What aspect does dead reckoning primarily rely on?

Dead reckoning primarily relies on historical position data, as it is a navigation method used to estimate one's current position based on a previously determined location. This technique factors in the heading, speed, and elapsed time since the last known point to calculate the current location.

In practice, after determining an initial position through methods such as GPS or a fixed landmark, dead reckoning then projects that position forward based on continuous movement data. Historical position data is critical because it serves as the foundation for subsequent calculations, allowing navigators to make informed estimates about their current whereabouts.

The other options, while related to navigation, do not capture the essence of dead reckoning. Environmental factors could influence movement but are not the primary basis of position estimation in dead reckoning. Extensive real-time data refers to ongoing information that may assist in navigation but is not essential for the basic principles of dead reckoning. Visual confirmation pertains to assessing one’s position through sight, which is outside the realm of dead reckoning's reliance on calculation from historical data.