I have spent sometime wandering around on the Channel Islands in the fog (sometimes I was in the fog even when the weather was clear, but that's another story). On a clear day the map would stay in my pocket, but let the fog roll in, and it was quite a different ball game. That is where GPS came in very handy. I got in the habit of taking a waypoint when I left the car at the beginning of a day, because you just never knew when the fog would come creeping in....

Knowing the country cold is the best way to go - no batteries required. I once had an early model GPS which gave me the display - "Change batteries within two minutes or lose data." I had a spare battery carrier, popped it in, only to find that those batteries were dead as well. I was very glad I had recorded waypoints already. Never forget Murphy's Law.