Learn & Understand

Finding Your Position Without GPS: Dead Reckoning

For centuries, mariners crossed oceans with no satellites to tell them where they were. They found their position by a disciplined kind of educated guessing called dead reckoning — starting from a known point and calculating where a known heading and distance must have carried them. It is exactly the calculation that finds a destination point from a start, bearing, and distance.

Reckoning From the Last Known Point

Dead reckoning begins with a known position and asks: if I travel in this direction, at this speed, for this long, where do I end up? By tracking heading and distance from the last certain fix, a navigator continually estimates the current position without any external reference. It is navigation by accumulation — each leg added to the last to keep a running estimate of location.

Measuring Speed and Heading

The method needed two measurements: direction and distance traveled. Direction came from the compass. Distance came from speed multiplied by time — and speed was measured by the “log,” a line with knots tied at intervals paid out behind the ship, the knots counted as they slipped past giving the vessel's speed. This is the origin of measuring a ship's speed in “knots.”

The Accumulating Error

Dead reckoning's weakness is that errors compound. Each leg's small mistakes in heading or speed carry forward and add up, so the estimated position drifts further from the truth the longer one goes without a fresh fix from the stars or land. Navigators corrected their reckoning whenever they could sight a landmark or take a celestial reading, resetting the accumulating drift.

The Same Math, Forward

Computing a destination point — where you arrive from a starting coordinate after traveling a set bearing and distance — is dead reckoning made exact. It is the forward version of finding distance and bearing between two known points, projecting a position ahead across the sphere. The satellites in orbit have made it effortless, but the underlying question is the mariner's oldest one.

Finding the Destination

To project a destination from a start, bearing, and distance, use the Destination Point Calculator. Find the bearing to steer with the Compass Bearing Calculator, and the distance with the Great Circle Distance Calculator.