Position Fixing Fundamentals

What is a Navigational 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. Every decision you make aboard a vessel depends on knowing where you are, and a fix is the only way to know rather than guess.

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. Independence matters because two LOPs derived from the same source share the same errors and do not truly confirm each other.

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. The rate of degradation depends on conditions: in open water with no current, a fix may remain useful for an hour; in a narrow channel with cross-currents, your fix can become dangerously stale in minutes. Developing a sense for when to take the next fix is one of the most important skills a coastal navigator can build.

Fixes are the backbone of safe navigation because they close the loop between where you planned to be and where you actually are. A dead reckoning position tells you where you should be based on course, speed, and time. A fix tells you where you are. The difference between the two reveals the effects of current, leeway, steering error, and instrument inaccuracies — information that is critical for adjusting your plan and arriving safely.

Lines of Position (LOPs)

A line of position (LOP) is the fundamental building block of every fix. An LOP is a line (or curve) along which the navigator knows the vessel must be located, based on a single observation. The observation might be a compass bearing to a charted object, a distance measurement from a radar target, a depth sounding matched to a charted contour, or the alignment of two charted objects in a transit. Each type of observation produces a different geometric shape on the chart, but they all serve the same purpose: constraining where the vessel can be.

Bearing LOPs are the most common type in visual coastal navigation. When you take a compass bearing to a lighthouse and read 045°, you know you are somewhere along the line extending from that lighthouse on a bearing of 225° (the reciprocal). Plotted on the chart, this line extends from the lighthouse through your approximate area. A bearing LOP is a straight line and is the basis for the classic two- or three-bearing fix. The accuracy of a bearing LOP depends on correct compass reading, proper deviation and variation correction, and correct identification of the charted object.

Distance circle LOPs are produced when you measure a distance to an object — typically with radar or by using a sextant to measure the vertical angle of an object of known height. The resulting LOP is a circle centered on the object with a radius equal to the measured distance. You are somewhere on the circumference of that circle. When a distance circle intersects a bearing line, the two possible intersection points usually reduce to one plausible position based on your general knowledge of where you are. Radar ranges are particularly valuable because they are generally more accurate than radar bearings.

Depth contour LOPs use your depth sounder reading matched to a contour line on the chart. If you read 10 fathoms and the 10-fathom contour runs roughly north-south in your area, you know you are somewhere along that line. Depth contours are especially useful in reduced visibility when visual bearings are limited or impossible. Transit LOPs (also called ranges) occur when two charted objects align visually — one directly behind the other. This produces an extremely precise LOP because no compass measurement is involved and therefore no compass error exists. Transits are the gold standard of visual LOPs and should be used whenever the opportunity presents itself.

The Cocked Hat and Fix Quality

When you plot three LOPs on a chart, they will almost never intersect at a single point. Instead, they form a small triangle called the cocked hat. This triangle is not a failure — it is expected and useful. Every observation contains some degree of error: slight compass reading inaccuracies, small identification mistakes, rounding of bearings, and the time elapsed between sequential observations. The cocked hat makes these combined errors visible. A small, tight cocked hat (sides less than about 0.3 nautical miles in coastal waters) indicates good-quality observations. A large, sprawling triangle signals that something is wrong.

Your position is most likely inside the cocked hat, but not necessarily at its center. If one bearing is more reliable than the others (for example, a transit versus a compass bearing), your position is likely closer to that LOP. When navigating near hazards, you should assume your position is at the worst-case corner of the cocked hat — the corner closest to danger. This is a fundamental principle of prudent navigation: when in doubt, assume the worst and navigate accordingly. Never assume you are at the favorable side of the uncertainty.

When should you re-shoot bearings? If the cocked hat is larger than you would expect for the conditions and distance to landmarks, recheck all three bearings. Start by verifying that you have identified the correct charted objects — misidentification is the most common source of large cocked hats. Next, check for compass error: has the deviation card changed, or have you placed a magnetic object near the compass? Finally, consider whether current or vessel movement between sequential bearings has introduced error. If you took three bearings over several minutes while moving at speed, the first bearing may have shifted significantly by the time you plot. Taking bearings quickly and in a consistent order (bow object, beam object, stern object) minimizes this effect.

Types of Fixes

There are several categories of fixes, each suited to different conditions and available resources. The simultaneous visual fix is the classic method: taking compass bearings to two or more charted landmarks at approximately the same time. It requires clear visibility, identifiable charted objects, and a reliable compass. Visual fixes are the foundation of coastal pilotage and every navigator should be proficient in them. When objects are well-spaced (60°–120° apart, ideally near 90°), a visual fix is both quick and accurate. Subsequent lessons in this series will cover visual fixing techniques in detail.

A running fix is used when only one landmark is available. You take a bearing at one time, note your course and speed, then take a second bearing to the same object after traveling a known distance. The first LOP is advanced along your course line and where it intersects the second bearing is the running fix. Running fixes are less accurate because they depend on knowing your exact course and speed between observations — current, leeway, and helm error all introduce uncertainty. However, a running fix is far better than no fix. Electronic fixes from GPS are now the most common type: the GPS receiver continuously computes position from satellite signals, providing latitude and longitude that can be plotted on a chart or displayed on a plotter. GPS is remarkably accurate (typically 3–10 meters) but is a single electronic system subject to failure.

Radar fixes use range and bearing measurements to charted features displayed on the radar screen. Radar ranges (distance arcs) are generally more accurate than radar bearings and are particularly useful at night or in fog when visual bearings are impossible. A radar fix from two or more ranges to identifiable headlands or isolated rocks can be plotted just like a visual fix. Finally, celestial fixes use observations of the sun, moon, stars, or planets with a sextant to produce LOPs. Celestial navigation is an advanced topic and the ultimate backup — it requires no electronics at all. Each of these fix types will be explored in dedicated lessons later in this series.

Plotting Fixes on a Chart

Knowing how to take bearings is only half the skill — you must also plot them accurately on the chart. The standard tool for plotting a compass bearing is the parallel ruler (or rolling plotter). Place the ruler on the nearest compass rose so that its edge passes through the center and aligns with the corrected bearing. Then walk the ruler across the chart until its edge passes through the charted position of the landmark. Draw the LOP as a light pencil line extending from the landmark through your approximate area. Repeat for each bearing. The intersection of the LOPs is your fix.

When plotting, always work with true bearings or magnetic bearings consistently — mixing the two without proper conversion is a common and dangerous error. If your compass reads magnetic bearings (as most do), apply the local variation (found on the chart's compass rose) and your vessel's deviation (from the deviation card) to convert to true before plotting from the outer ring of the compass rose. Alternatively, plot directly using magnetic bearings on the inner (magnetic) ring of the compass rose, which already accounts for variation but not deviation. Whichever method you choose, be consistent and systematic.

Label every fix on the chart with the time and the standard fix symbol: a small circle with a dot at the center, or a small circle at the intersection of the LOPs. Write the time (in 24-hour format) next to the fix, for example "1045 FIX". If it is a running fix, label it "1045 R FIX". Label each LOP with the time it was taken and the bearing value. Consistent labeling is not just good practice — it creates a navigational log on the chart that you can refer back to if conditions change or if you need to reconstruct your track. Keep your chart work neat: use a sharp pencil, draw thin lines, and erase old plots that are no longer needed to avoid clutter.