Understanding Tidal Heights, Chart Datum and Charted Depths

A practical guide to MHWS, MHWN, MLWN, MLWS, HAT, LAT and the terminology used on nautical charts and in tide tables.

When planning a passage, entering a shallow harbour or calculating the depth of water beneath a boat, it is essential to understand the difference between charted depth, height of tide and actual depth of water.

Nautical charts and tide tables use a common reference level known as Chart Datum. They also contain a number of abbreviations, including MHWS, MLWS, HAT and LAT, which describe different tidal levels.

Understanding these terms is an important part of the RYA Day Skipper Shorebased Course, the Yachtmaster Offshore Shorebased Course and of course the Yachtmaster Practical Exam.

1. Chart Datum (CD)

Chart Datum (CD) is the reference level from which soundings and depth contours shown on nautical charts and heights of tide are normally measured.

Around the UK, Chart Datum is generally set at Lowest Astronomical Tide (LAT), although this is not universally the case. In parts of the Baltic Sea (where tide is negligible) CD is often set at Mean Sea Level.

The purpose of using a low reference level is to provide mariners with a consistent, conservative basis for calculating water depths.

For example, if a chart shows a sounding of 3.2 m this means the seabed is 3.2 m below Chart Datum, which around the UK generally corresponds to Lowest Astronomical Tide (LAT).

If the height of tide is 2.0 m above Chart Datum, the actual depth of water will be approximately 5.2 m.

2. The Different Tidal Levels

The following diagram illustrates the relationship between the main tidal levels, Chart Datum and the seabed.

3. What Do the Abbreviations Mean?

HAT – Highest Astronomical Tide

HAT is the highest tide level that can be predicted to occur under average meteorological conditions, considering the full combination of astronomical influences. It is generally higher than MHWS and is not necessarily reached every year.

Importantly, HAT is not the highest water level that can ever occur. Strong winds and low atmospheric pressure can push sea levels above HAT.

MHWS – Mean High Water Springs

MHWS represents the average height of high waters during spring tides. Spring tides occur around the new and full moons, when the tide-generating effects of the Sun and Moon reinforce one another.

MHWS is not the maximum possible high water. Some spring tides will be higher, and others lower.

MHWN – Mean High Water Neaps

MHWN is the average height of high waters during neap tides. Neap tides occur around the first and third quarters of the Moon, when the Sun and Moon’s tide-generating effects partially oppose one another.

High water during neaps is generally lower than during springs.

MSL – Mean Sea Level

MSL is the average sea level over a long period, taking account of the rise and fall of the tide. It is an important reference for surveying and land elevations but should not be confused with Chart Datum.

MLWN – Mean Low Water Neaps

MLWN represents the average height of low waters during neap tides. Because neap tides have a smaller tidal range, low water generally does not fall as far as it does during springs. Consequently, MLWN is normally higher than MLWS.

MLWS – Mean Low Water Springs

MLWS is the average height of low waters during spring tides. Spring low waters are generally lower than neap low waters, making MLWS particularly relevant when considering shallow water, drying banks and harbour entrances. However, the tide can fall below MLWS.

LAT – Lowest Astronomical Tide

LAT is the lowest tidal level predicted to occur under average meteorological conditions and any combination of astronomical influences. Around the UK, LAT is commonly used as Chart Datum (CD).

LAT is not an absolute minimum water level. Unusual weather conditions, particularly strong offshore winds or high atmospheric pressure, can cause the water level to fall below LAT.

4. Charted Depth, Height of Tide and Actual Depth

Three measurements are particularly important when calculating the water available beneath a boat.

Charted Depth: The sounding shown on a nautical chart, measured below Chart Datum.

Height of Tide: The predicted height of the sea surface above Chart Datum at a particular time.

Actual Depth of Water: The distance from the seabed to the water surface at that time.

The relationship is: Charted Depth + Height of Tide = Depth of Water

Worked Example

A chart shows a sounding of 2.4 m. The predicted height of tide at the time of passage is 3.2 m.

A vessel with a draught of 1.5 m would therefore have a predicted under-keel clearance of 4.1 m.

These calculations assume the tide behaves as predicted and do not allow for squat, waves, surveying tolerances or meteorological effects.

5. Drying Heights

Some parts of the seabed, including sandbanks, rocks and mudflats, are above Chart Datum and will therefore become exposed when the tide falls sufficiently.

These are known as drying areas.

On nautical charts, drying heights are normally shown as underlined figures. They indicate how far a feature rises above Chart Datum.

To calculate the depth of water covering a drying feature:

Height of Tide – Drying Height = Water Depth

Worked Example

A sandbank has a drying height of 1.8 m. The predicted height of tide is 3.2 m.

The depth of water above the sandbank will be:

 

If the height of tide subsequently falls to 1.0 m the sandbank will be exposed by 0.8 m. This distinction between a charted depth and a drying height is particularly important when navigating shallow estuaries and tidal harbours.

 

6. Tidal Range

The tidal range is the vertical difference between high water and low water.

During spring tides, the range is generally greater. During neap tides, it is smaller. There are two useful reference ranges:

• Mean Spring Range: MHWS minus MLWS.

