4.2 Chart Datums, Lowest Astronomical Tide (LAT) & Soundings

4.2 Chart Datums, Lowest Astronomical Tide (LAT) & Soundings

Navigators must master vertical reference planes, understand how soundings and drying heights are measured, and execute accurate depth and clearance calculations.

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Chart Datum (CD) & Lowest Astronomical Tide (LAT)

Chart Datum (CD) is the zero reference level from which charted depths (soundings) and drying heights are measured.

UK tidal charts normally use a datum approximately at Lowest Astronomical Tide (LAT). IHO recommends LAT or a closely equivalent level in oceanic tidal areas, while permitting other national datums; mean sea level or a similar level is used where tides are negligible. Always read the chart's datum note and use matching tidal heights.

Definition & Astronomical Basis of LAT

Lowest Astronomical Tide (LAT): The lowest water level that can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.

LAT is computed using harmonic analysis across the full 18.6-year nodal regression cycle, during which the Moon's ascending node regresses through 360° and lunar declination cycles between 18.5° and 28.5°. Computing LAT over this complete astronomical cycle ensures that astronomical tides rarely drop below Chart Datum under standard weather.

Meteorological Deviations Below Chart Datum

LAT assumes average meteorological conditions for the place concerned:

1. Inverted Barometer Effect: As a rough estimate, a 1 hPa pressure change corresponds to an opposite sea-level change of about 1 cm. Relative to an illustrative average of 1013 hPa, 970 hPa gives about +0.43m and 1035 hPa about -0.22m; actual response also depends on wind and local conditions.
2. Wind Stress & Storm Surge: Strong offshore gales blow surface water out of shallow bays and estuaries.

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Charted Depths (Soundings)

Soundings are depth figures printed across navigable waters on nautical charts:

• Normally displayed in black sloping figures on white water (deep offshore areas) and pale blue water (shallow contours).
• Measured vertically downwards from Chart Datum to the seabed.
• A sounding of 5.2 records a surveyed depth of 5.2 metres below the stated Chart Datum. Actual clearance depends on water level, survey reliability and any seabed change.

Calculating Total Water Depth over Soundings

At any given moment, the sea surface rises above Chart Datum by the Height of Tide (HoT):

$\text{Total Water Depth} = \text{Charted Depth} + \text{Height of Tide}$

Example Calculation:

• Charted Sounding: 3.4m; Predicted Height of Tide (HoT): 2.8m.
• $\text{Total Water Depth} = 3.4\text{m} + 2.8\text{m} = \mathbf{6.2\text{m}}$.

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Drying Heights (The Intertidal Zone)

The intertidal zone consists of sandbanks, mudflats, and rocky ledges that are covered at high water but uncover (dry out) at low water.

On Admiralty charts, drying areas are rendered in a green tint. The figures printed on these drying features are Drying Heights:

• Distinguished from soundings by being underlined on the chart, such as an underlined 1.4 or 0.6.
• Measured vertically upwards from Chart Datum to the top of the seabed or rock.
• A drying height of 1.4m (charted as an underlined 1.4) means the seabed rises 1.4 metres above Chart Datum. When the tide falls to Chart Datum (0.0m), the sandbank protrudes 1.4 metres into the air.

Calculating Water Depth over Drying Features

$\text{Total Water Depth} = \text{Height of Tide} - \text{Drying Height}$

• If Height of Tide exceeds drying height, the bank is covered:
  • Height of Tide = 3.6m, Drying Height = 1.4m: $\text{Depth} = 3.6\text{m} - 1.4\text{m} = \mathbf{2.2\text{m}}$.
• If Height of Tide is less than drying height, the bank is dry:
  • Height of Tide = 0.9m, Drying Height = 1.4m: $\text{Depth} = 0.9\text{m} - 1.4\text{m} = -0.5\text{m}$ (bank dries 0.5m above water).

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High Water Datums & Overhead Clearances

High water datums govern vertical clearances above the sea surface:

• Highest Astronomical Tide (HAT): Highest predictable tidal level under average weather across the 18.6-year cycle.
• Mean High Water Springs (MHWS): Average spring high water; defines charted coastlines.
• Mean High Water Neaps (MHWN): Average neap high water level.

Overhead Clearances (Bridges & Cables)

To ensure safety errors fail on the side of caution, charted vertical clearances for bridges and power cables are heights above the stated high-water datum, commonly HAT (or MHWS on older charts). As the tide falls below HAT, available clearance beneath the bridge increases.

Overhead Clearance Formula

$\text{Clearance Margin} = \text{Charted Clearance} + (\text{HAT} - \text{Height of Tide}) - \text{Air Draught}$

Where:

• $\text{Charted Clearance}$ is published height from HAT to the lowest span.
• $(\text{HAT} - \text{Height of Tide})$ is extra vertical space gained because the tide is below HAT.
• $\text{Air Draught}$ is vessel height from waterline to masthead tip (including antennas).

Bridge Clearance Example:

• Charted Bridge Clearance (to HAT): 16.0m; Port HAT: 5.0m; Predicted HoT: 3.2m; Air Draught: 16.8m.
• $\text{Bridge Height Above Water} = 16.0\text{m} + (5.0\text{m} - 3.2\text{m}) = 17.8\text{m}$.
• $\text{Clearance Margin} = 17.8\text{m} - 16.8\text{m} = \mathbf{1.0\text{m}}$.

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Tidal Levels & Reference Planes