How is the MNF spatial coherence threshold calculated in ENVI's Spectral Hourglass Wizard?

In ENVI 3.5, the Spectral Hourglass Wizard was introduced as a guide through the ENVI "hourglass" processing flow to find and map image spectral endmembers from hyperspectral or multispectral data. Part of this process is to determine the inherent dimensionality of the data so that only the coherent portions are retained for further processing. This is accomplished by thresholding the MNF bands to separate the noise from the data, thus reducing the amount of data to be analyzed and improving spectral processing results. You can allow the Wizard to find this threshold and estimate the data dimensionality by using a Spatial Coherence measure. This Help Article discusses how the Spatial Coherence threshold is calculated in ENVI's Spectral Hourglass Wizard.

ENVI calculates the spatial coherence threshold based on the bands with spatial coherence values greater than a given floor, not greater than or equal to a given floor. If the threshold is set to 0, bands which score 0 will not be included. For example, if you input 50 bands into the MNF transformation and calculate the spatial coherence, it might return 36 out of 50 bands even though the threshold value is set to 0. This is because 14 of the bands have a spatial coherence value of 0. Presumably, a threshold greater than zero should be used, or the number of bands can be selected manually.

Additionally, the threshold is not a count of all the bands above or below a given threshold. Selecting the number of bands is accomplished by finding the first band which drops below the threshold and then including all the bands to the right of this threshold as "no good". The theory behind this is that the spatial coherence curve (as applied to MNF data) should become less and less as you move to higher MNF bands since the bands get noisier and noisier. So ENVI looks for the first band that dropped below the threshold and then throws out the rest.

Review on 12/31/2013 MM