What does dead-reckoning rely on to ascertain position?

Dead-reckoning is a navigation technique that determines the current position of a vessel based on its previously known position, factoring in the vessel's course and speed over time. This method relies on continuously calculating how far and in what direction the vessel has traveled from its last known position, which is determined by knowing the starting point, the vessel's heading, and the speed at which it is moving.

Using the vessel's course and speed allows mariners to estimate where they currently are, even in the absence of visual reference points or navigational aids. The accuracy of dead-reckoning can be affected by factors such as currents, wind, and any navigational errors, which is why it is often supplemented with other methods like GPS or visual navigation when available.

The other options do not accurately reflect the primary basis of dead-reckoning. Current position updates and predetermined waypoints relate more to active navigation and use of electronic systems rather than estimating position based on previous data alone. Geographic coordinates might provide a fixed location but do not relate to the calculation of current position using speed and course over time. Thus, the reliance on a vessel's course and speed from the starting point makes it the correct choice for understanding how dead-reckoning works.