What tidal datum is used on NOS surveys?
The tidal datum to which soundings from recent (post-1980) NOS surveys are referenced is Mean Lower Low Water (MLLW) while the currently used vertical datum on the Great Lakes is the International Great Lakes Datum of 1985. Pre-1980 surveys conducted along the Pacific coast and Alaska also utilized MLLW. The Mean Low Water (MLW) datum was used on the Atlantic coast of the US prior to 1980. Due to changes in sea level, the absolute elevation of these datums have been modified over the years to correspond to various tidal epochs (19-year periods). For more information on tidal datums contact the NOAA/NOS National Water Level Observation Network.
