Module 12 - Visual Aids to Navigation
Reading Light Descriptions on Charts
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.
- Chart format: Character Colour Period Height Range
- Example: Fl(2) WR 10s 15m 12M
- Period = time for one complete cycle
- Height = metres above the chart's stated height datum
- Nominal range uses the standard 10-nautical-mile meteorological-visibility convention
- Actual sighting range may be lower because of atmosphere, background or horizon
Optional quick check
Section 3 of 6
What does 12M state in a charted light description?
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