Returns tranlated and scaled geometries suitable for Mapbox vector
        tiles.

def pbf(self, bbox, geo_col=None, scale=4096):
        """Returns tranlated and scaled geometries suitable for Mapbox vector
        tiles.
        """
        col = geo_col or self.geo_field.name
        w, s, e, n = bbox.extent
        trans = self._trans_scale(col, -w, -s,
                                  scale / (e - w),
                                  scale / (n - s))
        g = AsText(trans)
        return self.annotate(pbf=g)