Indicator arrow drifts all over the map.

[M. Schrijver]

Most scenic route tunnels don't have fallback for GPS and cellphone.
Tunnels are normally known on the maps. And it is known GPS signal is gone in tunnels. So the software must take over. Every software has it own way of doing this. Mostly the software expects you drive at a constant speed through a tunnel and after leaving the tunnel (according to the map) they stay on this course for a while. In the mean time, the software tries to find the GPS signal again and links to the GPS again for a more accurate postion.