Navigation, Compass, Charts and Basic Meteorology
This domain asks you to move between the three norths, read a chart, and interpret simple weather signs. The arithmetic of compass error is the single most examined skill.
Three norths and compass error
- True north - geographic; magnetic north - where the compass needle points; compass north - what your compass reads.
- Variation is the angle between true and magnetic north, caused by the earth's magnetism; it is printed on the chart. Deviation is the error caused by the ship's own magnetism, taken from the deviation card.
- Compass error = variation + deviation (algebraic sum, East positive, West negative).
Charts
Mercator projection; latitude and longitude in degrees, minutes and seconds, with 1 minute of latitude ≈ 1 nautical mile; soundings in metres below chart datum; symbols for rocks, wrecks, buoys and depths. Course is the direction steered; track is the path made good.
Meteorology
- Beaufort scale describes wind force 0 (calm) to 12 (hurricane).
- Closely spaced isobars mean strong wind; a low brings cloud, rain and gales, a high brings settled fine weather.
- Cloud: cumulonimbus = thunderstorm and squalls; cirrus = high, fair; stratus = low grey layer.
Worked example
A chart gives variation 5° W; the deviation card gives 3° E. Compass bearing is 045°. Compass error = (+3) + (−5) = 2° W. True bearing = compass + error with East positive: 045° + (−2°) = 043°. The common error is adding 5 and 3 and getting 8° the wrong way - always apply the signs.
Traps
Adding variation and deviation arithmetically without signs; applying the correction in the reverse direction (true to compass); confusing variation with deviation; treating a minute of longitude as always one nautical mile.