Question 1

What is a "transit" (or leading line) in coastal navigation, and how can it be used to check for compass error?

Accepted Answer

A transit is formed when two charted objects (e.g. a church spire behind a headland) appear in line from the vessel's position; while the vessel is on the transit, taking a compass bearing of the transit and comparing it to the charted bearing between the two objects reveals any difference — this difference indicates the combined effect of compass error (variation and deviation) Transits (leading lines) are a classic, low-tech way to check compass error — because the charted bearing between the two objects is known precisely, any discrepancy with the compass bearing taken while on the transit must be due to compass error.

Question 2

When taking a visual position fix using two compass bearings of charted objects, why is a third bearing often taken if a suitable object is available?

Accepted Answer

Two bearings give a position at their intersection, but a third bearing forms a small triangle ("cocked hat") where the three position lines meet — the size of this triangle gives a visual indication of the likely accuracy of the fix and any compass error, helping the navigator judge how much to trust the result A two-bearing fix gives a single intersection point with no indication of error; a third bearing reveals a "cocked hat" whose size is a practical, visual guide to the fix's reliability.

Question 3

Using the "rule of twelfths" to estimate the height of tide between low water and high water, what fraction of the total tidal range is gained during the first hour after low water?

Accepted Answer

1/12 of the range The rule of twelfths approximates the rise/fall over each hour of a (roughly) 6-hour tide as 1, 2, 3, 3, 2, 1 twelfths — so the first hour accounts for 1/12 of the total range, the slowest rate of change near the turn of the tide.