Mountainlad
To summarise;
On an aircraft, everything is measured in nautical miles. Hence our groundspeed (which remember takes into account the effect of wind) is given in nautical miles per hour.
On a GPS equipped aircraft, the equipment gives a readout in nm/h NOT statute mph.
We do not really use maps for navigation anymore. We do have charts that will provide track and distance.
As we progress from one waypoint to another, the aircraft calculates the great circle distance (since this is the shortest distance over the curved earths surface) between the lat/long of each waypoint. The GPS feeds in the current a/c lat/long into the FMGS and therefore the a/c knows it is on track (this is a very simplisitic way of explaining this) The GPS feed also allows the a/c to deduce groundspeed. BUT we also have 3 inertial navigation systems backing up the GPS which do the same job, just slightly less accurately.
Hope this helps.
