND TGI indexes are useful for example when you do not have a multispectral camera. The multispectral camera has the NIR band, but an RGB camera not. However, you can still observe if the plant is healthier or not using the TGI and VARI indexes

So your understanding is correct with the VARI you check the leaf coverage “how green is the plant”, and higher values the higher the possibility plants are healthy, and TGI can be alternative when using RGB sensors Here more studies..

We would like you to be aware that there are many publications avaliable to read about indices, comparisons between them, and explaining that the results and the use of vegetation indices in general depend on many factors. For example, the results that you will get depend on the crop growth stages, crop density, crop type, etc.

Let’s take the NDVI as an example (but will work similarly to the other indices), sparse vegetation such as shrubs and grasslands or senescence crops may result in moderate NDVI values (approximately 0.2 to 0.5). Very high NDVI values (approximately 0.6 to 0.9) correspond to dense vegetation such as that found in temperate and tropical forests or crops at their peak growth stage.

Those indices in general help us to focus on the problematic areas, because you might see the low values, so we know that something is happening there, and for example some of our users instead of taking a soil sample of their whole field, they just take a soil sample of the areas where we know there is a problem. Also, most of our users do not choose one single index to analyze their field. In this case for example, now we know that the VARI will give us insights about the leaf coverage and the TGI about chlorophyll so that I would use both, but if I have a multispectral camera I get the NDVI directly and maybe look into other indices as well such as the NDRE.

Let me know if this helps.
Kind regards,