*3.2. Homogeneity*

Homogeneity, or spatial uniformity, quantifies the variability of a pixel. It is most straightforwardly represented by the percentage STD calculated using the pixel itself and its eight neighbors—that is, the STD of the value of the nine pixels in the 3 × 3 square divided by the value of the center pixel. A few options exist for its application—at pixel-based radiance of each sensor, at pixel-based ratios computed from collocated radiance pixels, or both; it is tested for this study that for as long as homogeneity is applied, the result differs very modestly only in rare events. For simplicity, this analysis applies homogeneity to radiance data.

The primary importance of homogeneity lies with it being a proxy to statistical quality of pixel-based data to be used in tandem with a sample size constraint condition, to be described below. Homogeneity in this analysis is not simply an imposed threshold, but is used to generate a sorting of pixel quality to allow a selection procedure under a sample size constraint. Using only a simple homogeneity threshold will include all pixels satisfying the threshold, and different SNO events will have different sample size. On the other hand, using size-constrained selection forces all qualified SNO events to contain the same number of pixels, and this has the advantage of allowing more straightforward interpretations and comparison among events. A threshold of homogeneity, such as 4.5% as a reasonable level can always be imposed, but its importance to contain noise or variability becomes secondary when sample size constraint, itself a mechanism of containment, is used.
