DH was trying to teach his Scouts map reading the other day. He was frustrated when they weren't very interested and one piped up 'i've got my mobile phone, why do I need to know this'.
As a rule, protocol for planning trips is NOT to rely on GPS or on mobiles for exactly this reason. DH gets frustrated by people who don't understand why you need a back up plan and why you need to understand the landscape (and how to find your way to safety without devices).
Because if you DO get into trouble, what do you do then?
Its fine if you are going to a busy place and sticking to a very clear path, but stray anywhere away from that, and it can start to be dangerous.
Even experienced people can forget this too.