The GPS in the newer phones is pretty accurate. I had a custom security application that I could actually see what office of the building my phone was in.
Don't rely on a peice of software that requires and active connection. Find an app that allows you to download maps and use your GPS signal. I use alltrails for my hiking activity and I have used it for offroad trails when the trail was available.
I can download maps of trails, put the phone in airplane mode, turn on location service and off to town. I could see my ground track very accurately, both driving and walking, and save it afterwards.
I am sure there are better overlanding apps available, I'm up for any suggestions.
Worst case: learn to download routes from Google maps and use it offline.