Flag semaphore spells a message letter by letter with two hand-held flags, each held at one of eight arm positions 45 degrees apart, and it works only by daylight between trained operators who can see each other. A plain flag colour can be read from farther away than an arm pose, because the eye must resolve a whole figure for a pose but only a coloured patch for a colour, so use colours for alarms and semaphore only for short messages at moderate range.
Part of Signal Systems and Communication
Semaphore is a flag alphabet in which the angle of each arm stands for part of a letter; the parent article’s colour code is a different and much simpler flag system.
A Pose Is Harder To Read Than A Colour
The parent article gives flags a range of 1 to 3 km and then describes a colour code (red, white, dark) with no semaphore in it. That distinction decides the range. Reading a colour needs the patch to be larger than the eye can resolve. Reading a pose needs the eye to resolve the position of two arms and two flags against a background, which takes a much larger apparent size.
The arithmetic uses the angle an object subtends: size divided by distance, in radians. A figure for good eyes is that the smallest detail resolved is about 1 arcminute, 0.000291 radians (a standard figure I am recalling, and the one the Observation Posts article also uses).
Worked Example: Colour Patch Against Arm Pose
The required apparent sizes below are assumptions for illustration, not measured values; only the method is fixed.
- Colour patch. A flag 1 m square that must subtend 3 arcminutes to be recognised as a colour: distance = 1 m / (3 x 0.000291) = 1 / 0.000873 = about 1,150 m, or 1.1 km.
- Arm pose. A signaller with arms and flags spread spans about 2 m. If the whole figure must subtend 10 arcminutes for the pose to be read: distance = 2 m / (10 x 0.000291) = 2 / 0.00291 = about 690 m.
- With binoculars. An 8x instrument multiplies the apparent angle by 8, so the same pose is readable at up to 8 x 690 = 5,500 m, if the air and the terrain allow it.
On these assumptions the parent article’s 1 to 3 km fits a large colour flag, or semaphore through binoculars, and does not fit semaphore with the naked eye. Test the real distance with your own people and write down what they can read.
What You Need
- Two flags, each a square of cloth on a stick or light pole, in colours that contrast with the usual background: bright against dark woods, dark against bright sky.
- A written chart of the alphabet, copied from a reliable source. Semaphore uses two arms, each at one of eight positions 45 degrees apart (recalled), and a letter is a pair of positions. Do not rebuild the alphabet from memory.
- A clear high stand with the receiver’s sky or ground behind the signaller, never the signaller against a cluttered background.
- Operators who practise weekly, as the parent article says.
Method: Sending A Short Message
Step 1: Face the receiver square on. An arm pointing toward or away from the watcher cannot be read; the signaller must stand with the body across the line of sight.
Step 2: Get attention first. Use a colour or a waved flag before spelling anything, so the receiver is looking.
Step 3: Send slowly and hold each pose. Hold every pose until it is acknowledged or for a counted pause. Speed comes from practice, not from rushing.
Step 4: The receiver reads aloud and a second person writes. Two people cut errors; a single watcher cannot watch and write.
Step 5: Acknowledge, then repeat back. A message is not received until it is read back and confirmed.
One Colour Collision To Avoid
The parent article makes a white flag mean “all clear, safe to approach”. A white flag is widely understood as truce or surrender (recalled), so a stranger who sees one at your gate will read it that way. Choose a colour with no existing custom, and write the choice into the code book so the code table is consistent.
Common Mistakes
- Reading the range off the colour code and applying it to poses. The pose range is a fraction of the colour range.
- A flag in sunlight against bright sky. Dark cloth silhouettes; pale cloth disappears.
- Wet or limp flags. A flag that hangs in calm air shows no colour pattern; use a stiff panel.
- No alphabet check. Two operators with different chart copies spell nonsense.
Safety
Wind, poles and overhead lines
A tall pole in wind can fall or whip the signaller off a platform; a metal pole near a power line or in a thunderstorm attracts lightning. Signal from the ground or a railed platform, and stop signalling when a storm approaches.
What’s Next
- Learn next: Mirror Heliograph — a narrower beam that reaches farther
- Combine with: Simple Code Tables — the shared meanings behind the flags