Chartwork and position fixing
Chartwork and position fixing is the domain where the examiner checks whether you can read a nautical chart, convert between true, magnetic and compass bearings, and plot a position you can defend. Most marks are lost through small conventions, not difficult maths.
Chart conventions and plotting
- Use a Mercator chart; measure distance on the latitude scale, never the longitude scale.
- Chart datum is the reference for soundings and drying heights: soundings are below it, drying heights above it. A drying height of 1.2 m means the seabed is 1.2 m above chart datum.
- Learn IALA Region A buoyage (red can to port, green cone to starboard when entering harbour) and common chart abbreviations.
- Plot with a sharp pencil: small circle for a fix, label time and log reading. A fix uses position lines; a dead-reckoning (DR) position uses course and distance; an estimated position (EP) adds leeway and tidal stream.
Compass conversion
True, magnetic and compass north differ. Variation is the angle between true and magnetic north; deviation is the angle between magnetic and compass north, caused by the boat. Use the sequence Compass → Magnetic → True. Going that way, add easterly deviation or variation and subtract westerly; reverse the signs when converting from true to compass.
Worked example: a hand-bearing compass reads 210°C. Deviation is 3°E, variation 6°W. Convert to true. Compass to magnetic: 210 + 3 = 213°M (east deviation added). Magnetic to true: 213 − 6 = 207°T (west variation subtracted). The true bearing is 207°T. The boat's deviation and local variation partly cancel.
Position fixing
A three-point fix uses two or three position lines; a small cocked hat is acceptable, a large one needs checking. A transit gives a precise line; a running fix uses one object at different times. Always ask whether the fix makes sense against soundings and the chart.
Traps: applying deviation to a true bearing; reversing the variation sign when converting true to magnetic; mixing degrees and decimal minutes; plotting a bearing from the wrong end of the line; treating a GPS position as chart-datum perfect; and ignoring leeway when labelling an EP.