Most survival shelter guides are written by people who are either trying to sell you a $400 bivy or teaching you how to survive naked in coastal Alaska. Neither one is useful to the guy who just realized he’s three miles into a trail in central PA and the sky went sideways in 45 minutes. Here’s how it actually goes: you’ve got something on you. Maybe a tarp you tossed in at the last second. Maybe a poncho. Maybe just what’s on your back and your two hands. The situation d