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CRISM Band Math


Rochdi
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Hello Experts,
 

I would like to learn how to calculate some indices to detect Gypsum and other minerals on Ritchey crater, I decided to start first with the formulations included in "CRISM Data Product Software Interface Specification" (as seen below) to seek out gypsum. However, I didn't clearly understand the syntax of the mentioned formulations, I mean (R, a, b ... etc), I found these formulations more detailed compared to the indices I've calculated before such as NDVI, EVI, SAVI ... (using Landsat8, Spot imageries).

Would you please explain to me the syntax of these formulations and what do I need to get started with before diving into this process?

Look forward to your reply,

Rochdi.

1.png

2.png

Edited by Rochdi
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Hi Jennifer,

Thanks a lot for the attached paper,
I found it useful for my Master thesis I'm working on. However, the mentioned indices above are still unclear to me for putting them into action.
What I'm wondering about is these questions:

Does R2210 mean reflectance at 2210nm? Do I need a value to assign it to R2210? if yes, could I use ENVI and CAT to get R2210 value, for example?

I look forward to your reply,

Rochdi.
 

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Yes, for each pixel in the scene R2210 means use the reflectance value at 2210 nm. ENVI's Band Math function will allow you to write an equation that when run produces the parameter map.The tool is in the ENVI5 series interfaces on the right hand side under Band Algebra. Also, the weightings a, b, etc. in the equations allow centering of the computation over the deepest part of the putative absorption feature, all relative to the wavelengths on the continuum on either side of the feature.

Ray Arvidson

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Hello Sir Ray Arvidson,

Thanks a lot, that's what I thought before.

I used the ENVI tool you mentioned, I tried to write the formulation of HCPINDEX just to learn.

  • HCPINDEX: (R1470 - R1050/R1470 + R1050) * (R1470 - R2067/R1470 + R2067)

As I wrote the expression, I refered R1470, R1050 and R2067 to b1, b2 and b3 successively (X1.png).
Then, a new window popped up to assign the variables (b1, b2 and b3) to their specific wavelengths.
The following is the bands that I assigned to the variables (X2.png):

  • b1 (R1470nm) => Band 73 (1473, 7nm)
  • b2 (R1050nm) => Band 09 (1053.8nm)
  • b3 (R2067nm) => Band  255 (2067.4nm)

However, I come up with this unexpected output result as seen below (X3.png)

Is something went wrong?

X1.png

X2.png

X3.png

Edited by Rochdi
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Yes, after I read the documentation for how to represent algebraic expression on ENVI Band Math function.
I realized that I need to modify the equation by adding float in the dominator as follows:

  • HCPINDEX: ((float(b1) - float(b2)) / (float(b1) + float(b2))) * ((float(b1) - float(b3)) / (float(b1) + float(b3)))

I come with this attached output result below.
So, where's Pyroxene, then? or the result should be in three color compsites?

 

 

Pyx.png

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That's it, Sir
Hope I'm not bothering you, as you know the L (IR)  cube is already converted into IOF.
If I want to extract the spectral signature of these bright areas which highlighted in the output above, could I use CAT toolkit for this purpose? or just a tool from ENVI toolbox?
 

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Rochdi: I am not sure what you are asking. CAT has spectral parameters for IR data. Use this function. I am not trying to be judgmental. However, I suggest that you need to read in detail the Viviano-Beck paper that was sent to you and that you become very familiar with CAT and its functions. Many of your queries would have been answered if you had done this homework in detail.

Ray Arvidson

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Hello Sir Ray Arvidson,

Sometimes I ask stupid questions, but I extracted the spectral signatures of those bright areas.
Thanks a lot for your tremendous help Sir Ray, and thanks to Jennifer G. Ward for the mentioned paper.

Rochdi.

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