Chart notation

Light characteristics

Every lit mark is charted as a compact string — class, colour, period, elevation, range. Learn the ten rhythm classes below, then decode any light and watch it blink.

Reading a charted light

Take the string apart left to right. Only the class is always present — everything else appears as needed. Lowercase m means metres of elevation; capital M means nautical miles of range.

Fl(2)

Class + group

group flashing, 2 flashes together

WRG

Colours

white, red and green sectors

10s

Period

the whole pattern repeats every 10 s

15m

Elevation

lantern 15 m above high water

9-6M

Range

seen from 9 M (brightest) down to 6 M

The rhythm classes

Ten classes cover nearly every light you will meet. Each demo blinks its example rhythm in real time.

F
Fixed
Continuously lit; never eclipses.
F RFixed: a steady light with no rhythm. Colour: red.
Fl
Flashing
One flash per period; total light clearly shorter than total dark.
Fl R 4sOne flash every 4 s — dark longer than light. Colour: red.
Fl(2)
Group flashing
A group of flashes close together, then a longer dark spell; the count is charted in brackets.
Fl(2) 10sA group of 2 flashes close together every 10 s.
LFl
Long-flashing
One long flash (≥ 2 s of light) per period — used on safe-water marks (LFl 10s).
LFl 10sOne long flash (at least 2 s) every 10 s.
Oc
Occulting
Total light clearly longer than total dark; the darkness is the event, not the flash.
Oc 4sA steady light with a brief eclipse every 4 s — lit longer than dark.
Iso
Isophase
On and off for identical durations; Iso 4s means 2 s lit, 2 s dark.
Iso 4sLight and dark phases are equal every 4 s.
Q
Quick flashing
About one flash per second, continuously (or in counted groups such as Q(3), Q(6), Q(9)).
QContinuous quick flashing.
VQ
Very quick flashing
About two flashes per second — cardinal marks may use VQ instead of Q.
VQContinuous very quick flashing.
Mo
Morse code
Light appears in dots (short) and dashes (long) spelling one letter each period.
Mo(A) 8sFlashes the Morse letter “A” every 8 s.
Al
Alternating
Shows one colour, then another, in a repeating sequence; charted with the colours after “Al”.
Al WR 4sAlternates white → red every 4 s.

Timing conventions per U.S. Chart No. 1, Section P — Lights (NOAA/NGA): quick = 50–60 flashes/min, very quick = 100–120/min, long flash ≥ 2 s.

Decoder — paste any charted light

Type a characteristic exactly as charted. Case matters for units: lowercase m is metres of elevation, capital M is nautical miles of range.

A group of 2 flashes close together every 10 s. Sector colours: white, red, green; the colour you see identifies the sector. The lantern is 15 m above high water. Nominal range: 9 M in the brightest sector, down to 6 M.

Class: Fl — Group flashing
Group: 2 flashes
Sector colours: white, red, green
Period: 10 s
Elevation: 15 m above high water
Nominal range: 9 – 6 nautical miles

See these rhythms on real marks in the buoyage explorer — cardinal marks alone teach you Q, Q(3), Q(6)+LFl and Q(9).