Module 15 - Restricted Visibility
Using Instruments in Fog
What you will learn
Combine position, depth, radar and AIS as bounded evidence, expose disagreement and keep one electronic source from becoming false confirmation.
Instruments can preserve orientation and reveal risk, but each answers a different question and can share data or failure modes with another display.
GNSS can provide a precise coordinate, but precision is not proof of accuracy, chart suitability, route safety or sensor integrity. Plot or monitor it on current appropriate charting, retain the recent DR or EP context and compare with evidence that can fail differently. Two screens using the same receiver, chart, route file or power source are not fully independent.
An echo sounder measures depth from its transducer or configured reference, not a unique chart position. Compare the reading with charted depth only after applying tide, datum, sounder offset, squat where relevant and seabed or sea-state uncertainty. A depth trend can expose a conflict or support a safe-water decision, but following a contour is not automatically a safe route and one depth cannot confirm position.
Radar can provide range and bearing to detectable contacts and identified charted features. AIS can add identity and transmitted motion information. Neither sees everything, and association between a radar echo and AIS symbol can be wrong. Keep collision-risk work based on systematic radar or visual observations where available, use AIS as supporting awareness and treat disagreement as a reason to increase margin rather than select the most reassuring display.
Worked example
Fictional depth cross-check: the charted depth is 8.0 m, predicted height of tide is 2.3 m and an uncorrected transducer is 0.5 m below the waterline. The display shows 7.2 m.
- 1Expected waterline-to-seabed depth: 8.0 m + 2.3 m = 10.3 m.
- 2Expected uncorrected transducer reading: 10.3 m - 0.5 m = 9.8 m.
- 3Difference from the observed display: 9.8 m - 7.2 m = 2.6 m shallower than expected.
- 4Protect safe water first, then check position, tide, chart datum, sounder reference and whether the seabed is changing faster than planned.
Sense check: The 2.6 m conflict is a warning, not a new position fix. Several locations or set-up errors could produce the same reading.
| Source | Useful evidence | Does not prove |
|---|---|---|
| GNSS and chart | Coordinate, track and route relationship | Independent accuracy, safe water or traffic clearance |
| Echo sounder | Depth and depth trend from a known reference | One unique position or an automatically safe contour |
| Radar | Range, bearing and systematic relative-motion evidence | Detection of every small target or immediate ARPA accuracy |
| AIS | Transmitted identity, position and motion data | Complete traffic picture or COLREG-compliant collision decision |
Resolve an instrument conflict
The GNSS track remains on the planned line, depth is shoaling faster than expected, a weak radar return persists ahead and an AIS symbol lies farther to starboard.
1. Protect margin
Slow, hold or move towards charted safe water as the circumstances allow before trying to explain every discrepancy.
2. Check each chain
Verify GNSS and chart set-up, tide and sounder reference, radar tuning and whether the echo persists across ranges and observations.
3. Keep targets separate
Do not assume the radar return and AIS symbol are the same object without supporting evidence.
4. Update the route decision
Use the unresolved conflict to increase margin or divert; do not continue because one display remains neat.
Sense check: Agreement between two displays is weak reassurance when both depend on the same GNSS, chart data or target association.
Common mistake or limitation
- Saying depth confirms position without accounting for tide, datum, transducer reference and the fact that many places share one depth.
- Following a charted depth contour as if it were a guaranteed safe lane rather than one bounded navigational technique.
- Treating a radar echo and nearby AIS symbol as one target without checking association and motion evidence.
- Using multiple screens as independent evidence when they repeat the same sensor, chart or power failure.
Recap
- Different instruments answer different questions and may share hidden failure modes.
- Depth needs tide, datum and sounder-reference control and cannot confirm a unique position alone.
- Protect margin when evidence conflicts; do not choose the most reassuring display.
Optional quick check
Section 4 of 10
The sounder reads 2.6 m shallower than the tide-adjusted chart expectation. What is the strongest response?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
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