Ground-to-air signs

From high up, the eye mainly recognises shapes and contrasts, hardly any details. In nature, lines are mostly curved and irregular: streams meander, forest edges are ragged. Straight lines, right angles and letters, on the other hand, look man-made – and that is exactly what makes searchers take notice.

Contrast decides whether a sign is visible at all. Dark material on light ground or light material on dark ground works; material of the same colour disappears. Shadows help too: if you pile up material or stamp deep trenches into snow, the edges cast shadows, especially when the sun is low. That makes the lines stand out even more clearly.

'SOS' is generally known as a distress signal and is easy to read from the air. There are also special codes for communicating with aircraft that survival manuals describe; they are deliberately not listed here. A large, clear SOS is easier to build and is understood.

The location is the most important thing. Look for an open, preferably level area that is clearly visible from above: a clearing, a meadow, a snowfield, a beach or a gravel bar. Under trees or in the shadow of a rock, nobody will see your sign. Make sure the spot itself is safe – no avalanche slope, no rockfall gully, no place that floods at high tide or high water.

Better one large, clear sign than several small ones. Exactly how large cannot be said in general – larger is better, as far as your energy allows. Use what is there: branches, stones and turf on snow or light sand; an emergency blanket, a light tarp or colourful clothing on grass. In snow you can stamp out lines and darken them with branches.

Weigh down sheets, tarps and blankets with stones. Wind can tear them away, and a helicopter's downwash is much stronger still. A piece of foil flying into the rotors endangers the rescuers.

Ground signs are worthwhile when you are likely to wait a long time and an air search is probable – for example in very remote terrain, after a breakdown in open country, or when others know you are overdue. In Europe people are usually located by phone today; ground signs matter most where that does not work.

In dense forest without a clearing or under low cloud, ground signs achieve little. And if you are cold, soaked or short of water, that comes first. Building a large sign takes a lot of energy; it must not leave you exhausted and chilled by the time rescuers arrive.

Pace yourself. Work in stages, take breaks and drink if you have water. Sweating in the cold is dangerous because wet clothing chills you later. Do not lay out your warm clothing as a sign if you need it to stay warm.

Some manuals also describe three fires in a triangle or a line as a distress signal. A signal fire is only an option where it is safe and permitted: mind wildfire danger and fire bans and check local rules. A fire that gets out of control endangers you and others.

Once you have been rescued, remove the sign if possible or tell the rescuers about it. Otherwise a later search flight might take it for a new emergency.

A ground sign is a quiet, lasting signal. When you hear an aircraft or helicopter, you can also become active: mirror in sunshine, lamp at dusk, waving a coloured cloth or showing the Y signal with your arms. Combining a fixed sign with an active signal increases the chance of being spotted.

If possible, stay close to your sign. Rescuers who spot it will search there first. If you have a phone with signal or a distress beacon, use those first – ground signs complement them but do not replace them.