Maritime Questions › Day Skipper Charts Tides

Why is a transit (two objects in line) particularly useful as a position line?

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. Transits apply only to leading lights in a harbour entrance

B. It needs no compass and is free of compass error — it gives an exact bearing from the chart if the objects are charted

C. A transit replaces all other position lines so only one is needed

D. It only works at night when lights align

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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