Module 15 - Restricted Visibility
Using Instruments in Fog
What you will learn
Combine GNSS, chart, depth, radar and AIS as bounded evidence, recognise conflicts and avoid turning one electronic source into a false position or collision-avoidance picture.
Instruments extend the lookout and navigation picture; they do not replace judgement. Each source answers a different question and has failure modes that must be checked.
GNSS can provide a precise-looking position, course and speed, but the display can still be affected by wrong chart or datum settings, poor route data, receiver or antenna faults, interference, or simple misinterpretation. Monitor the position against safe-water limits and compare it with genuinely independent evidence where available. A second screen fed by the same receiver is not an independent check.
An echo sounder supplies depth below its transducer, keel or surface according to its offset. To compare it with charted depth, account for that offset, the height of tide and the nature and slope of the seabed. One depth does not confirm a unique position because many places share it. A depth trend can support the wider picture, but blindly following a contour can lead towards a hazard or across an unsafe route.
If radar is fitted and operational, set it up, use suitable range scales and make systematic observations or plots; a single return or scanty radar information is not enough to establish risk. AIS may help identify a transmitting target, but some craft and objects do not transmit, entered data may be wrong, and AIS course and speed data are not a substitute for systematic visual or radar observations. VHF negotiations can misidentify vessels and waste time, so collision avoidance remains governed by the COLREG.
Worked example
Fictional training data: the chartplotter places the yacht on a 12 m charted sounding. Height of tide is 2.0 m. The echo sounder reads 7.5 m below the transducer, which is 0.5 m below the waterline.
- 1Expected water depth at the plotted position is about 12.0 m + 2.0 m = 14.0 m, before allowing for survey, tide and sea-state uncertainty.
- 2The sounder reading is 7.5 m below a transducer 0.5 m below the waterline, so approximate total water depth is 8.0 m.
- 3The roughly 6 m conflict is too large to call confirmation. Recheck sounder offset and display, tide and chart assumptions, GNSS source and route data while preserving safe water and safe speed.
- 4Use further independent evidence if available; do not average the two numbers into a plausible-looking position.
Sense check: If the charted depth is 12 m and the tide adds about 2 m, a total depth near 8 m cannot sensibly confirm that plotted position without an explained error or a different location.
| Source | What it can add | What it cannot prove alone |
|---|---|---|
| GNSS / chartplotter | Position, course, speed and route relationship | That the source, settings, chart data or route are correct |
| Echo sounder | Depth and depth trend with a known offset | A unique position or a safe contour-following route |
| Radar | Range and bearing observations of detectable targets and land | Risk from one return or when set-up and plotting are inadequate |
| AIS | Identity and transmitted static or dynamic data for participating targets | A complete traffic picture or a COLREG collision-avoidance decision |
Build an evidence stack
A GNSS track looks steady, the depth is shoaling faster than expected, one intermittent radar return lies ahead and an AIS target appears farther to starboard.
1. Protect time
Review safe speed and safe water before spending attention on menus or target labels.
2. Check each source
Verify chart and GNSS set-up, sounder offset and tide, radar gain/range and whether the return persists across systematic observations.
3. Keep sources separate
Do not assume the radar return and AIS target are the same object without evidence, and do not let AIS decide collision risk.
4. Act on uncertainty
Treat unexplained conflict as reduced confidence, preserve sea room and use the planned alternative or stop/anchor only where that is safe and lawful.
Sense check: More displays can repeat one faulty source; useful confidence comes from appropriate, understood and genuinely independent evidence.
Common mistake or limitation
- Saying an echo-sounder depth confirms position without tide, datum, offset, seabed shape and another line of position or constraint.
- Treating AIS as radar or assuming every vessel, buoy and obstruction will transmit accurate and current data.
- Making collision-risk decisions from one radar observation instead of a systematic plot or equivalent observation.
- Trying to agree an informal passing arrangement by VHF instead of identifying and acting under the COLREG.
Recap
- Use instruments as complementary evidence with known sources, settings and limitations.
- A depth reading needs offset, tide, chart and seabed context; it is not a position by itself.
- Radar needs proper set-up and systematic observation, while AIS supports awareness but does not decide collision risk.
Optional quick check
Section 4 of 5
The sounder depth strongly conflicts with the depth expected at the GNSS position. What should the skipper conclude?
Sources and factual review
Reviewed 1 September 2026 - Day Skipper Revision.
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