Upper panel: semi-major axis uncertainties for asteroids with occultations. The values obtained from the use of all observations are on the horizontal axis. On the vertical axis, we show the uncertainty obtained when fitting positions derived from occultation astrometry only, with the procedure described in the text. The line represents equal uncertainties. The symbol colour is related to the number of occultations used. Bottom panel: same as upper panel, but using pre-Gaia astrometry.