Function from_motion 

pub fn from_motion(
    heading_deg: f64,
    stw_kn: f64,
    cog_deg: f64,
    sog_kn: f64,
) -> Current

The current implied by the difference between the vessel’s motion through the water and over the ground.

Every quantity here is measured by a different instrument – heading from the compass, speed through water from the log, course and speed over ground from GPS – so the result carries the error of all four. A log reading 5% low produces a current that is not there.