Dead reckoning starts from a known point (the fix). Then the speed of the vehicle needs to be known. In old times, ships would throw a log over the side (tied to a rope) and count how long it took to pass this log (or to pull a rope with knots and count how many knots get pulled out in a certain amount of time). This would give the speed of the ship. Knowing the direction from a compass would give the direction. From these two numbers an estimate (a good guess) could be made about the current location of the vehicle. However, a wind pushes the vehicle to one side (called "drift") so this needs to be measured as well. Also there are currents in the water, even in the middle of the ocean. Over time this could make errors as well, so charts were made to correct for currents.
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