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ENVI

Select Band Subset

Select Band Subset

The Select Optimal Bands panel represents step 4 in the workflow.

Here, you will customize the band subset used to suppress your backgrounds. The BandMax algorithm calculates a significance value for each band in the input image. This unitless value ranges from 0 to 1, where a higher value indicates the band has a higher probability of distinguishing a target response from a background response. BandMax compares the significance value to a significance threshold to derive a subset of bands, based upon the current list of targets and backgrounds.

BandMax calculates the significance threshold by attempting to select 25% of the input bands, but never fewer than six bands. However, in some cases, even using a threshold of 0 (the lowest) does not provide an adequate number of bands. In this case, it is best to modify your selection of targets or backgrounds (typically by removing one or more) and run the BandMax algorithm again.

The results of the BandMax calculations are shown in the right part of the panel. You can customize the number of bands that will be chosen for SAM analysis (step 3 of the Wizard workflow) by modifying the values in the following steps.

  1. Review the Significant Bands list, which shows the bands that BandMax automatically determined were significant when you clicked Next in the Background Selection and Rejection panel.
  2. Modify the Band Significance Threshold value as needed. The threshold ranges from 0 to 1. Only bands with a significance value greater than or equal to the significance threshold will be used. Setting the Band Significance Threshold to a higher value results in fewer selected bands in the subset.

    The increase/decrease buttons change the threshold by increments of 0.01. An increase in the Band Significance Threshold value decreases the Number of Significant Bands value and updates the Significant Bands list. If a change of 0.01 is not enough to update the Number of Significant Bands, increase the increment until it does.

  3. You can also decrease the Number of Significant Bands value, which increases the Band Significance Threshold value and updates the Significant Bands list.

    The increase/decrease buttons change the number of bands by at least 1. If two or more bands have the same significance value, a greater increment is used to include all of these bands.

  4. Click Save Significant Bands to File if you want to save the band subset in the Significant Bands list to an ASCII file. When you have derived the subset of bands that effectively detects your targets, you may want to use this same band subset to perform a series of classifications on a set of images from the same sensor. You can use this output ASCII file as input when spectrally subsetting a file.
  5. Click Next in the Wizard. The Select SAM Maximum Angle Threshold panel appears. The bands in the Significant Bands list form a band subset that is used as input in the following SAM analysis.



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