Module 12 - Visual Aids to Navigation
Reading Light Descriptions on Charts
What you will learn
Decode character, colour, period, elevation and nominal range and explain why the printed range is not a promise of detection.
A chart compresses the expected light into a short ordered description. Decode every field, then check the chart notes and current publication rather than filling gaps from memory.
On a chart, a light is described in a standard format, e.g., 'Fl(2) WR 10s 15m 12M'. This reads: Group flashing (2 flashes), White and Red sectors, period 10 seconds, height 15 metres above the chart's stated height datum for that light, nominal range 12 nautical miles.
The period is the time for one complete cycle of the light pattern. Nominal range is the light's luminous range in a homogeneous atmosphere with meteorological visibility of 10 nautical miles. Actual detection can be limited by atmospheric visibility, background lighting, the light's intensity and the geographical horizon for the light and observer heights.
Light elevation is referred to the height datum stated by the chart or publication, often a high-water datum. Read the note rather than assuming Chart Datum or one universal reference. Corrections and temporary changes can also alter the operational picture.
Worked example
Decode Fl(2) WR 10s 15m 12M without overpromising what will be seen.
- 1Fl(2): a repeated group of two flashes. WR: white and red sectors are charted.
- 210s: one complete group-to-group period. 15m: elevation above the stated height datum.
- 312M: nominal range under the standard visibility convention, not guaranteed detection at twelve miles.
Sense check: A haze-reduced luminous range or a shorter geographical horizon can prevent sighting before the nominal range is reached.
Plan a positive and a negative cue
A charted 12M light is expected near a waypoint, but rain and shore lighting are reducing contrast.
1. Predict what can be tested
Record expected bearing, sector, group and period.
2. Do not use absence as a fix
Poor visibility can hide the light inside its nominal range.
3. Set a safe fallback
Use depth, position and a hold or abort limit if the aid is not positively identified.
Sense check: The printed range describes light performance under a convention; it does not certify the night's visibility or the observer's horizon.
Common mistake or limitation
- The period is the complete repeated pattern, not the duration of one flash.
- Light elevation is not automatically above Chart Datum; use the stated height datum.
Recap
- Decode character, colour, period, elevation and nominal range in order.
- Use the chart's height datum and current correction status.
- Treat nominal range as a standardised reference, not guaranteed visibility.
Optional quick check
Section 3 of 6
What does 12M state in a charted light description?
Continue studying Visual Aids to Navigation
This public lesson is part of Revision Module 12. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.