16th century compass. Illustration of a magnetic compass of variation designed by Archdeacon William Barlow in 1597, an improvement on previous marine compasses. The face of the compass was pivoted on gimbals to ensure that it remains horizontal so as to reduce errors as the ship moves. Barlow's compass allowed true north to be calculated from the indication of magnetic north if the variation was known. Variation is the angle between magnetic north and geographical (true) north, as they are in different locations. The magnitude of the variation depends upon the observer's geographical location.