Heavy overcast ceiling, fog, darkness, snow, and any of the previous paired with an absence of topography. Also its batteries never fail. It also helps to have some knowledge of the terrain and the relative positions of ones objectives. Fog and heavy ceilings have been my major issues and I once did a complete circle in Alaska's foggy alders before getting out my equipment. Still, one could sail the seas for years with a compass and never find Hawaii, so it's not the end all for serious long distance navigation.
GPS is the absolute mindless ultimate. With 5 minutes of schooling one could leave and tie up at the correct boat cleat in Hawaii. It's only shortcoming is batteries.
Last edited by 1minute; 08/22/18.
