We investigated the improvement of photographic astrometric observations of outer planets by use of Hipparcos Catalogue stars. The position of Jupiter on the plates was determined from measurements of the Galilean satellites, combined with ephemerides accurate to better than 10 mas. In a 6 contants model of photographic plates reduction, when images are near the center of the field, only offsets have to be computed, by considering the scale and orientation known. We applied this idea to the galilean satellites by using137 photographic plates realised between 1967 and 1974 by D. Pascu, with the Mc Cormick 26-inch refractor, Virginia, and its twin in the USNO, Washington DC. A study of the scale factors reveals an expected anisotropy of the scale factors. One possible hypothesis is that this effect is a consequence of the thermal expansions of the telescopic components. The use of Hipparcos data in the astrometric reduction allows to obtain an external error of around 40 mas on the determination of the observed absolute position of Jupiter. It is 5 times better than the classic computations using photographic or CCD reductions. Furthemore, with such results, badly-known effects can be easly estimated like the anisotropy of scale. At the end, important impacts on the fit of planetary ephemerides to the visual observations are expected.