Tide Datums Explained — MLLW, MHHW, NAVD88 & More
Every tide chart has a “zero” — but which zero? Here's what NOAA tidal datums mean and which one to use for what.
When a tide chart says “high tide 3.0 ft,” that's 3.0 ft above some reference point. That reference point is called a tidal datum, and NOAA uses several different ones for different purposes. If you're trying to figure out how deep an inlet is, how much clearance a boat has under a bridge, or what FEMA means by “1% annual flood elevation,” you need to know which datum the number refers to.
The six NOAA tidal datums
NOAA computes six standard tidal datums for every primary tide station, based on at least 19 years of observation (called the National Tidal Datum Epoch). Each is the average of a different tide-defining event over that 19-year period.
| Datum | Definition | Used for |
|---|---|---|
| MHHW | Mean Higher High Water — average of the higher of two daily high tides | Coastal regulation, FEMA flood maps |
| MHW | Mean High Water — average of all high tides | Property boundary in many states |
| MSL | Mean Sea Level — average of all hourly heights | Hydrographic surveys, sea level rise studies |
| MTL | Mean Tide Level — halfway between MHW and MLW | Engineering applications |
| MLW | Mean Low Water — average of all low tides | Older nautical charts |
| MLLW | Mean Lower Low Water — average of the lower of two daily low tides | Modern nautical charts, NOAA tide predictions |
MLLW — the “zero” on most tide charts
Mean Lower Low Water is the chart datum used by NOAA for tide predictions and nautical charts in the US. It's the average of the lowest of the two daily low tides over 19 years. When your tide chart says high tide is “3.0 ft” and low tide is “0.2 ft,” both numbers are measured above MLLW.
Practical implication: water can occasionally drop below MLLW on big spring tides — you'll see negative numbers on the tide chart (e.g., -0.3 ft). MLLW is an average, not an absolute minimum. Shoals charted at 0 ft on a nautical chart will be exposed at low tide during spring tides.
MHHW — used for flood elevations
Mean Higher High Water is the average of the daily higher high tide. FEMA flood maps, sea level rise projections, and most coastal regulatory programs use MHHW as their reference. When you read “OBX is projected to see 2 ft of sea level rise above MHHW by 2050,” that means 2 ft above the current average daily high tide.
NAVD88 — geodetic vertical datum
NAVD88 stands for North American Vertical Datum of 1988. Unlike the tidal datums above, NAVD88 is a geodetic datum — it's based on a fixed reference point on land (a benchmark in Rimouski, Quebec), not on the tides. NAVD88 is used in:
- Topographic maps and elevation data (USGS, lidar)
- FEMA Base Flood Elevations
- Highway engineering (NCDOT)
- Building codes that reference “feet above ground”
You can convert between NAVD88 and the tidal datums using NOAA's VDatum tool. At Duck, MLLW is roughly 1.5 ft below NAVD88. At Hatteras, MLLW is roughly 0.8 ft below NAVD88. The exact relationship varies by location because the ocean surface is not flat — even ignoring tides, sea level differs by inches from one OBX gauge to another.
Datum relationships at OBX gauges
| Station | MHHW above MLLW | MLLW below NAVD88 |
|---|---|---|
| Duck (8651370) | ~3.3 ft | ~1.5 ft |
| Oregon Inlet (8652587) | ~1.8 ft | ~1.1 ft |
| Hatteras (8654467) | ~1.5 ft | ~0.8 ft |
| Ocracoke (8654792) | ~1.5 ft | ~0.7 ft |
Values rounded — exact published values are on each NOAA station's “Datums” page.
Why this matters for you
- Boating: Use MLLW. Charted depths and bridge clearances reference MLLW. Add the current tide height to get real-time depth.
- Buying or building near the beach: FEMA flood maps reference NAVD88 elevations. Your survey will be in NAVD88. Your tide chart will be in MLLW. They are not the same number.
- Surf fishing: Just use the tide chart — MLLW reference is fine for picking when to fish.
- Sea level rise reading: Most projections reference MHHW. Be careful comparing claims that mix datums.