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Chart datum, drying heights and tidal levels

Every number on a chart is measured from somewhere, and it isn't always the same place. Depths come from one level, lighthouse heights from another and bridge clearances from a third. Mixing them up is how boats touch bottom or hit bridges.

Chart datum

Chart datum is the level that charted depths and drying heights are measured from. On UK charts it's approximately Lowest Astronomical Tide (LAT), so the tide very rarely falls below it. That's deliberate: the depth on the chart is close to the least water you're ever likely to find, and the tide almost always adds to it.

Weather can still take the sea below chart datum. High pressure and strong offshore winds can lower the level, so leave a margin.

The tidal levels

LevelMeaning
HATHighest Astronomical Tide: the highest level predicted under average weather conditions.
MHWSMean High Water Springs: the average height of high water at spring tides.
MHWNMean High Water Neaps: the average height of high water at neap tides.
MLWNMean Low Water Neaps: the average height of low water at neap tides.
MLWSMean Low Water Springs: the average height of low water at spring tides.
LATLowest Astronomical Tide: the lowest level predicted under average weather conditions.

From highest to lowest, they run HAT, MHWS, MHWN, MLWN, MLWS, LAT. Each standard port's values are given in the almanac, and many charts have a table of tidal levels for nearby ports.

What's measured from where

  • Charted depths (soundings) are measured down from chart datum.
  • Drying heights are measured up from chart datum, and shown as underlined figures. They mark ground that covers and uncovers with the tide.
  • Heights of land and of lights are generally measured above a high water level, MHWS on UK charts.
  • Bridge and cable clearances on newer charts are measured above HAT, the worst case. Some older charts still use MHWS. Check the chart's notes before trusting a clearance.

Putting it together

  • Depth of water = charted depth + height of tide.
  • Depth over a drying patch = height of tide − drying height.
  • Clearance under the keel = depth of water − your draught.

To work out the height of tide in the first place, see the standard port and secondary port guides, or the rule of twelfths for a quick estimate.

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Questions

What is chart datum?

The level that charted depths and drying heights are measured from. On UK charts it's approximately the Lowest Astronomical Tide, so the tide is very rarely below it.

What is a drying height on a chart?

The height above chart datum of ground that covers and uncovers with the tide. It's shown as an underlined figure. Subtract it from the height of tide to get the depth of water over it.

What are bridge clearances measured from?

On newer charts, from Highest Astronomical Tide (HAT), the worst case. Some older charts use MHWS, so check the chart's notes before relying on a clearance.

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Last checked 5 November 2026. DeckPrep is an independent revision tool and is not affiliated with, endorsed by, or operated by the RYA. Always check current requirements with the RYA.