I have always found the paper version of GPS (a map) was pretty reliable. Except, of course, in the Empty Quarter! In fact, using GPS once caused me a bit of a flutter when showing a couple R.N. types around Kuwait post Gulf War 1. We had a radio site literally on the border with Iraq and the matelots were busy picking up souvenirs from the debris left behind by the Iraqi forces. I looked at the GPS and was informed I was in Iraq by some distance. A quick check on the paper version (an aeronautical chart of the area), proved I was still in Kuwait.