Navigation and Position Fixing: Charts, Compass, Latitude/Longitude and Dead Reckoning
Charts and the information they carry
A coastal skipper must read a nautical chart as a working document, not a picture. Know the meaning of chart datum (usually LAT), soundings in metres and decimetres, drying heights, the magenta compass rose with true and magnetic rings, the variation note, and the difference between a leading line, a transit and a clearing bearing. Understand chart scale: large-scale charts (e.g. 1:20 000) show more detail for harbour work; small-scale charts cover more water with less detail. Trap: questions often mix up chart datum with mean sea level, or ask whether soundings are depths below the keel (they are depths below chart datum, not below your keel).
Compass work
Magnetic variation in South African waters is westerly and slowly changing; deviation is vessel-specific and read from the deviation card. Learn the correction sequence cold: True Virgins Make Dull Company (True + Variation = Magnetic; Magnetic + Deviation = Compass), with easterly added and westerly subtracted when going from true to compass, and reversed when going from compass to true. Trap: reversing the sign, or applying deviation to a true bearing.
Position fixing and dead reckoning
Latitude is measured north or south of the equator (0°–90°); longitude east or west of Greenwich (0°–180°). One minute of latitude equals one nautical mile - the basis of all distance measurement on a chart. A dead reckoning (DR) position is advanced from course and speed through the water only, ignoring wind and current; an estimated position (EP) also allows for leeway and set/drift. Fixes should use at least three position lines that cross at a small cocked hat, or a GPS position confirmed against a visual transit.
Rule of thumb
1 minute of latitude = 1 nautical mile; 1 minute of longitude = 1 nm × cos(latitude). At 34°S, one minute of longitude is about 0.83 nm.