19-29. If you lack other means, you can use natural materials to form a symbol or message that can be seen from the air. Build mounds that cast shadows; you can use brush, foliage of any type, rocks, or snow blocks.
19-30. In snow-covered areas, tramp the snow to form letters or symbols and fill the depression with contrasting material (twigs or branches). In sand, use boulders, vegetation, or seaweed to form a symbol or message. In brush-covered areas, cut out patterns in the vegetation or sear the ground. In tundra, dig trenches or turn the sod upside down.
19-31. In any terrain, use contrasting materials that will make the symbols visible to the aircrews. Orient the signal in a north-south fashion to attain the maximum benefit of the sun's shadow for contrast and recognition.