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Output's histograms - How to interpret each index?

#1

Good afternoon,

I’d like to know more about the meaning of each index through their respective histograms.
I’m using a Micasense Altum camera so I have few different outputs available to me. Since most of those indexes are related to vegetation’s health, where the minimum values mean unhealthy vegetation, soil, roads, etc and the maximum values mean healthy vegetation.
My question is: how about the Thermal output? It usually gives me a minimum value of 29000 and a maximum value of 30000. Does the histogram range from the minimum temperature to the maximum temperature captured in said Thermal output?
And if yes, how is this associated with ºC/ºF temperatures?

In another note, NDVI for example ranges from -1 to 1 but most of my generated NDVI’s range between other values, for example, -0.15 to 0.15. I assume fields tend to use the minimum and maximum registered values on each project, instead of using default values, in this case, -1 to 1 for NDVI.

Any feedback regarding these aspects would be highly appreciated!

Best regards,
James

#2

Hi James,

The thermal pixel values need to converted to degree C with the formula Thermal_ir=(lwir/100)-273.15

The values will be between that range -1 to +1. Since -0.15-0.15 is within that range it is correct. The range will differ for each project with min-max NDVI values as the range, as you mentioned.

#3

The values I have for the NDVI range is -0.28 to +0.38 and when the formula is used it does not make sense. Can you elaborate on this a bit, I’m very new to all this stuff?
In my case lwir= +0.38 correct?

Best regards
Mark

#4

Mark,

If you range of NDVI is from -0.28 to +0.38, it’s good. The NDVI range should be between -1 to +1. Stones, concrete, etc can have negative NDVI values and vegetation will have more than 0. Can you explain why you say “when the formula is used it does not make sense”?

For LWIR, +0.38 seems a very weird value, if after using the formula, Thermal_ir is 0.38, that would make sense as maybe the temperature of some object/pixel was 0.38 degree Celcius.

Can you send me a screenshot showing the NDVI and LWIR values?

#5

Momtanu,
Perhaps I should backtrack here a bit, I’m using the Sequoia camera and you are mentioning LWIR (Long-Wave Infrared).
Our goal is to estimate defoliation on Balsam fir and maybe I am using the wrong spectrum. Pardon me but I am quite new to this.
Here is a screen cap of my NDVI for one of my flights.

Pix4D%20capture-NDVI
Pix4D capture-NDVI.JPG1491×993 400 KB

#6

Oh I thought you are using Micasense altum, my bad! Then you can generate all the indices that have green, blue, red, rededge and NIR bands like the ones you have generated above. Your NDVI looks good, the range is within -1 to +1 which means it is correct.

Do not hesitate to ask more questions :slight_smile: