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Allendej

CRISM Summary Parameters

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Hello,

 

I've created some CRISM IR summary products using the CAT and am visualising them in combination with a HiRISE image. To highlight areas with the largest spectral response, in OLINDEX for example, I have read that typically stretching is performed using zero as the minimum realistic value and the 99th percentile as the maximum. The range of the OLINDEX parameter is very small:

Minimum -0.107, Maximum: 0.084, so Adjusted Min: 0, Adjusted Max: 0.043.

 

I was wondering what the typical expected values for these parameters are. What constitutes a significant response? For example, my 99th percentile value is 0.043 but this might still be a very small response compared with other areas.

 

Regards,

 

Elyse

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Elyse,

 

We have forwarded your question to the CRISM team and will post their response as soon as it's available.

 

 

Jennifer

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Hi Elyse,

 

From my experience, I would say values ~0.04 are marginal olivine indications for that parameter. Values closer to 0.1 are more robust but still could be an artifact. In particular there can be problems in shadows and on extreme albedos for the mafic paracmeters.

 

With values in that ~0.04 range for olindex you should definitely check the spectra to make sure there's a real olivine signature. You should see broad absorption around 1.0 micron in both VNIR and IR. It's not a bad idea to check the spectra in any case, even with relatively large summary parameter values.

 

If you're looking at hyperspectral data (FRT, HRL, etc) you can also check the hyperspectral summary parameter OLINDEX2. It avoids some of the problems of the mutlispectral parameter.

 

For the pyroxenes, values near ~0.05 tend to be more robust than for olivine I think. They are quite susceptible to problems in shadows though.

 

Frank M

 

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Hi Frank,

 

Thanks very much for your reply.

 

I know there is a list of detection thresholds for the OMEGA parameters in Poulet et al. (2007): 'Martian Surface Mineralogy from OMEGA/Mex', but I was wondering if there was such a list available for CRISM too? I know their formula are in Pelkey et al. (2007): 'CRISM Multispectral Summary Products' but I'd like to threshold each summary product with a realistic detection value - or is ~0.05/~0.1 applicable to most of the parameters?

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Hi Elyse,

There's no list of parameter thresholds available for CRISM. For most parameters something in the 0.05-0.1 range is probably about right. Zero may be appropriate for some of the BD parameters, or bump up to 0.005 for noisy scenes.

 

The Poulet et al thresholds may be pretty good for most of the CRISM multispectral parameters in the current CAT (up to 7.1), since the CAT parameters are mostly defined parallel to the OMEGA parameters.

 

Revisions to CRISM parameters are in development and appropriate thresholds for the updated formulations are a subject of active research.

 

Depending on what you're doing, you may not want to put too much emphasis on thresholding. Can be useful for statistical analysis or sifting through large amounts of data searching for something interesting, but once you focus on a particular observation a hard threshold might obscure interesting exposures.

Frank

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