| 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 |
| Term | Definition | Source |
|---|---|---|
| True North (T) | Geographic North Pole — the reference for all chart work | Fixed — Earth's axis |
| Variation (V) | Angle between True North and Magnetic North at a given location | Earth's magnetic field — varies by location and changes over time. Shown on chart compass rose with annual rate of change |
| Magnetic North (M) | Direction a compass needle points before vessel's own magnetism is applied | Earth's magnetic field |
| Deviation (D) | Angle between Magnetic North and the compass needle on a specific vessel — caused by the vessel's own magnetic field | Vessel's steel, electrical equipment — varies with the vessel's heading. Found from the deviation card |
| Compass North (C) | What the compass actually reads on board that vessel, on that heading | Combined effect of variation and deviation |
Compass to True: Reverse — errors WEST subtract, errors EAST add
090° + 3° (W variation, add) = 093°M. 093° − 2° (E deviation, subtract) = 091°C.
Steer 091° on the compass to make good 090° True.
- Standard port: Times and heights given directly in tide tables (NP 201)
- Secondary port: Apply time and height differences from the tables to the standard port data
- MHWS: Mean High Water Springs
- MHWN: Mean High Water Neaps
- MLWN: Mean Low Water Neaps
- MLWS: Mean Low Water Springs
- LAT: Lowest Astronomical Tide — datum for chart soundings
Approximates tidal rise/fall over each hour of a 6-hour tide cycle:
- Hour 1: 1/12 of range
- Hour 2: 2/12 of range
- Hour 3: 3/12 of range
- Hour 4: 3/12 of range
- Hour 5: 2/12 of range
- Hour 6: 1/12 of range
Use to find height at intermediate times when tidal curve not available
To find the height of tide at a given time between high and low water:
- Find the times and heights of HW and LW from the tide tables for the port
- Calculate the range (HW height minus LW height)
- Calculate the interval from HW to the required time
- Use the tidal curve for the port (Springs or Neaps curve, or interpolate)
- Read off the tidal factor and multiply by the range — add to LW height
| Source | How to use |
|---|---|
| Tidal stream atlas (NP 337 etc.) | 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 |
| Type | Purpose | Frequency |
|---|---|---|
| Weekly NtMs (UKHO) | Corrections to Admiralty charts and publications — new wrecks, light changes, buoy movements, depth changes | Weekly — available online at admiralty.co.uk. Must be applied before using charts for navigation |
| Annual Summary of NtMs | Cumulative corrections — useful for catching up a chart that has not been corrected recently | Annual |
| Temporary and Preliminary (T&P) notices | Short-term changes — temporary lights, survey operations, route alterations | As required — shown with (T) or (P) on chart |
| NAVAREA warnings (radio) | Urgent navigational warnings broadcast by NAVTEX and SafetyNET — new dangers, off-position buoys, survey vessels | As required — monitored via NAVTEX |
- Astronomical position fixing — sun, stars, moon sights
- Horizontal sextant angles — three-point fix from charted objects
- Vertical sextant angles — distance off a charted object of known height
- Compass check — azimuth of sun at known time and position
- Index error: Arc set to 0° does not show 0° — correct by applying index correction (IC) to all readings
- Side error: Horizon glass not perpendicular to the frame
- Perpendicularity error: Index mirror not perpendicular to the frame
- Collimation error: Telescope not parallel to the plane of the instrument
- Shade error: Shade glasses not parallel — affects accuracy when shades used
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.
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.
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.
| Position line | Worth |
|---|---|
| Transit (two charted objects in line) | Gold: no compass in it at all — and reading the compass as they touch measures your total error for free |
| Radar range of a fixed, steep-to feature | Excellent inshore — ranges beat visual bearings for accuracy |
| Visual compass bearing | Good — take the fastest-changing last, aim for 60–90° cuts |
| Depth contour from the sounder | The cheap arbiter running continuously between fixes |
| GNSS | Superb and singular — one system, cross-checked, never sole authority |
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.
| Datum | References | Bites you when |
|---|---|---|
| Chart datum (≈LAT) | Soundings and drying heights | Tide arithmetic — heights of tide are ABOVE it |
| Vertical land datum (e.g. MHWS) | 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 |
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.
Your fixes over three hours fall consistently 0.4 nm east of track although the helmsman is steering the planned course. Diagnose and act.
True course is 270°T. Variation is 5°W. Deviation is 3°W. What do you steer?
What is the Rule of Twelfths and when would you use it?
How do you correct a chart for weekly Notices to Mariners?