We used preliminary positions of the Yale Southern Proper Motion (SPM) catalogue (Platais, Girard et al., Astronomy from Large Database II, eds. Heck & Murtag, 1992) in a region of 5 fields around the South Galactic Pole to assess the astrometric accuracy of the mask solution (Taff, Lattanzi and Bucciarelli, ApJ 361, 667, 1990), which will be used (in combination with the subplate method) for the construction of the Guide Star Catalogue (GSC) version 1.2. Another semi-external check is done by direct comparison of GSC positions of stars lying onto overlapping plate areas. Results in tables 1 and 2 show that the average rms of the GSC–SPM differences is quite satisfactory (∼ 0.33 arcsecper coordinate), while an error degradation (up to ∼ 1 arcsecpositional) can occurr within a limited area at the plate corners, its signature varying from plate to plate. This can be cured on a plate-by-plate basis by the use of a filtering technique, e.g., as provided by the Collocation method (Bucciarelli, Lattanzi and Taff, ApJ Suppl. 84, 91, 1993), wherever a suitable reference star density is available.