MLLW, NAVD88, and what the numbers mean

A tide number is not, by itself, “how deep the water is under my boat” and not “how much space is under the bridge.” It is how high the water surface sits compared to a chosen zero line. That zero is called a datum.

Think of a mark painted on a piling. Every reading is “this many feet above (or below) that mark.” Change the mark, and the same water gets a different number.

When a station offers more than one datum, the app can switch that zero (for example MLLW, NAVD88, or a high-water datum). The water has not moved — only the ruler did.

When a station has measured local datums — the bottom of the channel and the underside of a bridge — those marks show on charts and calendars, and they can drive public or private alerts and calendar events. That is how the app turns a tide number into “is there water under the keel?” or “will I clear the bridge?” — not by guessing from MLLW alone.

Today, owners of Sentient Things tide stations get this. We are making the same measured-bottom and measured-bridge treatment a feature of NOAA, SECOORA, NERACOOS, and USGS stations as well — not a Sentient Things–only idea.

MLLW — the usual zero on US tide charts

MLLW means Mean Lower Low Water.

On most US coasts there are two low tides a day. One is lower than the other. NOAA (and similar agencies) average those lower lows over a long period — about 19 years. That average is 0.0 on a typical NOAA tide chart.

• 0.0 ft MLLW is a typical lowest-low at that gauge. It is not the bottom of the channel.
• +3.0 ft MLLW means the water surface is about 3 feet above that typical lowest-low.
• A negative tide (for example −0.5 ft MLLW) means the water is below that long-term average. The ocean can go lower than “zero.” That is normal on some days of the month.

How MLLW is measured

A water-level station records the tide for years. Analysts take the lower low from each day, average them, and call that MLLW. It is a statistic at the gauge, not a sounding of your slip, your boat ramp, or the middle of the creek.

How deep is the water?

You need two numbers, not one:

1. Charted depth on a nautical chart (in the US this is usually also relative to MLLW — how much water is over the bottom when the tide is at that typical lowest-low).
2. Today’s tide height in MLLW from the app.

Rough guide: water over the bottom ≈ charted depth + MLLW tide.

Example: the chart says 4 ft, and the app shows +2.5 ft MLLW → about 6.5 ft of water. If the tide is −1.0 ft MLLW → about 3 ft.

That is still an estimate unless the station has a measured bottom. A NOAA chart plus MLLW does not know your slip. Channels silt in. Storms move the bottom. Use official nautical charts and your own sounding for navigation when no local bottom datum is set.

How low is the bridge?

Bridge clearance is a different measurement from an MLLW tide height.

On nautical charts, vertical clearance under a bridge is usually measured from a high-water datum (often MHW or MHHW — a typical high tide) up to the lowest part of the structure. At high tide you have less room. At low tide you have more.

You cannot read “feet under the bridge” from an MLLW number alone unless that station has a measured bridge datum. Without that, you need the published clearance (and which high-water datum it uses), then where today’s water sits relative to that high-water mark.

If the station lets you switch the chart to a high-water datum, a reading near zero means you are near a typical high — the tight case for clearance. Water well below that zero means more room under the bridge.

Measured bottom and bridge

The channel bottom and bridge low steel (or similar) can be surveyed or measured and stored on a station.

Once those local datums are in place:

• Charts can show water relative to the bottom (depth) and relative to the bridge (clearance) — not only relative to MLLW or NAVD88.
• Calendars can show windows such as too shallow to get out or bridge too low, using those same marks.
• The same datums can feed public or private alerts and public or private calendars for that station (for example a marina calendar everyone can subscribe to, or a private feed).

Right now this is in place for Sentient Things station owners. It is not meant to stay a Sentient Things–only feature: the same measured datums, charts, calendars, and alerts are coming to NOAA, SECOORA, NERACOOS, and USGS stations too.

Stations that do not have these measurements yet still show the usual tidal zeros. This is not a threshold you type yourself in this version of the app. For a Sentient Things gauge today, see Hardware gauges or contact sales@sentientthings.com.

NAVD88 — a land-survey zero, not a tide zero

NAVD88 (North American Vertical Datum of 1988) is a nationwide elevation system used for maps, flood certificates, roads, and survey marks. 0 NAVD88 is not 0 MLLW. The difference is often a few feet and changes from place to place.

Some gauges — especially flood- or land-oriented ones — report in NAVD88 so the water lines up with ground elevations (“the road is at this many feet NAVD88; the water is at that”). A reading of 2.0 ft NAVD88 is not “2 feet of tide” in the MLLW sense.

To compare NAVD88 water to a nautical chart in MLLW, you need the local offset between those datums at that station. Use the datum control in the app when the station has both.

What to remember

• By themselves, tide numbers = water surface vs a zero, not depth under the keel and not the gap under a bridge.
• With measurements, the bottom and bridge can be stored as datums and shown on charts and calendars, and used for public or private alerts and calendars. Sentient Things owners have this today; NOAA, SECOORA, NERACOOS, and USGS stations are getting the same treatment.
• MLLW = typical lowest-low; the usual US chart and tide zero.
• Without a local bottom datum, depth ≈ charted depth + MLLW tide (rough, at that gauge’s waterway).
• Without a local bridge datum, bridge room depends on a published high-water clearance, not on MLLW by itself.
• NAVD88 = survey/flood elevation zero; different from MLLW.