In this case the ACT catalog was used, which had for example more stars and whereas the GSC 1.1 had 3rd order fit for plate distortions, the ACT has a five-order available. Another improvement of the GSC 1.1 was the GSC 1.2.
The ACT catalog was used in this instance, providing more stars than the GSC 1.1, which had third-order correction for plate distortions. The ACT offers a five-order solution, and the GSC 1.2 represented an improvement over the GSC 1.1.