◆ LIT — verified / checkable
Dead reckoning integrates position from heading and speed: p ← p + v·(cos θ, sin θ)·dt. Each leg's heading carries an independent small error, and because the errors accumulate rather than cancel, the gap between the true track and the reckoned track grows — the ‘circle of doubt’ widens roughly as √(legs) for random errors (faster if there is a bias). The instrument runs both tracks from the same port and measures the drift live: this is exactly why dead reckoning must be reset by a fix (a star sight, a landmark, a satellite) before the doubt swallows you.
▲ AMBER — the figure
A stylised 2-D voyage with a single tunable error scale; a real log-and-compass reckoning also fights current, leeway and speed error, and mariners bounded it with frequent fixes. The accumulating-error growth is the genuine, load-bearing fact.
a point is fixed from THREE — and the point they fix is Top, at the centre.