I've just had a quick glance at that David Wilson's site.
Very interesting, but unless I am missing something, it tells me what I already knew which is, basically, GPS is accurate to less than the width of the average motorway.
I'd say that's adequate for navigation, most of the time
I don't think anybody can accuse
me of hijacking this thread in favour of GPS though.
The error between WGS84 and some other commonly used datum should be of the order of a few hundred metres; not enough to make a difference in any practical enroute navigation application. If the unit was showing the correct lat/long coordinates but its moving map was showing the location some miles away, that's a pretty basic software problem. I saw that once, on an old version of Jeppesen FliteMap running on a tablet PC, somewhere in the Greek islands. The error was probably less than what one would have got using the nearest VOR. I reset the PC and it went away. The IFR GPS (KLN94) was fine all along.