How to Determine Your Position by Dead Reckoning

Main Explanation

What it is

Dead reckoning is a method for determining the estimated position of a vessel by advancing from a known fix of position along the vessel’s ordered course and speed. According to Bowditch’s American Practical Navigator, DR is an approximate position; it does not allow for the many errors that can cause a vessel to veer off course, such as helmsman error, compass error, or current and wind. The core purpose of dead reckoning is to project the ship’s position in the immediate future, predict landfall, determine distances to objects, predict arrival times, and evaluate the accuracy of electronic positioning information. In the context of recreational sailing, dead reckoning is an independent reference skill: it is not tied to any specific boat brand, sailing school, or equipment manufacturer. It relies only on basic navigation data—heading, speed through the water, and elapsed time—and a chart or plotting sheet.

Why it matters

Modern sailors face information overload, conflicting advice from forums, outdated printed resources, and the temptation to trust a single GPS screen. Dead reckoning cuts through that noise by giving you a simple, verifiable baseline. When your GPS fails, when you lose satellite signal in a fjord or under a hardtop, or when you want to cross-check an electronic fix, DR is the fallback. It also helps you understand what the boat is actually doing: by comparing a DR position with a GPS fix, you can estimate the set and drift of current and the effect of leeway. This is essential for safe passage planning, because a DR plot shows where you would be if no external forces acted on the boat—and the difference between that and your actual position reveals the forces at work. Inertial navigation systems, which provide very accurate directional information, use dead reckoning and are widely applied in commercial and military vessels, confirming the method’s enduring value.

How it works

The arithmetic behind dead reckoning is simple: distance = speed × time. If you sail due east at 10 knots for 3 hours, you have covered 30 nautical miles east, so your new position sits 30 nautical miles east of where you began. For any heading other than a cardinal direction, you resolve that distance into north-south and east-west components using basic trigonometry. Formally, from a start point (x₀, y₀) at bearing θ, the new position is x₁ = x₀ + d·sinθ and y₁ = y₀ + d·cosθ, where d is the distance run. In practice, you plot this on a nautical chart: draw a line from your last fix along your true course, measure off the distance traveled using the latitude scale, and mark the new DR position. Each new estimate becomes the start of the next, so the position is built up step by step without any outside reference. The key inputs are a known starting fix, an accurate compass course (corrected for variation and deviation), a reliable speed through the water from the log, and the elapsed time. A DR plot is usually maintained on the chart or a separate plotting sheet, with time labels at each DR position.

How to do it

Here is a practical walkthrough for a typical coastal passage. Suppose you depart a harbor entrance at 0900 and plan to sail a course of 045° true at 5 knots. Step 1: Obtain a precise fix at departure—use a GPS fix, a bearing fix on a charted object, or a known charted position. Mark it on the chart with a circle and the time. Step 2: Determine your true course. If your compass reads 040° and the local variation is 5° east, your true course is 045°. Step 3: Note your speed through the water from the log, not the GPS speed over ground. Step 4: Calculate the distance for a chosen time interval. At 5 knots for 1 hour, you travel 5 nautical miles. Step 5: From your fix, draw a line in the direction 045° true, measure 5 nautical miles along that line using the latitude scale, and mark the 1000 DR position. Step 6: Repeat every hour, or more often if conditions change. If you know the local tidal stream or current, you can apply a set and drift correction to the DR position to get an estimated position (EP). Always label each DR position with the time, and keep the plot neat so you can see at a glance where you expect to be. This method works for any boat, from a daysailer to a cruising yacht, and it requires no electronics beyond a compass, a log, and a chart.

