Navigational Fix

What is a Fix?

A navigational fix is a known, confirmed position determined from two or more independent sources of information. A fix answers the fundamental navigation question: exactly where am I right now? Without a reliable fix, all subsequent navigation — course plotting, distance calculations, arrival estimates — is built on an uncertain foundation.

The key word is 'independent.' A fix requires at least two lines of position (LOPs) that cross at a point. A single LOP tells you that you're somewhere along a line — a bearing from a lighthouse, a depth contour, a GPS waypoint distance ring. Two LOPs that cross give you a specific point. Three or more LOPs give a cocked hat (a small triangle) — the smaller the cocked hat, the more reliable the fix.

Fixes degrade over time. Once you have a fix, every minute that passes without an update means your position is estimated (dead reckoning), not confirmed. In pilotage waters with hazards, take fixes frequently — at minimum every time you change course.

Chart diagram showing two compass bearings from a lighthouse and a radio tower crossing to give a fix, with a third bearing producing a small cocked hat — Two lines of position crossing give a fix. Three LOPs produce a cocked hat — the smaller, the more accurate.

Why does a fix require at least two lines of position?

Regulations require redundancy for safety A single LOP only tells you you're somewhere along a line — two crossing LOPs give a specific point One LOP is never accurate enough due to compass error GPS requires two satellites for a position

What is a 'cocked hat' in navigation?

An error in compass calibration The small triangle formed where three lines of position cross — indicating the quality of the fix A type of chart symbol for a dangerous headland The offset between true and magnetic north

Visual Fixes

A visual fix uses compass bearings to two or more charted objects — lighthouses, beacons, towers, headlands, or other identifiable features. The bearing to each object is plotted as a line on the chart from that object's charted position. Where the lines intersect is your position.

For best accuracy: choose objects separated by 60–120° — close to 90° apart is ideal. Objects at nearly the same bearing give LOPs that are nearly parallel and cross at a very shallow angle, producing a huge position uncertainty. Simultaneous bearings are ideal but difficult alone; take them quickly in succession and note any boat movement between shots.

Transits (ranges) are even more precise than compass bearings. When two charted objects appear one directly behind the other, you're on the transit line — a position line that requires no compass measurement and has no compass error. Examples: a lighthouse lined up behind a headland, a steeple behind a water tower. Note these on your chart before departure and use them when available.

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When taking visual bearings, identify your objects before looking at the compass. Misidentifying an object — taking a bearing to the wrong lighthouse or chimney — creates a false LOP that looks plausible on the chart but puts you somewhere you're not.

For the best fix accuracy using two visual bearings, the objects should be approximately:

5–10° apart 30–45° apart 60–120° apart (ideally ~90°) 180° apart (directly ahead and astern)

What is a transit (range) and why is it particularly valuable?

A timed run between two known points — used to calculate boat speed Two charted objects appearing in line — a precise LOP requiring no compass measurement A GPS waypoint confirmed by a second electronic source A depth-contour line used as a position line when GPS fails

Electronic Fixes and GPS

GPS (Global Positioning System) provides continuous position fixes using signals from satellites. Modern GPS units are remarkably accurate — typically within 3–10 meters under normal conditions. For practical coastal navigation, this is more than sufficient. A GPS fix provides latitude and longitude that can be plotted directly on a chart or displayed on a chartplotter.

Despite its accuracy, GPS has failure modes: antenna or unit malfunction, battery failure, satellite signal blockage in fjords or near cliffs, and in rare cases, spoofing or jamming (more relevant in certain international waters). Never rely solely on GPS for critical navigation. Always cross-check with visual position if possible.

Depth as a position line: a depth sounder reading that matches a charted contour puts you somewhere along that contour line. Combined with a visual bearing, this creates a reliable fix without requiring two visible landmarks. In fog or poor visibility where only one landmark is visible, a depth contour plus a single bearing can confirm position.

