I think the accuracy is plenty good enough with GPS unless you intend taking off across the runway
How many times will a big bus be turned onto a runway pointing down it where the intent is not to take off in the length available infront of the aircraft? (and those times this is not the case, just ignore the alarm)
what gianmarko said, a master caution where the computers say "are you sure?" (but will let you carry on anyway)
and not set up to catch every small error or it'll become like the Scada control systems we have offshore, they flag up so many alarms so often the operators keep hitting "return" on the "accept alarm condition" panel such that they have accepted the next 5 alarms that come up sight unseen (did mention we in the patch had lost he plot didn't I)
things that catch gross errors, like being 30% off on actual All up Weight, when the aircraft is lined up on a runway (cars are positionally aware now to the extent that there really is no excuse for being nailed by a fixed speed camera in the UK)