A diagram consisting of a curve showing the variation of the horizontal component of the force of magnetism exerted upon the ship's compass-needle by the iron in the ship's composition while making a circuit or curve. Compasses are corrected (calibrated) on installation and periodically as modifications to the vessel are made, however this only applies in a straight line, since compass variation in a curve or circuit is non-linear, making the dygogram necessary for accuracy.

With the advent of GPS and similar navigation systems, this plot has been made redundant since corrections such as Cross-track-error(XTE) are performed automatically.

Source : The Oxford English Dictionary, memory.