You might want to think about bluetooth, if you already carry a bluetooth capable smartphone. The idea is that the GPS has no user interface; instead it just tells the phone where it is and the phone does all the clever stuff. This means the GPS can be very basic and small, and you can keep it in a ruck sack. You don't have to get it out to use it, you use the phone instead. Obviously the phone needs to be high spec and with the right software.

This isn't a recommendation, it's just something to consider. Last time I researched it, the products weren't quite there yet but that was 6 months ago.