Sorry to be slow to reply.
I did not say I was measuring the object "Picture Rock", I was only speaking of a particular point on the quadrangle MAP, the quadrangle map's name is Picture Rock. It doesn't matter, you can pick any point you like on that (or any?) paper quadrangle and compare its longitude on the paper map with the longitude of the same point on GE.
Anyway, I think I have a PARTIAL answer to my question, because I see the the paper map says "North American Datum 1927", but on trails.com, there is notation "NAD83/WGS84", which I suppose means North American Datum 1983. That particular map on trails.com agrees with GE, but they are both different than the USGS quadrangle on paper.
What I don't know is what is the definition of "North American Datum" and why it should ever change. Since the choice of zero longitude is completely arbitrary and was chosen in or near Greenwich, England many years ago, why would it ever change, except possibly for political reasons ? Changing something so fundamental wreaks havoc throughout communities who have established long traditions, work and documents (maps) based on the original standard.
We aren't talking about continental drift here which is only a few inches per year, the observed discrepancies are on the order of hundreds of feet.
Regards,
Bill Dempster