Histogram Scale on Plant Health Map

I am mapping a forest canopy that is a mix of evergreen and deciduous trees. I am able to change the color range on the Plant Health map via the Histogram sliders. The flight was taken in the fall season and the deciduous tree leaves were starting to change color. By using the histogram sliders, I am able to make the deciduous trees look healthy even with less green foliage (I’m using the RGB/VARI options). What I cannot figure out is what the histogram scale numbers represent. For my example, the left side range value is -0.09. The right side range value is 0.38. What do these numbers represent and how are they derived? What does a value of 0 mean on this scale?

Histogram scale

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Try reading the information in this article:

Also be sure to click on this link (which is also inside that article) to learn more about the map coloring:


Thank you very much for your reply. I had read most of the information you referenced. I’m not sure if the drone camera used was only RGB or was NIR. Under the current assumption that the camera is RGB, then I am limited to VARI. I did see the following under the Identifying Crop Variability with Drones page. The following was referenced when describing NDVI

The scale is from -1 to 1. I’m assuming that this same scale is used for all plant health maps. If that is true, then most of the forest I surveyed is considered unhealthy (less than 0.33) again using the RGB/VARI options. I understand, however, that open dirt is considered unhealthy and that the deciduous trees in the fall season appear less healthy.

The following is stated: “While some growers may find false-NDVI useful, we don’t recommend applying NDVI to RGB imagery because the algorithm was not designed for that purpose. As a result, this results in the reporting of inaccurate data. As mentioned above, the VARI formula is a much more useful way to calculate plant health using RGB imagery.”

I need to confirm that the camera is RGB and not NIR. If the camera is NIR compatible, then I can use some of the more advanced NI algorithms.

I think my question is answered, however, that the scale number is an evaluated health value from -1 to 1