Maritime Questions › Day Skipper Charts Tides

On an Admiralty chart, a feature shown with a depth figure underlined (e.g. 1.8) indicates what?

Q: On an Admiralty chart, a feature shown with a depth figure underlined (e.g. 1.8) indicates what?

A drying height — it dries 1.8 m above chart datum at LAT Underlined figures on a chart represent drying heights — the feature dries that many metres above LAT. Add tidal height to determine whether the feature is covered.

Q: On a UK Admiralty chart, depths are measured from which reference level?

Lowest Astronomical Tide (LAT) UK charts use LAT as chart datum — the lowest level the sea is predicted to fall under average meteorological conditions. All charted depths are below this, so actual depth is always at least the charted depth.

Q: Using the Rule of Twelfths, how much of the tidal range rises or falls in the SECOND hour of a tidal cycle?

2/12 of the range The Rule of Twelfths: 1st hour 1/12, 2nd hour 2/12, 3rd hour 3/12, 4th hour 3/12, 5th hour 2/12, 6th hour 1/12. The tidal flow is fastest mid-cycle.

A. The height of a coastal feature above MHWS

B. A drying height — it dries 1.8 m above chart datum at LAT

C. A depth of 1.8 m below chart datum

D. An uncertain sounding requiring a safety margin

On a UK Admiralty chart, depths are measured from which reference level?

A. Mean High Water Springs (MHWS)

B. Lowest Astronomical Tide (LAT)

C. Mean Low Water Neaps (MLWN)

D. Mean Sea Level (MSL)

Using the Rule of Twelfths, how much of the tidal range rises or falls in the SECOND hour of a tidal cycle?

A. 4/12 of the range

B. 1/12 of the range

C. 2/12 of the range

D. 3/12 of the range

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