Originally Posted by
pilotmike
The software also has a 'dead reckoning' mode, so that if you drive into a tunnel, it will continue to treat you as travelling in the same direction and speed while there is no raw data / signal. This often works well at putting you close to your actual position when you emerge from said tunnel.
Strangely I was noticing this exact feature only last night driving through the Dartford Tunnel. My Sat-nav handbook (system inbuilt in the car) describes it as one of the several ways it calculates the position, and as the tunnel alignment is in the system it keeps on track. It keeps on track better than the Heading Indicator sometimes does in the plane.
The new A1(M) alignment at Ferrybridge seems to be missing from many of the "updated" databases, even though it has been around for a few years now (the Mansfield bypass is the same). My hunch is that although there may be regular updates, the raw data has somehow not been provided to the Sat-Nav companies. It is probably provided separately by different authorities, eg councils, Highways Agency, etc, depending on the status of the new road.
An early in-car GPS I knew was always foiled by driving down the pier in Portland Harbour, Dorset, to a boat anchorage. The pier was not in its database at all and it would keep displaying "Attention your location is completely surrounded by water", as if anyone in such a situation would not have noticed !