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A GPS shows where the antenna is — not the whole boat. In tight quarters, a few meters of error matters. Always approach hazards slowly, confirm visually, and don't trust a 'close enough' GPS position when inches of clearance matter.

How can a depth sounder be used as a line of position?

The depth reading confirms the boat's speed through water A depth reading matching a charted contour places the boat somewhere along that contour line Depth sounders automatically update the GPS position Depth readings verify that the anchor is holding

What is the most important limitation of relying solely on GPS for coastal navigation?

GPS is only accurate to within 100 meters GPS cannot display hazards — it must be combined with a chart Electronics fail — a single system without backup creates single-point failure risk GPS does not work within 3 miles of shore

Running Fix

A running fix is used when only one object is visible for bearings — too few for a simultaneous fix. You take a bearing to the object at one time, advance the LOP along your course for a known distance and time, then take a second bearing. Where the advanced LOP crosses the new bearing is your running fix.

The accuracy of a running fix depends on how precisely you know your course and speed between the two bearings — any current, leeway, or speed error introduces proportional position error. A running fix is less reliable than a simultaneous fix but is far better than no fix at all.

Steps for a running fix: (1) Take bearing #1 to the known object and plot LOP #1. (2) Record time, course, and speed. (3) After traveling a known distance, take bearing #2 and plot LOP #2. (4) Advance LOP #1 along the course line by the distance traveled. (5) The intersection of the advanced LOP and LOP #2 is the running fix.

Why is a running fix less reliable than a simultaneous two-object visual fix?

Running fixes use older chart symbols that may be inaccurate The position depends on the accuracy of course, speed, and time between the two bearings — any error compounds Running fixes only work with reciprocal bearings GPS interference is stronger during a running fix

You take a bearing to a lighthouse at 0900, note your speed and course, then take another bearing at 0930 after traveling 1.5 miles. You advance the first LOP 1.5 miles along your course line. Where you draw the running fix is:

On LOP #1 only At the intersection of the advanced LOP #1 and LOP #2 On the midpoint of your distance traveled At the midpoint between the two plotted bearings

Summary

A fix is a confirmed position from two or more independent lines of position (LOPs). A single LOP is a line — two crossing LOPs give a point.

Three LOPs produce a cocked hat — the smaller the triangle, the better the fix.

Visual fixes use compass bearings to charted objects; best when objects are ~90° apart. Transits (ranges) are the most reliable visual LOPs — no compass needed.

GPS provides continuous, accurate fixes but should always be cross-checked visually. Depth contours can serve as LOPs when visual landmarks are limited.

A running fix uses two sequential bearings to one object with the first LOP advanced along the course line — less reliable than simultaneous fixes but better than nothing.

Key Terms

Fix: A confirmed position determined from two or more independent lines of position (LOPs)
Line of position (LOP): A line along which the vessel must be located — a compass bearing, depth contour, or GPS circle of position
Cocked hat: The small triangle formed when three LOPs cross — the smaller, the more reliable the fix
Transit (range): Two charted objects in direct alignment, providing a precise LOP without compass measurement
Running fix: A fix using two sequential bearings to one object, with the first LOP advanced along the course line
Advancing an LOP: Moving a line of position along the vessel's course by the distance traveled between bearings
Dead reckoning (DR): Estimating position from a known fix using course, speed, and elapsed time — not a fix

What is a Fix?

Visual Fixes

Electronic Fixes and GPS

Running Fix

Summary

Key Terms

Navigational Fix — Quiz

You plot three visual bearings and they form a large triangle on the chart. What should you do?

For the most accurate simultaneous visual fix, you should choose landmarks that are approximately:

In fog with limited visibility, only a headland 2 miles to the east is visible. Your depth sounder shows 40 feet. How can you create a fix?

A transit is more reliable than a compass bearing because:

In a running fix, what happens if there is a 1-knot unknown current running perpendicular to your course during the 30-minute run between bearings?

References & Resources

Related Links