def add_deviation(self, dev, td=None):
        """
        Add a deviation survey to this instance, and try to compute a position
        log from it.
        """
        self.deviation = dev
        try:
            self.compute_position_log(td=td)
        except:
            self.position = None
        return
Add a deviation survey to this instance, and try to compute a position
        log from it.