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. The range is the 'nominal range' — the distance the light can be seen in clear visibility (meteorological visibility of 10 nautical miles). Light heights are normally referred to a high-water height datum stated on the chart, so check the chart note rather than assuming one universal datum.
- 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
- Range = nominal range in nautical miles (in clear visibility)
Watch for this: Use the period to confirm identification: count the time for one complete cycle with a watch and match it to the chart.
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