Module 15 - Restricted Visibility
Using Instruments in Fog
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.
| 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 |
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 19 August 2026 - Day Skipper Revision.
Continue studying Restricted Visibility
This public lesson is part of Revision Module 15. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.