the true heading is set at the lubber's line, true bearings are observed directly. Similarly, compass bearings can be observed if the compass heading is set at the lubber's line, etc. However, the vessel must be on the heading to which the pelorus is set if accurate results are to be obtained, or else a correction must be applied to the observed results. Perhaps the easiest way of avoiding error is to have the steersman indicate when the vessel is on course. This is usually done by calling out "mark, mark, mark" as long as the vessel is within a specified fraction of a degree of the desired heading. The observer, who is watching a distant object across the pelorus, selects an instant when the vessel is steady and is on course. An alternative method is to have the observer call out "mark" when the relative bearing is steady, and the steersman note the heading. If the compass is swinging at the moment of observation, the observation should be rejected. The number of degrees between the desired and actual headings is added if the vessel is to the right of the course, and subtracted if to the left. Thus, if the course is 060° and the heading is 062° at the moment of observation, a correction of 2° is added to the bearing. See also Compass rose Navigation Pilotage Rhumbline network References Further reading J. Lagan (2005). The Barefoot Navigator: Navigating with the Skills of the Ancients. Dobbs Ferry, NY: Sheridan House. Tibbets, G.R. (1971). Arab Navigation in the Indian Ocean before the coming of the Portuguese. London: Royal Asiatic Society. Tolmacheva, M. (1980). "On the Arab System of Nautical Orientation", Arabica, vol. 27 (2), pp. 180–92. External links Bowditch Online See more examples in the online compass museum COMPASSIPEDIA Navigational equipment Orientation (geometry) Measuring instruments Navigational aids