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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.