Module 12 - Visual Aids to Navigation
Reading Light Descriptions on Charts
What you will learn
Decode character, colour, period, elevation and nominal range in order, then state which observations and limitations the charted description does not supply.
A charted light description is compact performance and identification data. It is not a promise that the light will be seen, a guarantee of safe water, or a substitute for update status and local notes.
Read Fl(2) WR 10s 15m 12M as one repeated group of two flashes, white and red sectors, a complete period of ten seconds, an elevation of 15 metres above the chart's stated height datum, and a nominal range of 12 nautical miles. Do not infer the duration of each flash or the bearings and purposes of the sectors from that abbreviation alone.
Nominal range is the luminous range in a homogeneous atmosphere with meteorological visibility of 10 nautical miles. It assumes a dark background free of background lighting. Actual detection can be shorter because of prevailing visibility, contrast, the geographical horizon, obstruction, serviceability and the observer's visual conditions.
Light elevation is referred to the height datum stated by the chart or publication, commonly a high-water datum in tidal waters. Chart Datum is not a universal answer. Check chart notes, corrections, Notices to Mariners and navigational warnings before using the description in a passage plan.
Worked example
Decode Fl(2) WR 10s 15m 12M and identify the planning limits.
- 1Fl(2) WR: two flashes per group, with charted white and red sectors.
- 210s: one complete group-to-group period. 15m: elevation above the stated height datum.
- 312M: nominal range under the standard atmospheric convention, not guaranteed detection at 12 miles.
Sense check: The abbreviation still does not state the sector bearings or prove that the light is currently operating; those checks remain outside the shorthand.
| Field | Meaning | Do not infer |
|---|---|---|
| Fl(2) | Two-flash group | Two seconds or two separate lights |
| WR | White and red sectors | Which colour is safe without sector bearings and purpose |
| 10s | Complete repeating period | Duration of one flash |
| 15m | Elevation above stated height datum | Automatic reference to Chart Datum |
| 12M | Nominal range | Guaranteed or geographical detection distance |
Plan for a light that may not appear
The chart shows a 12M light, but rain and a bright waterfront reduce contrast on the approach.
1. Predict measurable details
Record expected bearing, sector, group and period.
2. Set a no-sighting limit
Use depth and position to define where the vessel will hold or abort if the light remains unconfirmed.
3. Check current status
Review corrections, warnings and local information for altered or unreliable aids.
Sense check: A missing light inside 12M does not locate the vessel; it triggers the pre-briefed independent navigation response.
Common mistake or limitation
- Nominal range is a standardised luminous-range reference, not the maximum or guaranteed range for the night's conditions.
- The charted light elevation is not automatically above Chart Datum, and the printed sector colours do not state their local safe use by themselves.
Recap
- Decode every field without adding an unstated assumption.
- Read height datum, sectors and correction status from the applicable chart and publications.
- Make the no-sighting response part of the plan before the light is needed.
Optional quick check
Section 3 of 10
Which statement about the 12M in Fl(2) WR 10s 15m 12M is correct?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
Continue studying Visual Aids to Navigation
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