The North American Vertical Datum is a geodetic datum fixed by a single tide gauge bench mark in Quebec and carried across the continent by precise levelling. Its zero is not mean sea level anywhere in particular, so the offset to local tidal datums varies station by station and must come from a published datum relationship or conversion model. Engineers, floodplain maps and lidar all use it, which makes the tie to tidal datums a constant task.
MHHW at this station is 3.12 feet NAVD, so the seawall cap at 9 feet gives about six feet of freeboard.
The offset between NAVD and local mean sea level changes from station to station, even within one bay, so never reuse a conversion from the next station over.
Also heard as
- North American Vertical Datum
Related terms
Tidal datum
A local zero for heights, computed by averaging one stage of the tide, such as high or low water, across many years.
MSL
Mean Sea Level: the plain average of a station's hourly water levels across the tidal datum epoch.
Bench mark
A permanent, stable mark of known height near a tide station that preserves the station's datums.
Leveling
Precise measurement of height differences between bench marks and the gauge or staff using a level and rods.
Vertical land motion
Sinking or rising of the ground at a tide station, which changes the sea level trend the gauge records.