Dan Winterland's 2nd post (#8) gives an excellent explaination of the errors from the 'signal reception' point of view, but probably weren't the cause of your 'error'.
Tomtom, being a 'cheap' consumer product, has its own set of limitations, one of which is smoothing of the raw data. If it always displayed your position exactly where the raw data said, your path would appear to be rather jerky, and not always exactly on the road. So the signal is smoothed, and the software always tries to keep you travelling along the known route, at a reasonably steady speed, until this is obviously no longer applicable.
Witness the way it continues to take you off the main road at an intersection if you choose to continue straight on. It can take 10 seconds or more for it to realise that you are not going the route it thought you were, and for your position to magically 'jump' back to where you are.
The Ferrybridge incident you describe is one of these situations, exactly as TopBunk said - a re-route of the road. Similar problems occur on the A120 near Stansted (recently re-routed) and a bizarre quirk in the M49 near the Severn Bridge, to give just 2 such examples. Tomtom appear to be very slow to offer updated maps. Until such time, we will often find ourselves navigating across fields, apparently.
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.
PM
Last edited by pilotmike; 22nd August 2007 at 10:55.
Reason: more damned errors!