The following table will give some idea of the accuracy of the elevations measurements made by the differentially corrected Garmin 12XL GPS . Of course it's possible that the GPS measurement is more accurate than the numbers on the map, whose method of determination is not specified.
All numbers are in feet for ease of comparison to the map.
From each GPS measurement, I subtracted 3 feet to account for the height of the GPS above the ground level during the measurement.