(Bowditch, The American Practical Navigator, 2002)
A circle graduated in degrees, clockwise from 0° at the reference direction to 360° and sometimes also in compass points. Compass roses are placed at convenient locations on the Mercator chart to facilitate measurement of direction. The compass rose can be used by the mariner to help lay out a course with a protractor.
Compass roses on NOAA charts show two graduated scales. One is referenced to True North, while the other is referenced to Magnetic North. Gyro compasses used aboard vessels yield “True” headings, while magnetic compasses will yield magnetic headings. If a magnetic compass is in use, mariners must be able to convert back and forth between true and magnetic headings. They accomplish this by either adding or subtracting the magnetic variation—which is the angle between magnetic and geographic meridians.
Magnetic variation is given in degrees and minutes, with an easterly or westerly component. If converting from magnetic headings to true headings, a west variation is subtracted from the magnetic to obtain true. An easterly variation would be added.