'Clock-face' artillery aid

摘要

The pilot on an artillery observation sortie was given a photograph, or map, of his target marked as shown to assist him in making corrections. Due north was regarded as being at 12 o'clock. The distance of the shots from the centre of the target was noted with reference to the smallest circle within which they were contained and to the nearest clock hour. The letter code was used to speed up the transmission of results: OK = exact centre: Y = 10 yards out: Z = 25 yards out: A = 50 yards out: B = 100 yards out: C = 200 yards out: D = 300 yards out: E = 400 yards out and F = 500 yards out.