Directions.... (major spoilers)
Heather still had the compass, and must've had a rough idea of the route they needed to take. She would've planned the route before setting off, and had an idea of the map from memory. She says they go off the map at one point, but there must still be a basic logic to the directions.
Hadn't seen it for years, but that thought really took a lot of credibility away.
Should we put it down to Heather doing a really bad job of planning/managing the trip, and/or them all going out of their minds once they were out there?
Or was it the time travel theory that gets mentioned on here?