Navigation: Terrestrial, Celestial, Electronic and Passage Planning
Terrestrial work
A fix needs three position lines crossing at good angles; a small cocked hat is a check, a large one is a warning. A running fix advances an earlier position line along course and distance made good - use the ground track when current is known and the water track plus leeway when it is not, transferring the LOP parallel to itself. Doubling the angle on the bow and the four-point bearing give a position from one object and a log reading.
Compass error comes from amplitude or azimuth. Variation is read from the chart, deviation from the deviation card, and deviation = compass error minus variation, east positive.
Celestial
Intercept method: take GHA and declination from the almanac, LHA = GHA ± longitude, compute Hc and azimuth, then intercept = Ho − Hc, plotted towards the geographical position when positive. Correct sextant altitude for index error, dip, refraction, semi-diameter and parallax. Latitude from meridian altitude, or from Polaris in the northern hemisphere.
Electronic and planning
GNSS, DGPS, ARPA, AIS and ECDIS all need independent cross-checks; a single-source position is not a fix. Passage planning follows Appraisal, Planning, Execution and Monitoring, with no-go areas, safety margins, abort points, under-keel clearance policy and contingency anchorages.
Trap: sign errors in the intercept, and reversing the current when advancing a position line.
Rule of thumb: a true amplitude of E12°S against a compass amplitude of E18°S gives 6°W compass error; with 10°W variation the deviation is 4°E.