Navigation
Offline topo, GPS, and reading a map when the signal dies.
Offline first
You'll be out of signal for days at a time. Download the topo tiles before you go, and keep a paper map for the day your phone dies.
What to carry
topo offline tiles for the whole route gps phone + backup compass paper 1:50k or better for the crossings habits check the map before the junction
Why it matters
GPS tells you where you are. A map tells you where you're going. You want both, and you want them to agree.