2.7 Piloting: Bearings, Lines of Position & Fixes

Piloting: Bearings, Lines of Position & Fixes

Quick Answer: Piloting fixes your position using visible landmarks and aids. A bearing to a charted object gives a line of position (LOP) that you lie somewhere along; crossing two or more LOPs taken at the same time gives a fix. The best fixes come from LOPs that cross near a right angle, and a three-bearing fix reveals its own accuracy through the size of the triangle it forms.

Piloting is how a small-boat operator navigates within sight of land, and it feeds directly into the Chart Plot module.

Bearings: True, Magnetic, and Relative

A bearing is the direction from you to an object.

• A true bearing is measured clockwise from true north and is what you plot on the chart.
• A magnetic bearing is measured from magnetic north (convert with TVMDC before plotting).
• A relative bearing is measured clockwise from the vessel's bow (000 relative). An object broad on the starboard beam is at 090 relative; dead astern is 180 relative; on the port beam, 270 relative.

To turn a relative bearing into a true bearing, add the relative bearing to the vessel's true heading (subtract 360 if the total exceeds 360).

Worked example: Your vessel's heading is 040 degrees True and a buoy bears 030 degrees relative (30 degrees off the starboard bow). True bearing = 040 + 030 = 070 degrees True. If the heading were 350 and the object were 030 relative, the sum 380 would become 380 - 360 = 020 degrees True.

Lines of Position

A line of position (LOP) is a line drawn on the chart along which your vessel must be located. You get one from:

• A visual bearing to a charted object (plot the bearing line from the object).
• A range — two charted objects seen in line — which is an especially accurate LOP requiring no compass.
• A depth matching a charted contour, or a distance from a radar range.

A single LOP tells you that you are somewhere on that line, but not where on it. Crossing it with a depth sounding that matches a charted contour, or with a radar distance-off arc, upgrades it toward a full fix.

Fixes

A fix is your actual position, found where two or more LOPs taken at the same time cross. Quality depends on the angle of cut:

• LOPs crossing near 90 degrees give the strongest fix.
• Angles smaller than about 30 degrees (or larger than 150) give a weak, elongated fix because a small bearing error shifts the crossing point a long way.

When taking bearings for a two-bearing fix, sight the object that is changing bearing slowly first (near the bow or stern) and the fast-changing object (near the beam) last, so both bearings are effectively simultaneous. Mark and label every fix with a circled dot and the time, then start a fresh dead-reckoning plot from it.

The Three-Bearing Fix and the Cocked Hat

Three LOPs rarely meet at a perfect point. Instead they form a small triangle called a "cocked hat."

• A small triangle means good, consistent bearings — take your position at its center.
• A large triangle warns of an error (a mis-identified object, a bad bearing, or an uncorrected compass) — re-check before trusting it.
• For safety, when a cocked hat lies near a hazard, assume you are at the corner nearest the danger.

Running Fix and Distance Off

When only one object is available, you can still fix your position with a running fix: take a bearing, note the time, run your course and speed for a while, take a second bearing on the same object, then advance the first LOP by the distance and direction run and cross it with the second. The intersection is a running fix — less precise than a simultaneous fix but valuable when landmarks are scarce.

A related single-object trick is distance off by doubling the angle on the bow: note the run between the moment an object is at one relative angle off the bow and the moment that angle has doubled; the distance run then equals the distance off the object. The bow-and-beam case (45 degrees, then 90 degrees abeam) is the most common version and is covered in the Chart Plot module.

Danger Bearings and Danger Circles

You do not always need a full fix to stay safe:

• A danger bearing is a single bearing line drawn to a charted object that just clears a hazard. You keep the object's bearing on the safe side of that line — for example, "keep the lighthouse bearing not more than 040 degrees" to stay clear of a reef.
• A danger circle (or distance) keeps you outside a set range from a hazard, useful with radar: "stay more than 0.5 nautical miles off the shoal."

These techniques let a busy small-boat skipper monitor safety with a single glance at the compass or radar, instead of plotting continuously. Combined with regular fixes, they keep you both on track and out of danger.