Maritime Questions › Day Skipper Charts Tides

What is the difference between an Estimated Position (EP) and a Fix?

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. An EP is more accurate than a Fix because it accounts for tidal stream

B. A Fix can be plotted without any external references if you know your speed

C. An EP and a DR position are identical — both apply tidal stream

D. A Fix is derived from two or more position lines from external references; an EP is derived from a known position using course steered, speed, and tidal stream — no external reference

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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