When not to do it

Dead reckoning has clear limits. It does not account for current, wind, leeway, helmsman error, or compass error unless you explicitly add those corrections—and even then, the corrections are estimates. You should not rely solely on dead reckoning when navigating in close quarters, narrow channels, poor visibility, or areas with strong and variable currents. In those situations, use pilotage with visual bearings, radar, echo sounder, and GPS, and cross-check all sources. Dead reckoning is also not a substitute for professional training: a certified navigation course teaches you how to handle compass errors, plot fixes, and interpret chart symbols correctly. For offshore passages, celestial navigation and weather routing by a professional may be more appropriate than a simple DR plot. Finally, never use dead reckoning as your only means of navigation when a GPS or other electronic fix is available and reliable—use DR as a backup and a cross-check, not as a replacement for all other methods.

Visual

The following step sequence shows the dead reckoning process from start to finish. Use it as a checklist when plotting a DR position.

1. Obtain a known fix. Mark your exact position on the chart with a circle and the time (e.g., 0900).
2. Determine your true course. Convert the compass heading to true by applying variation and deviation.
3. Record your speed through the water. Read the log or estimate from engine RPM; do not use GPS speed over ground for DR.
4. Calculate distance for the time interval. Multiply speed by time (e.g., 5 knots × 1 hour = 5 nautical miles).
5. Plot the DR position. From the fix, draw a line along the true course, measure the distance using the latitude scale, and mark the new position with the time.
6. Repeat and update. Use each new DR position as the start for the next interval. If current or leeway is known, apply a set and drift correction to get an estimated position (EP).

Common Mistakes

Ignoring set and drift

Many sailors plot a DR position and treat it as their actual position, forgetting that current and wind will push the boat off the DR line. The consequence is a growing error that can put you miles off course, especially in tidal waters. The correct approach is to estimate set and drift from tide tables or observed differences between DR and GPS fixes, and apply that correction to get an estimated position.

Using speed over ground instead of speed through water

Dead reckoning is based on the boat’s motion through the water, not over the ground. If you use GPS speed over ground, you are already including the effect of current, which defeats the purpose of DR and makes it impossible to detect current. Always use the log speed or engine RPM through the water for DR calculations.

Failing to update the DR plot after a fix

Some sailors get a good GPS fix and then stop maintaining the DR plot, thinking the electronics will always work. The consequence is that when the GPS fails, there is no recent DR position to fall back on. The correct approach is to keep the DR plot current at all times, updating it at regular intervals and after every course or speed change, even when GPS is working.

Confusing magnetic and true headings

Plotting a magnetic course directly on a chart without correcting for variation and deviation leads to a systematic error that can be large in areas of high variation. The consequence is a DR line that points in the wrong direction. Always convert compass heading to true before plotting, and label the chart with the variation used.

Over-relying on a single electronic source

Using a phone or tablet as the only navigation tool, with no paper chart or DR plot, is a common mistake. The consequence is total loss of position awareness if the device fails, gets wet, or runs out of battery. The correct approach is to maintain a paper or waterproof chart with a DR plot as a backup, and to cross-check electronic fixes with DR at regular intervals.

Using generic checklists without adapting to the boat

Every boat behaves differently under sail or power, and a generic DR procedure may not account for your boat’s leeway, speed variations, or compass deviation. The consequence is an inaccurate DR plot that gives false confidence. The correct approach is to calibrate your log and compass, learn your boat’s leeway characteristics in different wind strengths, and adjust the DR procedure accordingly.

Safety Note

Dead reckoning is a powerful tool, but it must fit into a broader safety framework. Always cross-check critical navigation data—position, weather, tides, and currents—with official sources such as national hydrographic offices, coast guard broadcasts, and weather services. Never use a tablet or phone as your only navigation tool; keep paper charts and a handheld compass as backups, and know how to plot a DR position on paper. Take a certified navigation and safety course from a recognized sailing school or national federation, and practice DR plotting in good conditions before relying on it in poor visibility or heavy weather. Remember that digital tools and reference platforms complement—never replace—proper training, physical checks, and human judgment. Always follow the International Regulations for Preventing Collisions at Sea (COLREGS) and local maritime regulations, and maintain a proper lookout by all available means. If you are unsure of your position, slow down, use all available fixes, and do not hesitate to call for assistance.