The basic premise of compass adjusting (that is compensation aboard ship for magnetic fields caused by the vessel itself and the determination of residual deviation) is that a reference direction must be available during the operation. Normally visual ranges or the azimuth of a celestial body—usually the Sun—are used, but in some areas and climates reduced visibility often does not permit the use of either. Compass adjusters are frequently driven in desperation to use gyro-compass, radar, or radio bearings—with highly speculative results as the errors of these instruments are generally unknown, and fairly large compared to the accuracy required of the magnetic standard compass (± ½°) which is the primary direction standard aboard ship.

Just a century ago, in 1876, Patrick Weir, an officer of a vessel trading between London and Australia, conceived the idea of a diagram that might facilitate finding the Sun's true azimuth for the purpose of checking the magnetic compass. Some thirteen, years later Captain Weir's Diagram was the subject of a paper communicated by Sir William Thomson (later Lord Kelvin) to the Royal Society of Edinburgh. In his paper Weir outlined the train of reasoning by which he succeeded in constructing a novel diagram which was described by Professor P. G. Tait as ‘a singularly elegant construction which, not only puts in a new and attractive light one of the most awkward of the problems of spherical trigonometry, but it practically gives in a single-page diagram the whole content of the two volumes of Burdwood's Azimuth Tables’. Tait also remarked that the method supplied an interesting graphical plane construction of a function of three independent variables.