Maritime Questions › Day Skipper Charts Tides

What is the key advantage of a tidal stream atlas over tidal diamonds on a chart?

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. Tidal diamonds provide more accurate spring rates

B. Tidal stream atlases apply only to neap conditions

C. The atlas is updated daily; diamonds are not

D. The atlas shows the direction and rate of the tidal stream across a wide area for each hour, not just at fixed points

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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