def asbaseline(self, pos):
        """Convert a position measure into a baseline measure. No actual
        baseline is calculated, since operations can be done on positions,
        with subtractions to obtain baselines at a later stage.

        :param pos: a position measure
        :returns: a baseline measure

        """
        if not is_measure(pos) or pos['type'] not in ['position', 'baseline']:
            raise TypeError('Argument is not a position/baseline measure')
        if pos['type'] == 'position':
            loc = self.measure(pos, 'itrf')
            loc['type'] = 'baseline'
            return self.measure(loc, 'j2000')
        return pos
Convert a position measure into a baseline measure. No actual
        baseline is calculated, since operations can be done on positions,
        with subtractions to obtain baselines at a later stage.

        :param pos: a position measure
        :returns: a baseline measure