What is chart datum?
7 min read updated
The zero line every tide height is measured from — why it differs between countries, why our heights can go negative, and what that means when you read a depth.
The short version
A tide height is meaningless without the level it is measured from. Nautical charts use a low datum such as LAT or MLLW so that charted depths are pessimistic. Our predictions come from a global model whose heights are relative to mean sea level, so their lows go below zero — the timing and the shape of the curve are unaffected.
Why a datum exists at all
A tide height on its own says nothing. “Two metres” is only meaningful once you know two metres above what, and the level chosen as that zero is the datum. It is a bookkeeping decision rather than a property of the sea: the same low water can honestly be printed as a negative number by one authority and a positive one by another, because they are counting from different floors.
A depth printed on a nautical chart has to be safe to trust at any state of the tide, so hydrographic offices count from a level the sea almost never falls below. Add the predicted tide height to the charted depth and you get the water you can actually expect over that patch of seabed. Because the datum is low, the arithmetic is pessimistic, which is the whole point: an error in your favour is a grounding you did not have.
Heights on land are levelled from a different zero again — an ordnance or geodetic datum near mean sea level — which is why the height of a quay wall and the height of a tide cannot be compared until someone has told you which zero each one uses.
LAT, MLLW and the rest
There is no single low. Each authority picked a definition that suited its coast and its era. How far apart they sit depends on the local range: mean sea level stands 1.8 m above chart datum on the coast drawn below, and several times that where the tide is big. Their order never changes — highest astronomical tide at the top, mean sea level in the middle, the low-water datums beneath it.
| Datum | What it is | Where you meet it |
|---|---|---|
| LAT — Lowest Astronomical Tide | The lowest level astronomy alone can produce, assessed over about 19 years | Chart datum on most of the world’s charts, including the UK Hydrographic Office’s, and the level the International Hydrographic Organization recommends |
| MLLW — Mean Lower Low Water | The average of the lower of each day’s two low waters, over 19 years | Chart datum in the United States; the zero of NOAA’s tide tables |
| MLWS — Mean Low Water Springs | The average low water of spring tides | Older British and Commonwealth tables, and still quoted for some harbours |
| MSL — Mean Sea Level | The average level of the sea, in the middle of the range | The zero of the global model this site predicts from, and the level most land datums are set near |
The nineteen years in those definitions are not arbitrary. The Moon’s declination swings to about 28.6° north and south, and the size of that swing itself cycles over 18.6 years, so a datum meant to describe the true extremes has to watch a full cycle before it can claim to have seen the lowest tide astronomy can make.
This is why two honest sources can print different numbers for the same low water. The ladder above shows one: −1.2 m counted from mean sea level is +0.6 m counted from LAT, and neither figure can be used with the other’s chart.
Mean sea level, and our negative lows
Every prediction on this site comes from FES2022, a global tide model built from satellite altimetry: we take its harmonic constants — 34 of them — at the nearest ocean point to your station and run the tide forward from there. Those constants describe the water relative to mean sea level, so that is our zero, and it sits in the middle of the range rather than near the bottom of it.
The visible consequence is negative numbers. A low water printed as −1.2 m is 1.2 m below the local average sea level; on a chart whose zero sits below that low water, the identical event is printed as a positive figure. Nothing about the sea has changed. The datum ladder above shows the same low measured twice, and both readings are honest.
What a datum cannot do is bend the curve. Changing the zero slides the whole prediction up or down by a constant; it never moves a high water in time, never changes the interval between the tides, and never changes the tidal range, because subtracting one height from another cancels the datum out. Times and range survive the translation intact — only the absolute number needs care.
What this means in practice
Use the part of our prediction that travels, and take the part that does not from the authority that owns it.
- Times and shape: use ours. When high and low water fall, how long the water floods, when the flow turns — these are what the model is good at, and they do not depend on the datum at all. See how to read a tide chart.
- Absolute depth: use the chart. Never subtract our height from a charted depth without first checking that both are counted from the same zero. They usually are not.
- Offsets are roughly constant. If you know both numbers for one low water — ours and the official table’s — the difference is close to the offset between the two zeros at that place, and it will be about the same next week. It is a useful sanity check, not a survey.
- The weather has the last word anyway. Roughly a centimetre of sea level answers to each hectopascal of air pressure, and a storm can move the water half a metre or more for a day at a time, in either direction. No datum protects you from that; a margin does.
The datum decides what the number looks like. The astronomy decides when the water arrives.
Gauge stations are different
Some of our stations are real tide gauges — in this catalogue, the NOAA CO-OPS network, of which Boston and San Francisco (Golden Gate) are two. A gauge is an instrument with a measured record behind it, and a record like that is what an official datum is computed from in the first place. NOAA publishes those datums for its gauges, and its own official predictions on them, at tidesandcurrents.noaa.gov.
What is different is the station, not our arithmetic. The prediction we show on a gauge page is produced exactly like every other page on this site: FES2022 constants at the nearest ocean point, heights relative to mean sea level. The badge tells you that a measured record exists at that spot and that an official table is available for it — not that the figures you are reading are NOAA’s, and not that they are on NOAA’s datum. Our tide card names the zero we used on every page. How we compute spells the chain out.
So a gauge page is worth seeking out for two reasons. Its position is a real, surveyed one rather than a coastal place name, and there is an authoritative table to check ours against when the answer has to be exact. For a charted depth, an under-keel clearance or anything a harbourmaster would want in writing, use that official table. For deciding which afternoon to walk the flats, ours is the faster answer. Gauge vs. interpolated stations goes into how much the two differ.
Questions people ask
What is chart datum?
It is the zero level a tide height is measured from. Nautical charts use a low one, such as Lowest Astronomical Tide, so that a charted depth is pessimistic at any state of the tide.
Why is my tide height negative?
Because our heights are measured from mean sea level, which sits in the middle of the tidal range rather than near the bottom of it. A −1.2 m low is 1.2 m below the local average sea level, not a warning.
What is LAT?
Lowest Astronomical Tide — the lowest level astronomy alone can produce at a place, assessed over about 19 years so that the Moon’s 18.6-year cycle is covered. It is the chart datum on most of the world’s charts.
What is MLLW?
Mean Lower Low Water — the average of the lower of each day’s two low waters, taken over 19 years. It is the chart datum of the United States, and it is not the same level as LAT.
Is chart datum the same as sea level?
No. Mean sea level is the average level of the sea, in the middle of the range; chart datum is deliberately close to the bottom of it, so that the water rarely falls below the depths printed on the chart.