Replace the heading angles in one oce object with that from another, possibly with a constant adjustment.
useHeading(b, g, add = 0)object holding data from an instrument whose heading is bad, but whose other data are good.
object holding data from an instrument whose heading is good, and
should be interpolated to the time base of b.
an angle, in degrees, to be added to the heading.
A copy of b, but with b$data$heading replaced with
heading angles that result from linear interpolation of the headings in
g, and then adding the angle add.