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 GEOID03 to determine NAVD88.

Old standards like NGVD27 had serious inconsistancies over large distances which were greatly improved in NAVD88. For instance, in the Northampton area, NGVD27 is about 8 inches higher than NAVD88.

We encourage all surveyors to avoid NGVD27 control points and favor NAVD88 or GPS-based vertical control instead.

NGS control point G5X is the best physical vertical control point in Northampton. Our public GPS CORS station HAMP is the next generation of "portable" geodetic control, and is more accurate than physical control.