Entry & progression
Course: RYA shore-based navigation theory — the classroom knowledge behind the practical scheme, taught in three tiers: Day Skipper Theory, Coastal/Yachtmaster Offshore Theory, and Yachtmaster Ocean Theory. Entry: none for Day Skipper Theory; each higher tier assumes the level below. Feeds the practical Yachtmaster Coastal, Offshore and Ocean qualifications, which in turn feed the MCA Master (Code Vessels) <200 GT and the wider CoC pathway.
The chart and position
A nautical chart is a scaled picture of the sea packed with information: charted depths (soundings, reduced to chart datum), drying heights and clearances, the coastline, dangers, buoyage and much more, all in the standardised symbols of Admiralty chart 5011. Position is given as latitude (degrees and minutes north or south of the equator) and longitude (east or west of the Greenwich meridian). Latitude is read up the sides of the chart and longitude along the top and bottom. Because one minute of latitude equals one nautical mile, the latitude scale beside your position is also the distance scale — distance is never measured on the longitude scale.
- Measure distance on the latitude scale, level with your position.
- Charted depths are reduced to chart datum, so the real depth is the charted depth plus the height of tide.
- Learn the common symbols and abbreviations from chart 5011.
- Note the chart's datum: a GNSS position may need a small shift to match an older chart.
The compass and its errors
The steering compass points to magnetic north, which is not true (geographic) north, and is disturbed further by the boat's own metal. Two corrections convert between compass, magnetic and true. Variation is the angle between true and magnetic north, taken from the chart's compass rose and changing with place and slowly with time. Deviation is the boat's own magnetic error, taken from a deviation card and changing with the boat's heading. The memory hook 'error east, compass least; error west, compass best' fixes the signs when converting.
TIP — Latitude scale for distance, always The single most common beginner's error is measuring distance on the longitude scale. Distance is always taken from the latitude scale at the side of the chart, level with where you are.
NOTE — Two norths, two corrections True north is fixed; magnetic north is where the compass points; the boat's metal adds deviation on top. Variation and deviation together are the compass error you apply to every course and bearing.