Definitions of Mean Sea Level (MSL) have changed and improved substantially over the years. The current standard is NAVD88. Users of survey-grade GPS should use GEOID12A to determine NAVD88.
Old standards like NGVD27 had serious inaccuracies which were greatly improved in NAVD88. In the Northampton area, elevations based NGVD29 datum are about 8 inches (0.669 ft, 0.204m) higher than NAVD88.
We encourage all surveyors to avoid NGVD27 control points and favor NAVD88 or GPS-based vertical control. In any case, label the vertical datum used on plans.
NGS control point G5X is the best physical control point in Northampton.
Our public GPS CORS station HAMP is the next generation of geodetic control and is more accurate than physical control.