[Proj] NGS Control Points

[Greg Troxel]

  I have noticed that control points from the National Geodetic Survey (USA)
  NAD83 to NAD27 transform values do not agree with transforms done using BM.

BM?

Coordinates for stations in both NAD27 and NAD83 are the result of
large-scale adjustments.  All coordinates have errors, generally more so
in NAD27.  Datum shifts are meant to apply over areas, and will not
match any particular control point.

Someone more knowledgable with your area will have to comment on whether
a 1m discrepancy indicates a problem; my guess is that it does not.