Thanks, Hill, for giving the other thread. The comment in that thread that is most pertinent is that of unruh on 3/16/08 which points out that the assignment of 0 longitude is completely arbitrary. That anyone would ever tamper with the definition of 0 longitude once established is an abomination. But, I still don't know why there would still be further changes as between North American Datum 1927 and something more recent (like WGS84?) unless it was discovered that there were inconsistencies in the 1927 reference and it became necessary to make changes to reconcile worldwide mapping. ??