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This is discussed in the ENVI Help:
The PPI Threshold Value is used to define how many pixels are marked as extreme at the ends of the projected vector. The threshold value should be approximately two to three times the noise level in the data (which is 1 when using MNF transformed data). Larger thresholds cause the PPI to find more extreme pixels, but they are less likely to be “pure” endmembers.
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