Visual signals between nearby posts are silent and need a clear line of sight: hand signals work to a few hundred metres, flags and panels farther by day, and a shuttered lamp by night. Make a lamp signal less visible from outside by facing it inward so the settlement’s own walls and buildings block it from the sides, send only the agreed table patterns, and remember that any signal can be seen by anyone with the same line of sight.
Part of Watch and Patrol Scheduling
Visual signals between posts are the hand, flag and lamp signals used over short distances where there is a clear line of sight and quiet is wanted.
Silent, But Not Private
The earlier articles on visual signalling cover the main methods and their range (Visual Signal Methods). Between posts the distances are shorter, so the question is not range but exposure: a signal that is fine at 400 m can still be seen from the same angle by a stranger. The camouflage article gives a candle as visible at about 1.5 km at night (Camouflage and Concealment), many times the distance between two posts.
Worked Example: How Far Can A Hand Signal Be Read?
An arm and hand signal occupies about 0.7 m. If it must subtend 10 arcminutes to be read (an assumed requirement, not a measured one) and 1 arcminute is 0.000291 radians, then the reading distance is 0.7 / (10 x 0.000291) = 0.7 / 0.00291, about 240 m. A flag 1.0 m square, held up so its whole face is seen, read as a colour at 3 arcminutes, is about 1.0 / (3 x 0.000291), about 1,150 m. So hand signals work between posts a couple of hundred metres apart and flags between posts a few times as far; test each with real people.
What You Need
- A clear line of sight between each pair of posts, checked in daylight.
- Flags or panels in colours that contrast with the background, and a stiff frame so they show in calm air.
- A shuttered lamp in a hood open only toward the receiver (Lantern Signals).
- The single table of patterns (Simple Code Tables).
- A reply signal so each message is acknowledged.
Method: Setting Up A Short-Range Visual Link
Step 1: Walk the line of sight from the receiving post and note what is behind the sender: sky, trees or wall. Choose a flag colour that shows against it.
Step 2: Orient lamps inward. A lamp in a hood facing the camp or the neighbouring post is screened from the outside by the post’s own wall, roof or terrain behind it. Check by walking round the outside at night and looking at the post.
Step 3: Fix the pattern length to the table (four pulses) and send each message twice with a gap.
Step 4: Always acknowledge. The receiver answers with the acknowledgement pattern; no reply means send again or use the whistle.
Step 5: Keep hand signals for contact within sight of the receiver and when silence matters. Use a few big, slow movements.
Common Mistakes
- A lamp visible from the road. Walk the outside and look.
- Hand signals given facing away, or against a cluttered background.
- Colours chosen for looks. White flags mean truce or surrender to a stranger.
- No acknowledgement. Without it the sender does not know.
Safety
Lamp fires and glare
A lamp in a hood can start a fire in a hut or on a platform; keep it on a fireproof stand with a lid or sand to hand (Lamp Fire Prevention). A mirror flash is bright enough to dazzle and to damage eyes; never aim it at a face.
What’s Next
- Learn next: Audio Signals — signals that carry without a sight line
- Combine with: Runner Systems — carrying the detail a signal cannot