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Ralph Vidal

Best stretch values for CRISM parameters

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Hi,

I read in a CRISM doc that following:

        

Quote

For most band depth and similar measures of absorption,

  • Zero is the minimum realistic value
  • The 99th percentile is typically a good maximum, although there is a empirically-determined “minimum maximum” that varies by parameter (e.g., 0.02 for D2300)

Is there a table/document where such minimum/maximum  values can be found,  or is it image dependent and has to be found individually each time ?

crism_stretch_values.JPG

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Hi Ralph: The min and max stretch values for parameter maps are scene dependent. Just make sure that you do not set the minimum to less than zero because then the negative values would correspond to bumps and not absorption dips. I typically set values for min and max based on comparing the stretched parameter maps to what can be seen in the false color CRISM images and whatever CTX and HiRISE data one can find. Mapping contiguous pixels in the parameter maps that overlay areas that look like geomorphic or geologic units gives me some certainty that I am mapping some mineral parameter that makes sense. Random detections across the scene just mean that you are mapping noise. Remember for your crater you should be looking for rim and crater ejecta deposits that are windows beneath the regolith, sand, and dust covers. Small areas but pixels should map close to one another.

Ray Arvidson

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