• Mean Neap Range: MHWN minus MLWN.

For example, if MHWS is 5.0 m and MLWS is 0.5 m the mean spring range is 4.5 m.

These are mean values, not necessarily the range of a particular day’s tide. Actual predicted high and low waters should be taken from tide tables.

RYA students often confuse height of tide and range. Height of tide is the vertical distance between Chart Datum and the water surface at a particular time. Tidal range is the difference between high-water and low-water heights during a particular tidal cycle.

7. Under-keel clearance formula

 

This is the predicted static under-keel clearance. A navigator must additionally allow for squat, heel, waves, tidal uncertainty and other relevant factors.

8. Charted Heights and Overhead Clearances

So far, we have considered depths below the water’s surface. However, boaters must also consider heights above it, particularly when passing beneath bridges, overhead cables and other obstructions.

On modern UK ADMIRALTY charts, the usual conventions are:

These conventions are not universal, so always check the datum notes on the chart.

Worked Example: Passing Under a Bridge

Suppose a bridge has a charted vertical clearance of 12.0 metres above HAT.

HAT is 5.5 metres above Chart Datum, and the predicted height of tide at the time of passage is 3.5 metres.

The predicted clearance beneath the bridge is:

If your vessel has an air draught of 11.0 m, the theoretical clearance above the vessel is 3.0 m

Remember that a suitable safety margin is essential, and overhead electrical cables demand particular caution. Some cable clearances are published as safe clearances rather than simple physical clearances.

9. Why Predicted and Actual Heights May Differ

Tide tables predict astronomical tides. They do not fully account for prevailing weather conditions.

The actual height of tide can be affected by:

• Atmospheric pressure: low pressure tends to raise sea levels, while high pressure tends to lower them.

• Wind direction and strength: persistent winds can push water towards or away from the coast.

• Storm surges: weather systems can cause substantial departures from predicted tidal levels.

• Local conditions: river flows and local geography can also influence observed water levels.

For this reason, tidal predictions should never be treated as guarantees of minimum water depth.

10. Southampton Water and the Solent

The Solent provides an excellent practical example of why understanding tidal datums is important.

Southampton’s double high water and young flood stand make the local tidal curve more complicated than the simple rise and fall taught during basic tidal theory.

However, the underlying principles remain the same. Charted depths are referenced to Chart Datum, and tidal heights are added to establish the predicted available water depth.

When operating in the Solent, remember that the height of the tide and the strength or direction of the tidal stream are separate considerations. High water does not necessarily coincide with slack water.

For more information, read our guides to spring and neap tides and the Rule of Twelfths.

Frequently Asked Questions

What is the difference between Chart Datum and Lowest Astronomical Tide?

Chart Datum is the reference level used on a nautical chart. Lowest Astronomical Tide is the lowest predicted astronomical tidal level under average weather conditions. Around the UK, Chart Datum is normally approximately equal to LAT.

What is the difference between MHWS and HAT?

MHWS is the mean high-water level during spring tides. HAT is the highest astronomical tide predicted under average weather conditions, so it is normally higher than MHWS.

Can the tide fall below Chart Datum?

Yes. Although Chart Datum is selected as a low reference level, unusual astronomical or meteorological circumstances can cause the water level to fall below it.

Does a charted depth include the tide?

No. Charted depths are referenced to Chart Datum. The height of tide must normally be added to calculate the predicted depth of water.

What does an underlined depth mean on a nautical chart?

It normally indicates a drying height. The feature lies above Chart Datum and can become exposed at low tide.

Is mean sea level the same as Chart Datum?

No. Mean Sea Level is a long-term average sea level, whereas Chart Datum is normally a considerably lower reference level in tidal UK waters.

Are spring tides always the highest tides of the year?

No. Spring tides occur approximately every fortnight, but their heights vary. Some astronomical spring tides are considerably larger than others.

Why are nautical chart depths measured from Chart Datum rather than Mean Sea Level?

Nautical chart depths are generally referenced to a deliberately low tidal datum to give navigators a conservative indication of the water available. In most UK tidal waters, Chart Datum is approximately Lowest Astronomical Tide. Using this reference makes it straightforward to add the predicted height of tide to calculate the water depth at a particular time. Mean Sea Level is used in some parts of the world.

How do I calculate the clearance beneath a bridge at low tide?

Check the vertical clearance and its reference datum on the nautical chart. Where clearance is referenced to HAT, subtract the predicted height of tide from the HAT height, then add the result to the charted clearance. Compare this with the vessel’s air draught and allow an appropriate safety margin. Never assume that bridge and overhead cable heights use the same datum without checking.

Learn More About Tidal Navigation

Understanding Chart Datum, tidal heights and charted depths is essential for safe navigation.

At Chieftain Training, we teach these principles on our RYA Day Skipper and RYA Yachtmaster Offshore navigation courses, alongside tidal calculations, passage planning and practical pilotage.

Students learn not just how to calculate tidal heights, but how to apply that knowledge when navigating real boats in real tidal waters.

Doug Innes
RYA Navigation Instructor Trainer & Yachtmaster Instructor
The author has been teaching tidal theory for 35 years.