How do you determine position on a chart when you know a landmark bearing?

• A. Plot a line of bearing from the landmark and intersect with additional lines or known fixes.
• B. Guess your position by sighting the landmark.
• C. Use depth soundings from the landmark.
• D. Use only GPS for placement.

Answer: (A)

You determine your position on a chart by using a line of position from a known landmark. When you know the bearing to a fixed landmark, your vessel lies somewhere along the line that passes through that landmark in the reciprocal direction of that bearing (the bearing from the landmark to you). By plotting that line on the chart, you create a locus where you could be. To fix your exact location, you need at least one more independent line of position from another landmark or fixed feature, and you find where the lines intersect. That intersection gives your precise position.

For example, if you sight a lighthouse at a bearing of 045°, you would plot a line through the lighthouse at 225° on the chart. That line shows all possible positions your vessel could occupy relative to that lighthouse. Then you measure a bearing to a second landmark and plot its reciprocal line as well; where the two lines cross is your fix. Depth soundings or guessing don’t provide a definite position on their own, and while GPS can give position, the technique demonstrated here is about triangulating position using charted landmarks and lines of position.