Tides / Guides / NAVD88 and mean lower low water
NAVD88 and mean lower low water
Updated October 27, 2026.
A survey and a tide chart can both say "8 feet" and mean two different heights. The survey is usually measuring the land. The tide chart is measuring the water against a zero that sits on the tide. Those zeros are not the same line, and the distance between them is different in Boston than it is in Galveston.
NAVD88 is a land height
NAVD88 is the North American Vertical Datum of 1988. People sometimes write it NAV 88. It is the height a surveyor uses for a house, a road, or a benchmark in the United States. Zero is a land reference, tied to gravity and to benchmarks, not to this morning's tide. If the finished floor is 8.00 feet NAVD88, that number stays 8.00 whether the tide is in or out.
It is not mean sea level, even when the two are close. Mean sea level is the average height of the water at that harbor. NAVD88 is one surface for the continent. On the NOAA sheet for Seattle, mean sea level sits 4.30 feet above NAVD88. On the Boston sheet it sits 0.30 feet below NAVD88. A form that says "sea level" has not told you which zero it used.
Mean lower low water is the tide-chart zero
On a United States tide chart the zero is mean lower low water, written MLLW. Most days have two low tides. Mean lower low water is the average of the lower one, over a 19-year stretch called a tidal epoch. The epoch on current NOAA sheets is 1983–2001. A prediction of 0.0 feet is that average low. It is not an empty harbor, and it is not the height of the land.
Mean low water, written MLW, is a different zero. It averages every low tide, not only the lower one each day. Where the two lows are almost the same height, the zeros are close. Where one low falls much farther, they are not. At Seattle, mean low water sits 2.83 feet above mean lower low water. At Galveston the gap is 0.30 feet. A note that says "mean low" is not automatically the zero on this site. The United States charts here use mean lower low water.
The same flood number is not the same water
A coastal flood report in the United States is often a height above mean lower low water. A flood map is often NAVD88, and so is the survey of a house. Comparing them takes the gap at a gauge that publishes both.
NOAA's sheet for Seattle puts mean lower low water 2.34 feet below NAVD88. A mark at 8.00 feet NAVD88 is 10.34 feet above mean lower low water there. A flood report of 3.00 feet mean lower low water is 0.66 feet NAVD88 there.
The Galveston sheet puts mean lower low water only 0.31 feet below NAVD88, so that same 8.00-foot mark would be 8.31 feet above mean lower low water. At San Francisco the two zeros are 0.06 feet apart, and mean lower low water sits slightly above NAVD88. At Boston, mean lower low water sits 5.50 feet below NAVD88. One number for the whole country would be the wrong number in every one of these harbors.
These figures are the published station datums for the 1983–2001 epoch. A gauge a few miles away can print a different gap. Some gauges publish mean lower low water and do not publish NAVD88. When the sheet has only one of them, there is nothing honest to subtract.
What other countries print
A tide table still needs a zero, and a survey still needs a land height. The names change. The job does not.
Outside the United States, the zero on a nautical chart is called chart datum. The usual choice is lowest astronomical tide: the lowest tide the moon and the sun alone would produce, with no wind and no storm. Mean lower low water is a close idea and a different line. United States pages on this site stay on mean lower low water. Pages in other countries name their datum in the first paragraph, often chart datum or lowest astronomical tide, and the heights are metres.
Australia surveys land on the Australian Height Datum. New Zealand surveys land on NZVD2016. Canada uses CGVD2013, and older maps may still say CGVD28. In Britain the land height is Ordnance Datum Newlyn. Each of those is a land height. The tide chart beside it is chart datum for that port, and the gap is published port by port. This site does not carry those land-to-tide offsets.
A survey in Mexico is a land height. The tide table is the chart datum for that coast. There is no single offset for the country on this page.
What a height on this site is not
The number on a tide chart is a prediction of the water against that chart's zero. It is not the depth under the boat. Wind, air pressure, and a storm surge are not inside it. A flood gauge, when a page has one, is the water that is there. This guide does not turn a tide prediction into a survey.
Convert a height at one United States gauge
Type the town and the height. Say whether that height came from a survey or from a tide or flood report. The other number is the published gap at the gauge you pick. It is not a statewide offset, and it does not convert a chart outside the United States.