A Fog Of War Problem

[5cats]

I like Fow, really I do, but one thing bugs me:
When you're sailing around, lets say with a marauding Submarine, you "discouver" that there's a resource in Madagascar (for example). Why doesn't this STAY discouvered??? Next turn you sail by and re-discouver it, over and over again. This slows things down, and prevents 'take back' for no particular reason [:@]
Why can't map features stay there? Sure it's possible they could change after you sail away, but how likely is that? Couldn't they at least stay there, and if you sail by and they are changed then that would stop movement??