Description

This is an "on/off" function. When switched “off�, the GPS is used to calculate the position of your vessel. When “on�, the position is calculated based on the current speed and the input from the course gyro.

In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, by using estimations of speed, heading direction and course over elapsed time. [..] Dead reckoning is subject to cumulative errors. Advances in navigational aids that give accurate information on position, in particular satellite navigation using the Global Positioning System, have made simple dead reckoning by humans obsolete for most purposes. However, inertial navigation systems, which provide very accurate directional information, use dead reckoning and are very widely applied. [..] In marine navigation a "dead" reckoning plot generally does not take into account the effect of currents or wind.

Aboard ship a dead reckoning plot is considered important in evaluating position information and planning the movement of the vessel.

https://en.wikipedia.org/wiki/Dead_reckoning (November 2020)

When dead reckoning is switched off, the vessel movement and track line are calculated from the GPS data. These are relative to the bottom. When switched on, the inputs from the gyro and speed log are used to positioning the vessel relative to the water.