MLLW and tide datums, explained

Every tide height is measured from somewhere. When a table says a high tide will reach 6.1 ft, the obvious questions are "6.1 ft above what?" and "why does the table go negative?" The answer is a tidal datum: a reference level defined by the tide itself and averaged over many years. U.S. tide tables, this site included, use MLLW, Mean Lower Low Water. This page explains what that is, how it relates to sea level, land elevations and nautical charts, and what the full set of datums tells you about a place.

What MLLW is

Most of the California coast gets two low tides a day, and one is lower than the other (why that happens is on Why California gets two unequal tides a day). Take the lower of the two lows each day, average them over 19 years, and you have Mean Lower Low Water.

MLLW is set to 0.0 ft. So:

• 0.0 ft is an average lower low tide, a perfectly ordinary low.
• Positive heights are above it. Every high tide and most low tides are positive.
• Negative heights are below it. A minus tide is a lower low that dips below its own average, which happens most days around spring tides and exposes ground that is usually submerged.

MLLW is not the sea floor, the lowest possible tide, or sea level. It is a statistical line drawn on a pier.

The datum ladder

NOAA publishes a full set of datums for each station with a long enough record. Here they are for Port San Luis, all in feet above MLLW:

fig · datum ladder, port san luis

Datum	Name	What it averages
MHHW	Mean Higher High Water	The higher of each day’s two highs
MHW	Mean High Water	All high tides
MTL	Mean Tide Level	Halfway between MHW and MLW
MSL	Mean Sea Level	Every hourly water level
MLW	Mean Low Water	All low tides
MLLW	Mean Lower Low Water	The lower of each day’s two lows

NOAA also publishes ranges between them. The great diurnal range (GT, MHHW minus MLLW) is the span of an average day from its highest point to its lowest: 5.33 ft at Port San Luis. The mean range (MN, MHW minus MLW) is 3.58 ft. The diurnal inequalities say how unequal the two daily tides are on average: the two highs differ by about 0.71 ft and the two lows by about 1.04 ft.

Reading the ladder on this site

The Now panel in the console places the current water level between MLLW and MHHW, so you can see at a glance whether the water is near an average low, mid-tide, or close to an average higher high. The datum ladder on Tide Intelligence draws the station’s main datums with the current water level and the highest and lowest tides of the next 14 days beside them. Water above MHHW is a higher-than-average high tide, and a foot or more above it, with a storm on top, is what floods low-lying streets during king tides.

Santa Cruz and Ventura have no published datums, so their pages hide the ladder.

Why 0.0 ft is not sea level

Mean sea level sits in the middle of the tide range: 2.80 ft above MLLW at Port San Luis, and between about 2.7 and 3.7 ft along the California coast. So when someone says "the water rose 2 ft above sea level", they mean something like 4.8 ft on a tide table.

Tide tables use MLLW instead of sea level for a practical reason: safety at sea. Nautical charts print water depth as soundings measured down from the chart datum. If the chart datum were mean sea level, the water would be shallower than the chart says for about half of every day. With MLLW, the water is at least as deep as charted almost all the time; only minus tides drop below it. The U.S. adopted MLLW as the single chart datum for all its coasts in 1980 (the West Coast had used it long before), and tide tables use the same zero so the two can be added directly: charted depth plus tide height equals the actual depth.

Land elevations: NAVD88

Maps, surveys, flood maps and GPS heights use a different kind of datum, a geodetic (land) datum. In the U.S. it is the North American Vertical Datum of 1988 (NAVD88). It has nothing to do with the tide; it is tied to a network of survey marks across the continent.

On the California coast, by coincidence, NAVD88 sits very close to MLLW, from a few tenths of a foot below to about 0.4 ft above:

Station	NAVD88 above MLLW
Crescent City	+0.38 ft
Point Reyes	+0.02 ft
San Francisco	−0.06 ft
Monterey	−0.14 ft
Port San Luis	+0.08 ft
Santa Barbara	+0.13 ft
Los Angeles	+0.20 ft
San Diego	+0.43 ft

That closeness is convenient but easy to over-trust. On the East Coast the two differ by several feet, and anyone combining tide data with elevation maps (for sea-level rise or flood studies) should convert properly with NOAA’s VDatum tool rather than assuming they match.

The 19-year epoch

Why average over 19 years? The moon’s orbit is tilted, and that tilt wobbles through an 18.6-year nodal cycle that makes the tides slightly bigger in some years and smaller in others. Averaging over a full cycle, rounded up to 19 years, cancels it out. That period is the National Tidal Datum Epoch.

The datums on this site are on the 1983–2001 epoch, the one NOAA currently publishes for these California stations. Because sea level is rising, the water now averages a little higher against those numbers than it did during the epoch, and NOAA periodically moves to a newer epoch to catch up. When it does, every datum shifts by a few hundredths of a foot to a few tenths, and tables move with it.

Datums along the coast

Datums are local. The range and mean sea level change from station to station with the shape of the coast and the continental shelf:

fig · tide range, north to south

Station	GT (MHHW − MLLW)	MSL above MLLW
Crescent City	6.87 ft	3.70 ft
North Spit (Humboldt Bay)	6.86 ft	3.70 ft
Arena Cove	5.87 ft	3.14 ft
Point Reyes	5.77 ft	3.10 ft
San Francisco	5.84 ft	3.12 ft
Half Moon Bay	5.60 ft	2.99 ft
Monterey	5.34 ft	2.83 ft
San Simeon	5.24 ft	2.76 ft
Port San Luis	5.32 ft	2.80 ft
Santa Barbara	5.39 ft	2.78 ft
Santa Monica	5.43 ft	2.79 ft
Los Angeles	5.49 ft	2.82 ft
Newport Beach	5.41 ft	2.78 ft
La Jolla	5.33 ft	2.73 ft
San Diego	5.72 ft	2.94 ft

Because each station has its own MLLW, a 6.0 ft high at Crescent City is roughly an average higher high, while 6.0 ft at Port San Luis is 0.7 ft above average. Compare stations by how far the water is above or below their own datums, not by raw feet.

Minus tides in practice

Minus tides are the reason most people learn what MLLW means.

• How low they go On the California coast the lowest predicted tides of the year are around −1.5 to −2 ft. A −0.5 ft tide uncovers the upper reef; −1.0 ft and below opens up the mid and low intertidal, where the sea stars, anemones, nudibranchs and octopus live.
• When they happen Around spring tides, so near new and full moons, and on the low that is already the lower low of the day. From late spring through summer they fall in the early morning; from late fall through winter, in the late afternoon and evening.
• What can spoil them A big swell, which washes over exposed reef no matter what the table says, and a deep low-pressure system, which can hold the water several inches above the prediction. The residual tells you whether today’s tide is running high or low.

The other end of the ladder matters too. When predicted highs exceed MHHW by a foot or more, as during winter king tides, beaches narrow, coastal trails at sea level can flood, and a storm on top can push water into low-lying neighborhoods.

Summary

• Tide heights on this site are feet above MLLW, the 19-year average of each day’s lower low.
• 0.0 ft is an ordinary low, not sea level; mean sea level is about 2.8 ft above it on the Central Coast.
• Negative numbers are minus tides, the best tidepooling.
• MHHW is the average higher high; water above it is a high high tide.
• Each station has its own datums, so compare places by their datums, not by raw feet.