Originally Posted by
LookingForAJob
Not sure what inaccuracy you are thinking about. As far as radar processing and display is concerned, the altitude displayed will be based on the appropriate QNH that has been put into the display system. This may be different to that shown on the aircraft altimeter, but this is not uncommon in the circumstances originally described.
Simply that the "spot" qnh at the departure aerodrome might be significantly different to the area qnh, in changing weather ie, a frontal passage.