Trigonometric distances of planetary nebulae

Trigonometric parallaxes of three planetary nebulae (A21, A29, and He 2-11) have been determined. The method of observation and reduction is briefly p resented. The results for A21 and A29 are within the formal errors of the v alues published by USNO Flagstaff Station. The trigonometric method is disc ussed in comparison with other methods generally used for distance determin ation. We have found that statistical distances show no correlation with tr igonometric distances, while other methods, which do not involve any assump tion about the central stars of the nebulae, give in general fairly good co rrelations.