Dead reckoning is navigating by nothing but a compass bearing and a counted distance: from here, walk 120 paces on 075°, plot the new spot, repeat. It is how sailors crossed oceans and how you cross a fogged ridge with no GPS.
Every leg adds a little error — a few degrees of compass wobble, a slightly long or short stride. Those errors accumulate: the gold ellipse is your honest uncertainty, and like any random walk it only swells. A 3° error over a kilometre is already 50 metres sideways.
A terrain fix resets it. The moment you positively recognise a feature — a stream crossing, a saddle on the ridge — you know where you are again, and the ellipse collapses. That is a Kalman update in the field.
So the rule every navigator learns: fix early and fix often. Uncertainty is cheap to erase when it is small and ruinous when it is large. The same maths governs robots, spacecraft and your phone's step counter.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.