Calculate the great circle distance between two points
        on the earth (specified in decimal degrees)

def distance(self, loc):
        """
        Calculate the great circle distance between two points
        on the earth (specified in decimal degrees)
        """
        assert type(loc) == type(self)

        # convert decimal degrees to radians
        lon1, lat1, lon2, lat2 = map(radians, [
            self.lon,
            self.lat,
            loc.lon,
            loc.lat,
        ])

        # haversine formula
        dlon = lon2 - lon1
        dlat = lat2 - lat1
        a = sin(dlat/2)**2 + cos(lat1) * cos(lat2) * sin(dlon/2)**2
        c = 2 * asin(sqrt(a))
        r = 6371000 # Radius of earth in meters.
        return c * r