"And many runways are around 45m wide - so 20m accuracy puts a wheel on the grass in most airliners!"
20m accuracy (is usually better than that) is more than enough to figure out in which airport and rwy the aircraft is. probably enough to establish if taking of from an intersection.
could easily implement some device telling the pilot something like "yo buddy, are you really sure you want to take off from here and at this weight or am i getting something wrong?"
personally, i prefer computers suggesting the pilots what to do or what not to do, rather than computers between pilot and control surfaces. FBW is a wonderful, amazing technology, but having worked as professional tester in large software and hardware projects, im not 100% sure i want it on liners...