Which fix to trust: reliability and cross-checks
1The lesson in plain words
Every fix has errors. Rank your methods, know what goes wrong with each, and check any position with a second, independent method.
A rough ranking
With certain marks, good crossing angles and sound observations, a rough order is: a sensitive transit crossed near 90° by an independent line; radar ranges, or a bearing and range of a close, steep object; a three-point compass fix checked for common errors; a two-bearing fix; a bearing with a depth contour. Geometry and observation quality matter more than the method’s name. GNSS usually gives a precise position, but still needs an independent check.
Where errors come from
Misidentification is the biggest: the wrong headland or tower gives a confident, wrong fix. Then come compass errors, such as variation applied the wrong way or unknown deviation; bearings taken too slowly while the boat moves; old or poor chart surveys; and plotting slips. Distance from the object magnifies every angular error.
Independent checks
Check a fix with something it does not depend on. Reduce the sounding to chart datum before comparing it with charted depths. Compare a GNSS position with a compass bearing or a radar range. Compare the fix with your estimated position (EP), worked out from heading, distance, leeway and tidal stream. A large difference means something is wrong: find out what before you act on either.
How often
Choose an interval short enough to detect an error and act before you could reach danger. Base it on speed, position uncertainty and distance to hazards. Strong streams and poor visibility can increase uncertainty. Near danger, monitor continuously and obtain frequent independent fixes. Fix before altering course near danger.
Paper and electronics
In open water, plotting the GNSS position on the paper chart every hour may give a useful recent starting point if the electronics fail. Shorten that interval when conditions require it. The Electronic navigation unit covers GNSS errors and interference.
2Key data
- Especially strong
- Sensitive transit crossed near 90° by an independent line
- Least reliable
- Bearing and depth contour over a gentle slope
- Biggest error
- A misidentified object
- Cross-checks
- Depth, a second method, the EP
3Key points
- 1
With certain marks and good observations, transits and ranges rank highly; geometry matters more than the method’s name.
- 2
Misidentifying an object is the most dangerous error.
- 3
Check independently; reduce soundings to chart datum before comparing them with the chart.
- 4
Fix more often near danger and before altering course.
4Frequent mistakes
Wrong: A GNSS position needs no checking.
Right: Check it independently with a reduced sounding, a compass bearing or a radar range.
Wrong: If the fix and the EP disagree, trust the fix.
Right: Find the cause first: the fix may use a misidentified object, or the EP a wrong stream.
5Practice case & exam keys
Approaching an unfamiliar coast in haze, your GNSS position puts you 2 M off, in about 40 m of water. The sounder reads 12 m. You can just see a headland you think is the one on the chart.
Show the answer
Reduce speed or stop if safe. Avoid closing the coast; do not choose a new heading from the suspect GNSS fix. Retrace a known-clear approach track only if safe. Reduce the sounding for sounder offset and tide. Positively identify the headland before using its bearing and radar range. Compare with your EP and another independent check. Resolve the discrepancy before continuing.
Exam keys
- Good geometry and certain marks matter more than the method’s name.
- Misidentification causes the largest errors.
- Cross-check with an independent method: depth, bearing, radar, EP.
- Fix more often near danger.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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