.. come to think of it ... why would we want to identify the precise (as opposed to a reasonably accurate, but nonetheless approximate) touchdown position ? In my approach to data reduction and monitoring (which we have only recently started doing) I am far more concerned with overly short/long touchdowns .. can't say that I can see any need for identifying the touchdown better than, say, 30-50m for practical purposes .. and I reckon I can get that sort of notional accuracy without too much effort on many occasions .. and, comfortably, 100m. All depends on the data one has available with which to play for the particular kit, of course ..
.. always looking to learn, though, if I might ask for comments on the why ? Upon reflection .. training for naval carrier ops might be a case in point ... but for routine airline-style operations ?