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Dead Reckoning: Plotting Where You'll Be
💡 Dead reckoning projects your position forward from course, speed, and time alone — deliberately ignoring wind and tide until you upgrade it to an estimated position.
Recall how a chart lets you fix your position from landmarks. But what happens between fixes — or when fog swallows the coast? Dead reckoning (DR) is how you estimate your position using only three things you already control: your course steered, your speed through the water, and the time elapsed. Start from a known position, project forward, and you get a DR position — your best guess of where you are without any outside reference.
The engine is the classic distance formula: distance = speed × time. If you steer 090° (due east) at 6 knots (6 nautical miles per hour) for 30 minutes, you cover 6 × 0.5 = 3 nautical miles. On the chart, you lay off a line from your last fix along 090° and step off 3 miles with the dividers — that endpoint is your DR position.
The crucial honesty of DR: it uses course steered and speed through the water only — it deliberately ignores wind, tide, and current. That makes it a projection, not the truth. When you fold in tidal set and leeway, the DR becomes an estimated position (EP), which sits closer to reality. DR still matters because it's fast, needs no instruments, and gives you a running position to sanity-check every GPS reading against.