Maritime Questions › Day Skipper Charts Tides

When taking a 3-bearing fix, what is the ideal angle between bearings and why?

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. Angle does not matter as long as three objects are visible

B. 120° apart — spreading bearings equally avoids parallax error

C. 90° apart — right angles give mathematically exact positions

D. 60° apart — maximises the triangle's area and makes any bearing error more visible

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

A. Lowest Astronomical Tide (LAT)

B. Mean High Water Springs (MHWS)

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. 2/12 of the range

B. 1/12 of the range

C. 4/12 of the range

D. 3/12 of the range

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