Parse altitude managing units conversion

def parse_altitude(cls, distance, unit):
        """
        Parse altitude managing units conversion
        """
        if distance is not None:
            distance = float(distance)
            CONVERTERS = {
                'km': lambda d: d,
                'm': lambda d: units.kilometers(meters=d),
                'mi': lambda d: units.kilometers(miles=d),
                'ft': lambda d: units.kilometers(feet=d),
                'nm': lambda d: units.kilometers(nautical=d),
                'nmi': lambda d: units.kilometers(nautical=d)
            }
            try:
                return CONVERTERS[unit](distance)
            except KeyError:
                raise NotImplementedError(
                    'Bad distance unit specified, valid are: %r' %
                    CONVERTERS.keys()
                )
        else:
            return distance