Napier diagram

Napier diagram

[′nā·pē·ər ′dī·ə‚gram]
(navigation)
A diagram on which compass deviation is plotted for various headings, and the points are connected by a smooth curve, permitting deviation problems to be solved quickly without interpolation; it consists of a vertical line, usually in two parts, each part being graduated for 180° of heading, and two additional sets of lines at an angle of 60° to each other and to the vertical lines.