#Compass bearings.# A compass bearing of a single object, as a lighthouse or a tangent to a point of land, laid down on the chart, shows that the vessel is somewhere on that line, and when combined with other information, as with a Sumner line or the course by dead reckoning or the distance by a vertical angle, will give a position whose correctness of course depends on the accuracy of the data used. Bearings of two objects not in the same direction give two lines on the chart whose intersection is the position. This will be very weak if the angle of intersection is acute, and will become stronger as it approaches a right angle. A bearing of a third object should be taken when practicable, as it affords a valuable check in that the three lines should intersect in the same point; if they do not do so when plotted the error is either in the observations, or the compass, or the plotting, or the chart. (Fig. 40). All compass bearings are of course dependent upon the accuracy of the compass and the knowledge of its errors due to the local magnetic effect of the ship, and also upon the correctness with which the magnetic variation from true north is known. Bearings of near objects should therefore always be preferred, and those of distant objects considered as giving only approximate positions. An error of one degree in the bearing of an object 30 miles away will deflect the plotted line about one-half mile. Because of the facility with which they may be taken compass bearings are much used for inshore navigation, but in point of reliability they are inferior to some of the other methods. A single or "danger" bearing of an object is often a valuable guide in avoiding a danger. For example, a reef may lie to the westward of a line drawn South 10° East from a lighthouse; in approaching a vessel will pass safely to the eastward of the reef if the lighthouse is not allowed to bear any to the northward of North 10° West. (Fig. 41.)