'Corrects the heading by the given variation. This is sometimes known as
the true heading.
@return: The heading, corrected by the variation. If the variation or
the angle are unknown, returns L{None}.
@rtype: C{float} or L{None}'

@property
def correctedHeading(self):

    if ((self._angle is None) or (self.variation is None)):
        return None
    angle = ((self.inDecimalDegrees - self.variation.inDecimalDegrees) % 360)
    return Angle(angle, Angles.HEADING)
