Position fixing · Variation & deviation · Compass courses · Tidal calculations · Notices to Mariners · Sextant
13 lessonsabout 14 min reading5 visuals and tools10 scenario questions4 oral questions
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Lesson 1 of 13 · 1 min
Position Fixing Methods
Method
How
Accuracy / Notes
Cross-bearings
Take compass bearings of two or more fixed charted objects. Convert to true. Plot on chart — position is where the lines intersect
Three bearings preferred — creates a "cocked hat". The true position is within the cocked hat
Radar range and bearing
Take radar range and bearing to a charted object. Plot range as arc, bearing as line
Radar range is more accurate than radar bearing — use range + visual bearing where possible
Running fix
Take a bearing of one object. Travel a known distance on a known course. Take another bearing of the same or different object. Transfer first bearing by the run
Accuracy degrades with distance run — use with caution over long legs
Transferred position line
Use a transit, clearing bearing, or range to establish a position line
Quick and useful in pilotage waters
GPS/GNSS
Satellite-derived position — latitude and longitude
Highly accurate but must be cross-checked. GPS can be jammed or spoofed
Echo sounder
Compare depth with charted soundings and contours
Useful as a check — particularly approaching a coast or bar
Say it out loud. Talk “Position fixing methods and the fix cycle” through with the live Tutor.
Chartlets showing set and rate at each hour relative to HW Dover or local port. Find the page for the required time and read set and rate from the arrows
Tidal diamonds on chart
Table giving set and rate at Spring and Neap tides for each hour relative to HW. Interpolate for intermediate range
Say it out loud. Talk “Course to steer with tidal stream” through with the live Tutor.
090°C, deviation 3°W, variation 5°W. Apply toward true, ADD east / SUBTRACT west: 090 − 3 = 087°M; 087 − 5 = 082°T. Sanity check with "error west, compass best": the compass number should be the bigger one — it is.
True → Compass
Make good 140°T, variation 2°W, deviation 4°E. Reverse the signs: 140 + 2 = 142°M; 142 − 4 = 138°C. Deviation is taken from the card for the heading you will actually STEER — re-enter the card if the correction moves you across its bands.
Say it out loud. Talk “Compass error by transit, azimuth and amplitude” through with the live Tutor.
Track to make good 000°T, stream setting 270°T at 3 kn, boat speed 12 kn. Construction: lay the stream vector (3 nm toward 270°) from your position for one hour; from its end, swing the one-hour speed arc (12 nm) to cut the track line; the line from stream-end to intersection is the course to steer — here about 014°T, aimed UP-tide, with ground speed a touch under 12 kn read off the track. Two vocabulary points that mark understanding: course to steer is through the water, track made good is over the ground — muddling them reads as muddled understanding; and the degree-per-knot shortcut is wrong — the offset is asin(cross-stream ÷ boat speed) and depends on the geometry.
Say it out loud. Talk “Course to steer with tidal stream” through with the live Tutor.
One identifiable object only: take a bearing, note log and time; run a known course and distance; take a second bearing; transfer the FIRST position line forward parallel to itself by the run — where it cuts the second bearing is the fix. Its honesty problem: the "run" embeds course steered, log accuracy, leeway and your tidal estimate, so an unallowed-for set skews the fix — the running fix silently audits your DR discipline. Refinements: let the bearing change 30–60° between observations, and if radar range is available, a simultaneous bearing-plus-range beats the whole construction. Related single-object gift: the rising/dipping range — the moment a light lifts above the horizon, its geographic range (from height of eye and light, 2.03√h summed, or the tables) gives you a distance; with the bearing, that is a landfall fix from one lighthouse.
Say it out loud. Talk “The running fix” through with the live Tutor.
Charted heights of lights, bridges, overhead cables
Air draught — the clearance shrinks as the tide falls is WRONG: charted vertical clearance is usually from MHWS, so you normally have MORE than charted; check the title block
Horizontal datum (WGS84 vs local)
The lat/long grid itself
Plotting GNSS positions on older/foreign paper charts — errors can exceed a channel width; read the datum note, apply the shift, or navigate by radar/visual relative to the charted land
Say it out loud. Talk “Chart datums, symbols and publications” through with the live Tutor.
Keeping the Chart True — The Correction Chain by Speed
Navtex/NAVAREA warnings within hours (the new wreck, the extinguished light) → Temporary & Preliminary notices for evolving situations (pencil, tracked, erased when cancelled) → permanent weekly Notices to Mariners (violet ink, logged chart-by-chart in the record of corrections) → new editions when accumulation demands. ENCs mirror it with weekly update files, applied and logged per cell. The passage-planning connection is direct: appraisal includes confirming every chart and publication on the route is corrected to date — navigating on an uncorrected chart is navigating on a rumour, and the correction record is the first thing a Port State inspector opens at the chart table.
Say it out loud. Talk “Variation, deviation and compass correction” through with the live Tutor.
Scenario questions from the oral, with the reasoning after each one. The answers that look nearly right are there on purpose.
Module 11
Oral questions
Answer each one out loud before you open the model answer. If you can't say it, you don't know it yet.
Q1Your fixes over three hours fall consistently 0.4 nm east of track although the helmsman is steering the planned course. Diagnose and act.Model answer ▾
Safety first: is the displacement standing us into danger before I finish diagnosing? Then the consistency IS the information — a systematic offset means an unallowed-for set or leeway, or an instrument error. Quantify it from the fixes: direction and rate by simple vector difference, then correct at the CAUSE — a new course to steer that pre-compensates, not repeated dog-legs back to track. And audit the other suspects while I'm there: compass error checked against a transit, the log over-reading, helmsman bias. The habit under examination is the fix discipline that caught it: regular, plotted, COMPARED — a fix that isn't compared with the DR is a photograph, not navigation.
Q2True course is 270°T. Variation is 5°W. Deviation is 3°W. What do you steer?Model answer ▾
True to Magnetic: 270° + 5° (West, add) = 275°M. Magnetic to Compass: 275° + 3° (West, add) = 278°C. Steer 278° on the compass to make good 270° True.
Q3What is the Rule of Twelfths and when would you use it?Model answer ▾
The Rule of Twelfths approximates the rise or fall of the tide over each hour of a six-hour tidal cycle. The rise or fall follows the pattern 1, 2, 3, 3, 2, 1 twelfths of the range. I would use it as a quick mental calculation when I do not have tidal curve tables available — for example, to estimate the height of tide at an intermediate time to check UKC before approaching a shallow bar or anchorage.
Q4How do you correct a chart for weekly Notices to Mariners?Model answer ▾
Download or obtain the weekly NtMs from the UKHO (admiralty.co.uk). Check the index to identify any corrections affecting charts on board. Apply permanent corrections in violet ink using the standard symbols and the chart correction tracing if supplied — Temporary and Preliminary (T&P) corrections go on in pencil only, so they can be rubbed out when cancelled. Note the NtM number and date in the bottom left corner of the chart. Update the chart correction record. For ECDIS, apply ENC updates via the AVCS update service — check update status before every passage.
Ready?
Sit a live oral on Chartwork & Navigation.
The live Examiner asks the questions out loud, coaches you when you miss, and gives you a debrief.