Tides came up at work, and we were trying to figure out what the tide numbers were relative to. You would think this would have been easy…but it took a lot of searching to find.


Tides are given in feet above (sometimes below) chart datum. Chart datum is defined (in the United States) by Mean Lower Low Water (MLLW). MLLW is “The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch” (http://www.baysail.com/tide_terms/gloss_m.html). Lower low water is the lower of the two low tides (or single low tide) in a day. The National Tidal Date Epoch is 19 years, and currently is the period from 1983-2001 (http://co-ops.nos.noaa.gov/datum_update.shtml). I’m mainly posting this just to remind myself since I spent so much time figuring it out.