Fix by two compass bearings

How it works

A bearing tells you the direction in which you see a conspicuous object, for example a lighthouse or church tower marked on the chart. If you see the lighthouse at 060°T, the boat is on a line running from the lighthouse towards 240°T, the reciprocal bearing. One position line is not enough: you know you are along the line, not where. With two objects you get two lines, and the fix is where they cross.

Take the bearings with the hand-bearing compass, held away from iron, the engine and electrical equipment, and note the time. Choose two objects so that the angle between the lines is between 30° and 150°, ideally close to 90°. With small angles the lines are almost parallel and an error of a few degrees moves the crossing by many miles. Take first the object nearest ahead or astern (it changes slowly), last the one abeam (it changes fastest), one after the other within a few seconds.

On the chart you work in degrees true, so convert. From compass to magnetic you apply deviation, from magnetic to true variation; easterly (E) is added, westerly (W) subtracted. Example with variation 4°W, hand-bearing compass with no deviation: lighthouse A read at 064°M becomes 064 − 4 = 060°T; church B read at 142°M becomes 142 − 4 = 138°T. If you take bearings with the ship's compass, also apply the deviation from the table for your heading before the variation.

Now the drawing. Lines are drawn from the charted object, not from the boat, using the reciprocal bearing: A 060°T becomes 240°T, B 138°T becomes 318°T (reciprocal = bearing ± 180°). With protractor or parallel rule, carry the direction from the compass rose and draw a line a few miles long from each object towards the boat. The crossing is the fix: circle it, write the time and the log reading, then read latitude and longitude. Here the angle between the lines is 138 − 60 = 78°: excellent.

A third bearing checks the other two. Example: object C read at 217°M, that is 213°T, reciprocal 033°T. Three lines rarely meet at one point: they form a small triangle, the cocked hat. If it is small, the fix is in the middle; if it is large, there is an error in reading, in identifying the object, in variation or in conversion, and the bearings are retaken. For safety you assume the boat is at the corner of the triangle nearest the danger.