Tides

What Is Chart Datum? The Reference Level Explained

Published · Last checked October 2026

Chart datum (CD) is the reference level from which charted depths, drying heights and predicted tide heights are measured. On UK ADMIRALTY charts it is set at, or very close to, Lowest Astronomical Tide (LAT), the lowest level the tide is predicted to reach under average weather. Because the tide rarely falls that far, the water is almost always deeper than the charted depth. Real depth equals charted depth plus the height of tide above chart datum at that moment.

Why charts need a datum

The sea surface rises and falls twice a day, so a chart cannot simply show 'the depth'. Instead it shows depth below a fixed, low reference level. Tide tables then give the height of the water above that same level. Add the two together and you have the depth of water at any time. Because both numbers share one reference, the sum works anywhere on the chart.

Diagram showing chart datum, charted depth, drying height, height of tide and heights above MHWS
Chart datum is the baseline for depths below and drying heights above.

Chart datum and LAT

In UK and many other waters the UK Hydrographic Office sets chart datum at approximately LAT. This is deliberately cautious: on all but the most extreme days the tide stays above it. Strong offshore winds and high atmospheric pressure can push the level below predictions, so a low spring tide in a big high can briefly fall below chart datum. Older charts and some foreign charts used other datums, so check the chart's title block.

Levels measured from chart datum
TermMeasuredShown as
Charted depth (sounding)Downwards from CDPlain figures, e.g. 4₂ = 4.2 m
Drying heightUpwards from CDUnderlined figure, e.g. 1̲₅ = 1.5 m above CD
Height of tideUpwards from CDTide tables and curves
Heights of lights, bridgesUsually from MHWS or HATNot chart datum, check the chart notes

Using chart datum in practice

  1. Read the charted depth (or drying height) at your position.
  2. Find the height of tide for the time from the tide table or tidal curve.
  3. Depth of water = charted depth + height of tide. For a drying patch, depth = height of tide − drying height.
  4. Subtract your draught and keep a safety margin for swell, squat and forecast error.

For example, a charted depth of 1.8 m with a height of tide of 2.6 m gives 4.4 m of water. With a 1.6 m draught you have 2.8 m under the keel.

Practise these sums in the tidal heights lesson and see how to read a tide table.

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Frequently asked questions

Is chart datum the same as low water?

No. Chart datum is close to the lowest predicted tide (LAT). Ordinary low waters, especially at neaps, are well above it, so there is usually more water than charted.

Can the tide go below chart datum?

Yes, occasionally. High pressure and strong offshore winds can lower the sea level below the predicted height, so allow a margin on very low spring tides.

Is chart datum the same as Ordnance Datum?

No. Ordnance Datum Newlyn is the land-map reference near mean sea level. Chart datum is much lower and varies from port to port.

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