"The Threshold Alpha command converts semi-transparent areas of the active layer into completely transparent or completely opaque areas, based on a threshold you set, between 0 and 255." That's what the GIMP documentation. Whoopee! So what?
I have landed an airplane in 2 states:
I have played a round of golf in 5 states:
I know I have been in (not just layovers) 30 states:
(http://www.epgsoft.com/VisitedStatesMap/)