GPS on the Culloden battlefield

If you take a tour of Culloden battlefield near Inverness you are given a Smartphone device that triggers a commentary automatically at numbered sites as you walk round. It knows where you are! GPS works by having fixed satellites with very accurate atomic clocks. They send out coded signals that are picked up by the handset. This lets the handset work out how long the signal took to arrive from a given satellite. By combining time-of-flight data from different satellites of known location in space, it can compute where you are. It is said to be accurate to 7.8 metres. In other words, the locations on my tour must be over 7.8 metres apart. I'd say about 30 metres, but perhaps less than that on some parts of the tour. It would work on a battlefield but not inside a museum because the objects are much closer.